timer.c 40.6 KB
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/*
 * This file is part of the Micro Python project, http://micropython.org/
 *
 * The MIT License (MIT)
 *
 * Copyright (c) 2013, 2014 Damien P. George
 *
 * Permission is hereby granted, free of charge, to any person obtaining a copy
 * of this software and associated documentation files (the "Software"), to deal
 * in the Software without restriction, including without limitation the rights
 * to use, copy, modify, merge, publish, distribute, sublicense, and/or sell
 * copies of the Software, and to permit persons to whom the Software is
 * furnished to do so, subject to the following conditions:
 *
 * The above copyright notice and this permission notice shall be included in
 * all copies or substantial portions of the Software.
 *
 * THE SOFTWARE IS PROVIDED "AS IS", WITHOUT WARRANTY OF ANY KIND, EXPRESS OR
 * IMPLIED, INCLUDING BUT NOT LIMITED TO THE WARRANTIES OF MERCHANTABILITY,
 * FITNESS FOR A PARTICULAR PURPOSE AND NONINFRINGEMENT. IN NO EVENT SHALL THE
 * AUTHORS OR COPYRIGHT HOLDERS BE LIABLE FOR ANY CLAIM, DAMAGES OR OTHER
 * LIABILITY, WHETHER IN AN ACTION OF CONTRACT, TORT OR OTHERWISE, ARISING FROM,
 * OUT OF OR IN CONNECTION WITH THE SOFTWARE OR THE USE OR OTHER DEALINGS IN
 * THE SOFTWARE.
 */

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#include <stdint.h>
#include <stdio.h>
#include <string.h>

#include <stm32f4xx_hal.h>
#include "usbd_cdc_msc_hid.h"
#include "usbd_cdc_interface.h"

#include "nlr.h"
#include "misc.h"
#include "mpconfig.h"
#include "qstr.h"
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#include "gc.h"
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#include "obj.h"
#include "runtime.h"
#include "timer.h"
#include "servo.h"
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#include "pin.h"
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/// \moduleref pyb
/// \class Timer - periodically call a function
///
/// Timers can be used for a great variety of tasks.  At the moment, only
/// the simplest case is implemented: that of calling a function periodically.
///
/// Each timer consists of a counter that counts up at a certain rate.  The rate
/// at which it counts is the peripheral clock frequency (in Hz) divided by the
/// timer prescaler.  When the counter reaches the timer period it triggers an
/// event, and the counter resets back to zero.  By using the callback method,
/// the timer event can call a Python function.
///
/// Example usage to toggle an LED at a fixed frequency:
///
///     tim = pyb.Timer(4)              # create a timer object using timer 4
///     tim.init(freq=2)                # trigger at 2Hz
///     tim.callback(lambda t:pyb.LED(1).toggle())
///
/// Further examples:
///
///     tim = pyb.Timer(4, freq=100)    # freq in Hz
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///     tim = pyb.Timer(4, prescaler=0, period=99)
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///     tim.counter()                   # get counter (can also set)
///     tim.prescaler(2)                # set prescaler (can also get)
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///     tim.period(199)                 # set period (can also get)
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///     tim.callback(lambda t: ...)     # set callback for update interrupt (t=tim instance)
///     tim.callback(None)              # clear callback
///
/// *Note:* Timer 3 is reserved for internal use.  Timer 5 controls
/// the servo driver, and Timer 6 is used for timed ADC/DAC reading/writing.
/// It is recommended to use the other timers in your programs.

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// The timers can be used by multiple drivers, and need a common point for
// the interrupts to be dispatched, so they are all collected here.
//
// TIM3:
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//  - flash storage controller, to flush the cache
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//  - USB CDC interface, interval, to check for new data
//  - LED 4, PWM to set the LED intensity
//
// TIM5:
//  - servo controller, PWM
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//
// TIM6:
//  - ADC, DAC for read_timed and write_timed

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typedef enum {
    CHANNEL_MODE_PWM_NORMAL,
    CHANNEL_MODE_PWM_INVERTED,
    CHANNEL_MODE_OC_TIMING,
    CHANNEL_MODE_OC_ACTIVE,
    CHANNEL_MODE_OC_INACTIVE,
    CHANNEL_MODE_OC_TOGGLE,
    CHANNEL_MODE_OC_FORCED_ACTIVE,
    CHANNEL_MODE_OC_FORCED_INACTIVE,
    CHANNEL_MODE_IC,
} pyb_channel_mode;

STATIC const struct {
    qstr        name;
    uint32_t    oc_mode;
} gChannelMode[] = {
    { MP_QSTR_PWM,                TIM_OCMODE_PWM1 },
    { MP_QSTR_PWM_INVERTED,       TIM_OCMODE_PWM2 },
    { MP_QSTR_OC_TIMING,          TIM_OCMODE_TIMING },
    { MP_QSTR_OC_ACTIVE,          TIM_OCMODE_ACTIVE },
    { MP_QSTR_OC_INACTIVE,        TIM_OCMODE_INACTIVE },
    { MP_QSTR_OC_TOGGLE,          TIM_OCMODE_TOGGLE },
    { MP_QSTR_OC_FORCED_ACTIVE,   TIM_OCMODE_FORCED_ACTIVE },
    { MP_QSTR_OC_FORCED_INACTIVE, TIM_OCMODE_FORCED_INACTIVE },
    { MP_QSTR_IC,                 0 },
};

typedef struct _pyb_timer_channel_obj_t {
    mp_obj_base_t base;
    struct _pyb_timer_obj_t *timer;
    uint8_t channel;
    uint8_t mode;
    mp_obj_t callback;
    struct _pyb_timer_channel_obj_t *next;
} pyb_timer_channel_obj_t;

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typedef struct _pyb_timer_obj_t {
    mp_obj_base_t base;
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    uint8_t tim_id;
    uint8_t is_32bit;
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    mp_obj_t callback;
    TIM_HandleTypeDef tim;
    IRQn_Type irqn;
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    pyb_timer_channel_obj_t *channel;

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} pyb_timer_obj_t;
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// The following yields TIM_IT_UPDATE when channel is zero and
// TIM_IT_CC1..TIM_IT_CC4 when channel is 1..4
#define TIMER_IRQ_MASK(channel) (1 << (channel))
#define TIMER_CNT_MASK(self)    ((self)->is_32bit ? 0x3fffffff : 0xffff)
#define TIMER_CHANNEL(self)     ((((self)->channel) - 1) << 2)

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TIM_HandleTypeDef TIM3_Handle;
TIM_HandleTypeDef TIM5_Handle;
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TIM_HandleTypeDef TIM6_Handle;
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// Used to divide down TIM3 and periodically call the flash storage IRQ
static uint32_t tim3_counter = 0;

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// Used to do callbacks to Python code on interrupt
STATIC pyb_timer_obj_t *pyb_timer_obj_all[14];
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#define PYB_TIMER_OBJ_ALL_NUM MP_ARRAY_SIZE(pyb_timer_obj_all)
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STATIC mp_obj_t pyb_timer_deinit(mp_obj_t self_in);
STATIC mp_obj_t pyb_timer_callback(mp_obj_t self_in, mp_obj_t callback);
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STATIC mp_obj_t pyb_timer_channel_callback(mp_obj_t self_in, mp_obj_t callback);
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void timer_init0(void) {
    tim3_counter = 0;
    for (uint i = 0; i < PYB_TIMER_OBJ_ALL_NUM; i++) {
        pyb_timer_obj_all[i] = NULL;
    }
}

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// unregister all interrupt sources
void timer_deinit(void) {
    for (uint i = 0; i < PYB_TIMER_OBJ_ALL_NUM; i++) {
        pyb_timer_obj_t *tim = pyb_timer_obj_all[i];
        if (tim != NULL) {
            pyb_timer_deinit(tim);
        }
    }
}

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// TIM3 is set-up for the USB CDC interface
void timer_tim3_init(void) {
    // set up the timer for USBD CDC
    __TIM3_CLK_ENABLE();

    TIM3_Handle.Instance = TIM3;
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    TIM3_Handle.Init.Period = (USBD_CDC_POLLING_INTERVAL*1000) - 1; // TIM3 fires every USBD_CDC_POLLING_INTERVAL ms
    TIM3_Handle.Init.Prescaler = 84-1; // for System clock at 168MHz, TIM3 runs at 1MHz
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    TIM3_Handle.Init.ClockDivision = TIM_CLOCKDIVISION_DIV1;
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    TIM3_Handle.Init.CounterMode = TIM_COUNTERMODE_UP;
    HAL_TIM_Base_Init(&TIM3_Handle);

    HAL_NVIC_SetPriority(TIM3_IRQn, 6, 0);
    HAL_NVIC_EnableIRQ(TIM3_IRQn);

    if (HAL_TIM_Base_Start(&TIM3_Handle) != HAL_OK) {
        /* Starting Error */
    }
}

/* unused
void timer_tim3_deinit(void) {
    // reset TIM3 timer
    __TIM3_FORCE_RESET();
    __TIM3_RELEASE_RESET();
}
*/

// TIM5 is set-up for the servo controller
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// This function inits but does not start the timer
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void timer_tim5_init(void) {
    // TIM5 clock enable
    __TIM5_CLK_ENABLE();

    // set up and enable interrupt
    HAL_NVIC_SetPriority(TIM5_IRQn, 6, 0);
    HAL_NVIC_EnableIRQ(TIM5_IRQn);

    // PWM clock configuration
    TIM5_Handle.Instance = TIM5;
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    TIM5_Handle.Init.Period = 2000 - 1; // timer cycles at 50Hz
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    TIM5_Handle.Init.Prescaler = ((SystemCoreClock / 2) / 100000) - 1; // timer runs at 100kHz
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    TIM5_Handle.Init.ClockDivision = TIM_CLOCKDIVISION_DIV1;
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    TIM5_Handle.Init.CounterMode = TIM_COUNTERMODE_UP;
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    HAL_TIM_PWM_Init(&TIM5_Handle);
}

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// Init TIM6 with a counter-overflow at the given frequency (given in Hz)
// TIM6 is used by the DAC and ADC for auto sampling at a given frequency
// This function inits but does not start the timer
void timer_tim6_init(uint freq) {
    // TIM6 clock enable
    __TIM6_CLK_ENABLE();

    // Timer runs at SystemCoreClock / 2
    // Compute the prescaler value so TIM6 triggers at freq-Hz
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    uint32_t period = MAX(1, (SystemCoreClock / 2) / freq);
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    uint32_t prescaler = 1;
    while (period > 0xffff) {
        period >>= 1;
        prescaler <<= 1;
    }

    // Time base clock configuration
    TIM6_Handle.Instance = TIM6;
    TIM6_Handle.Init.Period = period - 1;
    TIM6_Handle.Init.Prescaler = prescaler - 1;
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    TIM6_Handle.Init.ClockDivision = TIM_CLOCKDIVISION_DIV1; // unused for TIM6
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    TIM6_Handle.Init.CounterMode = TIM_COUNTERMODE_UP; // unused for TIM6
    HAL_TIM_Base_Init(&TIM6_Handle);
}

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// Interrupt dispatch
void HAL_TIM_PeriodElapsedCallback(TIM_HandleTypeDef *htim) {
    if (htim == &TIM3_Handle) {
        USBD_CDC_HAL_TIM_PeriodElapsedCallback();
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        // Periodically raise a flash IRQ for the flash storage controller
        if (tim3_counter++ >= 500 / USBD_CDC_POLLING_INTERVAL) {
            tim3_counter = 0;
            NVIC->STIR = FLASH_IRQn;
        }

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    } else if (htim == &TIM5_Handle) {
        servo_timer_irq_callback();
    }
}

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/******************************************************************************/
/* Micro Python bindings                                                      */

STATIC void pyb_timer_print(void (*print)(void *env, const char *fmt, ...), void *env, mp_obj_t self_in, mp_print_kind_t kind) {
    pyb_timer_obj_t *self = self_in;

    if (self->tim.State == HAL_TIM_STATE_RESET) {
        print(env, "Timer(%u)", self->tim_id);
    } else {
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        print(env, "Timer(%u, prescaler=%u, period=%u, mode=%s, div=%u)",
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            self->tim_id,
            self->tim.Init.Prescaler,
            self->tim.Init.Period,
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            self->tim.Init.CounterMode == TIM_COUNTERMODE_UP     ? "UP" :
            self->tim.Init.CounterMode == TIM_COUNTERMODE_DOWN   ? "DOWN" : "CENTER",
            self->tim.Init.ClockDivision == TIM_CLOCKDIVISION_DIV4 ? 4 :
            self->tim.Init.ClockDivision == TIM_CLOCKDIVISION_DIV2 ? 2 : 1);
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    }
}
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/// \method init(*, freq, prescaler, period)
/// Initialise the timer.  Initialisation must be either by frequency (in Hz)
/// or by prescaler and period:
///
///     tim.init(freq=100)                  # set the timer to trigger at 100Hz
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///     tim.init(prescaler=83, period=999)  # set the prescaler and period directly
///
/// Keyword arguments:
///
///   - `freq` - specifies the periodic frequency of the timer. You migh also
///              view this as the frequency with which the timer goes through
///              one complete cycle.
///
///   - `prescaler` [0-0xffff] - specifies the value to be loaded into the
///                 timer's Prescaler Register (PSC). The timer clock source is divided by
///     (`prescaler + 1`) to arrive at the timer clock. Timers 2-7 and 12-14
///     have a clock source of 84 MHz (pyb.freq()[2] * 2), and Timers 1, and 8-11
///     have a clock source of 168 MHz (pyb.freq()[3] * 2).
///
///   - `period` [0-0xffff] for timers 1, 3, 4, and 6-15. [0-0x3fffffff] for timers 2 & 5.
///              Specifies the value to be loaded into the timer's AutoReload
///     Register (ARR). This determines the period of the timer (i.e. when the
///     counter cycles). The timer counter will roll-over after `period + 1`
///     timer clock cycles.
///
///   - `mode` can be one of:
///     - `Timer.UP` - configures the timer to count from 0 to ARR (default)
///     - `Timer.DOWN` - configures the timer to count from ARR down to 0.
///     - `Timer.CENTER` - confgures the timer to count from 0 to ARR and
///       then back down to 0.
///
///   - `div` can be one of 1, 2, or 4. Divides the timer clock to determine
///       the sampling clock used by the digital filters.
///
///   - `callback` - as per Timer.callback()
///
///  You must either specify freq or both of period and prescaler.
    STATIC const mp_arg_t pyb_timer_init_args[] = {
    { MP_QSTR_freq,         MP_ARG_KW_ONLY | MP_ARG_INT, {.u_int = 0xffffffff} },
    { MP_QSTR_prescaler,    MP_ARG_KW_ONLY | MP_ARG_INT, {.u_int = 0xffffffff} },
    { MP_QSTR_period,       MP_ARG_KW_ONLY | MP_ARG_INT, {.u_int = 0xffffffff} },
    { MP_QSTR_mode,         MP_ARG_KW_ONLY | MP_ARG_INT, {.u_int = TIM_COUNTERMODE_UP} },
    { MP_QSTR_div,          MP_ARG_KW_ONLY | MP_ARG_INT, {.u_int = 1} },
    { MP_QSTR_callback,     MP_ARG_KW_ONLY | MP_ARG_OBJ, {.u_obj = mp_const_none} },
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};
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#define PYB_TIMER_INIT_NUM_ARGS MP_ARRAY_SIZE(pyb_timer_init_args)
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STATIC mp_obj_t pyb_timer_init_helper(pyb_timer_obj_t *self, mp_uint_t n_args, const mp_obj_t *args, mp_map_t *kw_args) {
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    // parse args
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    mp_arg_val_t vals[PYB_TIMER_INIT_NUM_ARGS];
    mp_arg_parse_all(n_args, args, kw_args, PYB_TIMER_INIT_NUM_ARGS, pyb_timer_init_args, vals);
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    // set the TIM configuration values
    TIM_Base_InitTypeDef *init = &self->tim.Init;
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    if (vals[0].u_int != 0xffffffff) {
        // set prescaler and period from frequency

        if (vals[0].u_int == 0) {
            nlr_raise(mp_obj_new_exception_msg(&mp_type_ValueError, "can't have 0 frequency"));
        }

        // work out TIM's clock source
        uint tim_clock;
        if (self->tim_id == 1 || (8 <= self->tim_id && self->tim_id <= 11)) {
            // TIM{1,8,9,10,11} are on APB2
            tim_clock = HAL_RCC_GetPCLK2Freq();
        } else {
            // TIM{2,3,4,5,6,7,12,13,14} are on APB1
            tim_clock = HAL_RCC_GetPCLK1Freq();
        }

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        // Compute the prescaler value so TIM triggers at freq-Hz
        // On STM32F405/407/415/417 there are 2 cases for how the clock freq is set.
        // If the APB prescaler is 1, then the timer clock is equal to its respective
        // APB clock.  Otherwise (APB prescaler > 1) the timer clock is twice its
        // respective APB clock.  See DM00031020 Rev 4, page 115.
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        uint32_t period = MAX(1, 2 * tim_clock / vals[0].u_int);
        uint32_t prescaler = 1;
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        while (period > TIMER_CNT_MASK(self)) {
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            period >>= 1;
            prescaler <<= 1;
        }
        init->Prescaler = prescaler - 1;
        init->Period = period - 1;
    } else if (vals[1].u_int != 0xffffffff && vals[2].u_int != 0xffffffff) {
        // set prescaler and period directly
        init->Prescaler = vals[1].u_int;
        init->Period = vals[2].u_int;
    } else {
        nlr_raise(mp_obj_new_exception_msg(&mp_type_TypeError, "must specify either freq, or prescaler and period"));
    }

    init->CounterMode = vals[3].u_int;
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    init->ClockDivision = vals[4].u_int == 2 ? TIM_CLOCKDIVISION_DIV2 :
                          vals[4].u_int == 4 ? TIM_CLOCKDIVISION_DIV4 :
                                               TIM_CLOCKDIVISION_DIV1;
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    init->RepetitionCounter = 0;

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    if (!IS_TIM_COUNTER_MODE(init->CounterMode)) {
        nlr_raise(mp_obj_new_exception_msg_varg(&mp_type_ValueError, "Invalid counter_mode (%d)", init->CounterMode));
    }

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    // init the TIM peripheral
    switch (self->tim_id) {
        case 1: __TIM1_CLK_ENABLE(); break;
        case 2: __TIM2_CLK_ENABLE(); break;
        case 3: __TIM3_CLK_ENABLE(); break;
        case 4: __TIM4_CLK_ENABLE(); break;
        case 5: __TIM5_CLK_ENABLE(); break;
        case 6: __TIM6_CLK_ENABLE(); break;
        case 7: __TIM7_CLK_ENABLE(); break;
        case 8: __TIM8_CLK_ENABLE(); break;
        case 9: __TIM9_CLK_ENABLE(); break;
        case 10: __TIM10_CLK_ENABLE(); break;
        case 11: __TIM11_CLK_ENABLE(); break;
        case 12: __TIM12_CLK_ENABLE(); break;
        case 13: __TIM13_CLK_ENABLE(); break;
        case 14: __TIM14_CLK_ENABLE(); break;
    }
    // set the priority (if not a special timer)
    if (self->tim_id != 3 && self->tim_id != 5) {
        HAL_NVIC_SetPriority(self->irqn, 0xe, 0xe); // next-to lowest priority
    }
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    HAL_TIM_Base_Init(&self->tim);
    if (vals[5].u_obj == mp_const_none) {
        HAL_TIM_Base_Start(&self->tim);
    } else {
        pyb_timer_callback(self, vals[5].u_obj);
    }

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    return mp_const_none;
}

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/// \classmethod \constructor(id, ...)
/// Construct a new timer object of the given id.  If additional
/// arguments are given, then the timer is initialised by `init(...)`.
/// `id` can be 1 to 14, excluding 3.
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STATIC mp_obj_t pyb_timer_make_new(mp_obj_t type_in, mp_uint_t n_args, mp_uint_t n_kw, const mp_obj_t *args) {
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    // check arguments
    mp_arg_check_num(n_args, n_kw, 1, MP_OBJ_FUN_ARGS_MAX, true);

    // create new Timer object
    pyb_timer_obj_t *tim = m_new_obj(pyb_timer_obj_t);
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    memset(tim, 0, sizeof(*tim));

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    tim->base.type = &pyb_timer_type;
    tim->callback = mp_const_none;
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    tim->channel = NULL;
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    // get TIM number
    tim->tim_id = mp_obj_get_int(args[0]);
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    tim->is_32bit = false;
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    switch (tim->tim_id) {
        case 1: tim->tim.Instance = TIM1; tim->irqn = TIM1_UP_TIM10_IRQn; break;
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        case 2: tim->tim.Instance = TIM2; tim->irqn = TIM2_IRQn; tim->is_32bit = true; break;
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        case 3: nlr_raise(mp_obj_new_exception_msg(&mp_type_ValueError, "Timer 3 is for internal use only")); // TIM3 used for low-level stuff; go via regs if necessary
        case 4: tim->tim.Instance = TIM4; tim->irqn = TIM4_IRQn; break;
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        case 5: tim->tim.Instance = TIM5; tim->irqn = TIM5_IRQn; tim->is_32bit = true; break;
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        case 6: tim->tim.Instance = TIM6; tim->irqn = TIM6_DAC_IRQn; break;
        case 7: tim->tim.Instance = TIM7; tim->irqn = TIM7_IRQn; break;
        case 8: tim->tim.Instance = TIM8; tim->irqn = TIM8_UP_TIM13_IRQn; break;
        case 9: tim->tim.Instance = TIM9; tim->irqn = TIM1_BRK_TIM9_IRQn; break;
        case 10: tim->tim.Instance = TIM10; tim->irqn = TIM1_UP_TIM10_IRQn; break;
        case 11: tim->tim.Instance = TIM11; tim->irqn = TIM1_TRG_COM_TIM11_IRQn; break;
        case 12: tim->tim.Instance = TIM12; tim->irqn = TIM8_BRK_TIM12_IRQn; break;
        case 13: tim->tim.Instance = TIM13; tim->irqn = TIM8_UP_TIM13_IRQn; break;
        case 14: tim->tim.Instance = TIM14; tim->irqn = TIM8_TRG_COM_TIM14_IRQn; break;
        default: nlr_raise(mp_obj_new_exception_msg_varg(&mp_type_ValueError, "Timer %d does not exist", tim->tim_id));
    }

    if (n_args > 1 || n_kw > 0) {
        // start the peripheral
        mp_map_t kw_args;
        mp_map_init_fixed_table(&kw_args, n_kw, args + n_args);
        pyb_timer_init_helper(tim, n_args - 1, args + 1, &kw_args);
    }

    // set the global variable for interrupt callbacks
    if (tim->tim_id - 1 < PYB_TIMER_OBJ_ALL_NUM) {
        pyb_timer_obj_all[tim->tim_id - 1] = tim;
    }

    return (mp_obj_t)tim;
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}

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STATIC mp_obj_t pyb_timer_init(mp_uint_t n_args, const mp_obj_t *args, mp_map_t *kw_args) {
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    return pyb_timer_init_helper(args[0], n_args - 1, args + 1, kw_args);
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}
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STATIC MP_DEFINE_CONST_FUN_OBJ_KW(pyb_timer_init_obj, 1, pyb_timer_init);
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/// \method deinit()
/// Deinitialises the timer.
///
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/// Disables the callback (and the associated irq).
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/// Disables any channel callbacks (and the associated irq).
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/// Stops the timer, and disables the timer peripheral.
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STATIC mp_obj_t pyb_timer_deinit(mp_obj_t self_in) {
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    pyb_timer_obj_t *self = self_in;

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    // Disable the base interrupt
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    pyb_timer_callback(self_in, mp_const_none);

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    pyb_timer_channel_obj_t *chan = self->channel;
    self->channel = NULL;

    // Disable the channel interrupts
    while (chan != NULL) {
        pyb_timer_channel_callback(chan, mp_const_none);
        pyb_timer_channel_obj_t *prev_chan = chan;
        chan = chan->next;
        prev_chan->next = NULL;
    }

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    HAL_TIM_Base_DeInit(&self->tim);
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    return mp_const_none;
}
STATIC MP_DEFINE_CONST_FUN_OBJ_1(pyb_timer_deinit_obj, pyb_timer_deinit);

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/// \method channel(channel, mode, ...)
///
/// If only a channel nunber is passed, then a previously initialized channel
/// object is returned.
///
/// Othwerwise, a TimerChannel object is initialized and returned.
///
/// Each channel can be configured to perform pwm, output compare, or
/// input capture. All channels share the same underlying timer, which means
/// that they share the same timer clock.
///
/// Keyword arguments:
///
///   - `mode` can be one of:
///     - `Timer.PWM` - configure the timer in PWM mode (active high).
///     - `Timer.PWM_INVERTED` - configure the timer in PWM mode (active low).
///     - `Timer.OC_TIMING` - indicates that no pin is driven.
///     - `Timer.OC_ACTIVE` - the pin will be made active when a compare
///        match occurs (active is determined by polarity)
///     - `Timer.OC_INACTIVE` - the pin will be made inactive when a compare
///        match occurs.
///     - `Timer.OC_TOGGLE` - the pin will be toggled when an compare match occurs.
///     - `Timer.OC_FORCED_ACTIVE` - the pin is forced active (compare match is ignored).
///     - `Timer.OC_FORCED_INACTIVE` - the pin is forced inactive (compare match is ignored).
///     - `Timer.IC` - configure the timer in Input Capture mode.
///
///   - `callback` - as per TimerChannel.callback()
///
///   - `pin` None (the default) or a Pin object. If specified (and not None)
///           this will cause the alternate function of the the indicated pin
///      to be configured for this timer channel. An error will be raised if
///      the pin doesn't support any alternate functions for this timer channel.
///
/// Keyword arguments for Timer.PWM modes:
///
///   - 'pulse_width' - determines the initial pulse width to use.
///
/// Keyword arguments for Timer.OC modes:
///
///   - `compare` - determines the initial value of the compare register.
///
///   - `polarity` can be one of:
///     - `Timer.HIGH` - output is active high
///     - `Timer.LOW` - output is acive low
///
/// Optional keyword arguments for Timer.IC modes:
///
///   - `polarity` can be one of:
///     - `Timer.RISING` - captures on rising edge.
///     - `Timer.FALLING` - captures on falling edge.
///     - `Timer.BOTH` - captures on both edges.
///
/// PWM Example:
///
///     timer = pyb.Timer(2, freq=1000)
///     ch2 = timer.channel(2, pyb.Timer.PWM, pin=pyb.Pin.board.X2, pulse_width=210000)
///     ch3 = timer.channel(3, pyb.Timer.PWM, pin=pyb.Pin.board.X3, pulse_width=420000)
STATIC const mp_arg_t pyb_timer_channel_args[] = {
    { MP_QSTR_callback,         MP_ARG_KW_ONLY | MP_ARG_OBJ, {.u_obj = mp_const_none} },
    { MP_QSTR_pin,              MP_ARG_KW_ONLY | MP_ARG_OBJ, {.u_obj = mp_const_none} },
    { MP_QSTR_pulse_width,      MP_ARG_KW_ONLY | MP_ARG_INT, {.u_int = 0} },
    { MP_QSTR_compare,          MP_ARG_KW_ONLY | MP_ARG_INT, {.u_int = 0} },
    { MP_QSTR_polarity,         MP_ARG_KW_ONLY | MP_ARG_INT, {.u_int = 0xffffffff} },
};
#define PYB_TIMER_CHANNEL_NUM_ARGS MP_ARRAY_SIZE(pyb_timer_channel_args)

STATIC mp_obj_t pyb_timer_channel(mp_uint_t n_args, const mp_obj_t *args, mp_map_t *kw_args) {
    mp_arg_check_num(n_args, n_args - 3, 3, MP_OBJ_FUN_ARGS_MAX, true);

    pyb_timer_obj_t *self = args[0];
    mp_int_t channel = mp_obj_get_int(args[1]);

    if (channel < 1 || channel > 4) {
        nlr_raise(mp_obj_new_exception_msg_varg(&mp_type_ValueError, "Invalid channel (%d)", channel));
    }

    pyb_timer_channel_obj_t *chan = self->channel;
    pyb_timer_channel_obj_t *prev_chan = NULL;

    while (chan != NULL) {
        if (chan->channel == channel) {
            break;
        }
        prev_chan = chan;
        chan = chan->next;
    }
    if (kw_args->used == 0) {
        // Return the previously allocated channel
        if (chan) {
            return chan;
        }
        return mp_const_none;
    }

    // If there was already a channel, then remove it from the list. Note that
    // the order we do things here is important so as to appear atomic to
    // the IRQ handler.
    if (chan) {
        // Turn off any IRQ associated with the channel.
        pyb_timer_channel_callback(chan, mp_const_none);

        // Unlink the channel from the list.
        if (prev_chan) {
            prev_chan->next = chan->next;
        }
        self->channel = chan->next;
        chan->next = NULL;
    }

    // Allocate and initialize a new channel
    mp_arg_val_t vals[PYB_TIMER_CHANNEL_NUM_ARGS];
    mp_arg_parse_all(n_args - 3, args + 3, kw_args, PYB_TIMER_CHANNEL_NUM_ARGS, pyb_timer_channel_args, vals);

    chan = m_new_obj(pyb_timer_channel_obj_t);
    memset(chan, 0, sizeof(*chan));
    chan->base.type = &pyb_timer_channel_type;
    chan->timer = self;
    chan->channel = channel;
    chan->mode = mp_obj_get_int(args[2]);
    chan->callback = vals[0].u_obj;

    mp_obj_t pin_obj = vals[1].u_obj;
    if (pin_obj != mp_const_none) {
        if (!MP_OBJ_IS_TYPE(pin_obj, &pin_type)) {
            nlr_raise(mp_obj_new_exception_msg(&mp_type_ValueError, "pin argument needs to be be a Pin type"));
        }
        const pin_obj_t *pin = pin_obj;
        const pin_af_obj_t *af = pin_find_af(pin, AF_FN_TIM, self->tim_id);
        if (af == NULL) {
            nlr_raise(mp_obj_new_exception_msg_varg(&mp_type_ValueError, "pin %s doesn't have an af for TIM%d", qstr_str(pin->name), self->tim_id));
        }
        // pin.init(mode=AF_PP, af=idx)
        const mp_obj_t args[6] = {
            (mp_obj_t)&pin_init_obj,
            pin_obj,
            MP_OBJ_NEW_QSTR(MP_QSTR_mode),  MP_OBJ_NEW_SMALL_INT(GPIO_MODE_AF_PP),
            MP_OBJ_NEW_QSTR(MP_QSTR_af),    MP_OBJ_NEW_SMALL_INT(af->idx)
        };
        mp_call_method_n_kw(0, 2, args);
    }

    // Link the channel to the timer before we turn the channel on.
    // Note that this needs to appear atomic to the IRQ handler (the write
    // to self->channel is atomic, so we're good, but I thought I'd mention
    // in case this was ever changed in the future).
    chan->next = self->channel;
    self->channel = chan;

    switch (chan->mode) {

        case CHANNEL_MODE_PWM_NORMAL:
        case CHANNEL_MODE_PWM_INVERTED: {
            TIM_OC_InitTypeDef oc_config;
            oc_config.OCMode       = gChannelMode[chan->mode].oc_mode;
            oc_config.Pulse        = vals[2].u_int;
            oc_config.OCPolarity   = TIM_OCPOLARITY_HIGH;
            oc_config.OCNPolarity  = TIM_OCNPOLARITY_HIGH;
            oc_config.OCFastMode   = TIM_OCFAST_DISABLE;
            oc_config.OCIdleState  = TIM_OCIDLESTATE_SET;
            oc_config.OCNIdleState = TIM_OCNIDLESTATE_SET;

            HAL_TIM_PWM_ConfigChannel(&self->tim, &oc_config, TIMER_CHANNEL(chan));
            if (chan->callback == mp_const_none) {
                HAL_TIM_PWM_Start(&self->tim, TIMER_CHANNEL(chan));
            } else {
                HAL_TIM_PWM_Start_IT(&self->tim, TIMER_CHANNEL(chan));
            }
            break;
        }

        case CHANNEL_MODE_OC_TIMING:
        case CHANNEL_MODE_OC_ACTIVE:
        case CHANNEL_MODE_OC_INACTIVE:
        case CHANNEL_MODE_OC_TOGGLE:
        case CHANNEL_MODE_OC_FORCED_ACTIVE:
        case CHANNEL_MODE_OC_FORCED_INACTIVE: {
            TIM_OC_InitTypeDef oc_config;
            oc_config.OCMode       = gChannelMode[chan->mode].oc_mode;
            oc_config.Pulse        = vals[3].u_int;
            oc_config.OCPolarity   = vals[4].u_int;
            if (oc_config.OCPolarity == 0xffffffff) {
                oc_config.OCPolarity = TIM_OCPOLARITY_HIGH;
            }
            oc_config.OCNPolarity  = TIM_OCNPOLARITY_HIGH;
            oc_config.OCFastMode   = TIM_OCFAST_DISABLE;
            oc_config.OCIdleState  = TIM_OCIDLESTATE_SET;
            oc_config.OCNIdleState = TIM_OCNIDLESTATE_SET;

            if (!IS_TIM_OC_POLARITY(oc_config.OCPolarity)) {
                nlr_raise(mp_obj_new_exception_msg_varg(&mp_type_ValueError, "Invalid polarity (%d)", oc_config.OCPolarity));
            }
            HAL_TIM_OC_ConfigChannel(&self->tim, &oc_config, TIMER_CHANNEL(chan));
            if (chan->callback == mp_const_none) {
                HAL_TIM_OC_Start(&self->tim, TIMER_CHANNEL(chan));
            } else {
                HAL_TIM_OC_Start_IT(&self->tim, TIMER_CHANNEL(chan));
            }
            break;
        }

        case CHANNEL_MODE_IC: {
            TIM_IC_InitTypeDef ic_config;

            ic_config.ICPolarity  = vals[4].u_int;
            if (ic_config.ICPolarity == 0xffffffff) {
                ic_config.ICPolarity = TIM_ICPOLARITY_RISING;
            }
            ic_config.ICSelection = TIM_ICSELECTION_DIRECTTI;
            ic_config.ICPrescaler = TIM_ICPSC_DIV1;
            ic_config.ICFilter    = 0;

            if (!IS_TIM_IC_POLARITY(ic_config.ICPolarity)) {
                nlr_raise(mp_obj_new_exception_msg_varg(&mp_type_ValueError, "Invalid polarity (%d)", ic_config.ICPolarity));
            }
            HAL_TIM_IC_ConfigChannel(&self->tim, &ic_config, TIMER_CHANNEL(chan));
            if (chan->callback == mp_const_none) {
                HAL_TIM_IC_Start(&self->tim, TIMER_CHANNEL(chan));
            } else {
                HAL_TIM_IC_Start_IT(&self->tim, TIMER_CHANNEL(chan));
            }
            break;
        }

        default:
            nlr_raise(mp_obj_new_exception_msg_varg(&mp_type_ValueError, "Invalid mode (%d)", chan->mode));
    }

    return chan;
}
STATIC MP_DEFINE_CONST_FUN_OBJ_KW(pyb_timer_channel_obj, 3, pyb_timer_channel);

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/// \method counter([value])
/// Get or set the timer counter.
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mp_obj_t pyb_timer_counter(mp_uint_t n_args, const mp_obj_t *args) {
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    pyb_timer_obj_t *self = args[0];
    if (n_args == 1) {
        // get
        return mp_obj_new_int(self->tim.Instance->CNT);
    } else {
        // set
        __HAL_TIM_SetCounter(&self->tim, mp_obj_get_int(args[1]));
        return mp_const_none;
    }
}
STATIC MP_DEFINE_CONST_FUN_OBJ_VAR_BETWEEN(pyb_timer_counter_obj, 1, 2, pyb_timer_counter);

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/// \method prescaler([value])
/// Get or set the prescaler for the timer.
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mp_obj_t pyb_timer_prescaler(mp_uint_t n_args, const mp_obj_t *args) {
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    pyb_timer_obj_t *self = args[0];
    if (n_args == 1) {
        // get
        return mp_obj_new_int(self->tim.Instance->PSC & 0xffff);
    } else {
        // set
        self->tim.Init.Prescaler = self->tim.Instance->PSC = mp_obj_get_int(args[1]) & 0xffff;
        return mp_const_none;
    }
}
STATIC MP_DEFINE_CONST_FUN_OBJ_VAR_BETWEEN(pyb_timer_prescaler_obj, 1, 2, pyb_timer_prescaler);

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/// \method period([value])
/// Get or set the period of the timer.
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mp_obj_t pyb_timer_period(mp_uint_t n_args, const mp_obj_t *args) {
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    pyb_timer_obj_t *self = args[0];
    if (n_args == 1) {
        // get
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        return mp_obj_new_int(__HAL_TIM_GetAutoreload(&self->tim) & TIMER_CNT_MASK(self));
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    } else {
        // set
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        __HAL_TIM_SetAutoreload(&self->tim, mp_obj_get_int(args[1]) & TIMER_CNT_MASK(self));
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        return mp_const_none;
    }
}
STATIC MP_DEFINE_CONST_FUN_OBJ_VAR_BETWEEN(pyb_timer_period_obj, 1, 2, pyb_timer_period);
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/// \method callback(fun)
/// Set the function to be called when the timer triggers.
/// `fun` is passed 1 argument, the timer object.
/// If `fun` is `None` then the callback will be disabled.
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STATIC mp_obj_t pyb_timer_callback(mp_obj_t self_in, mp_obj_t callback) {
    pyb_timer_obj_t *self = self_in;
    if (callback == mp_const_none) {
        // stop interrupt (but not timer)
        __HAL_TIM_DISABLE_IT(&self->tim, TIM_IT_UPDATE);
        self->callback = mp_const_none;
    } else if (mp_obj_is_callable(callback)) {
        self->callback = callback;
        HAL_NVIC_EnableIRQ(self->irqn);
        // start timer, so that it interrupts on overflow
        HAL_TIM_Base_Start_IT(&self->tim);
    } else {
        nlr_raise(mp_obj_new_exception_msg(&mp_type_ValueError, "callback must be None or a callable object"));
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    }
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    return mp_const_none;
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}
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STATIC MP_DEFINE_CONST_FUN_OBJ_2(pyb_timer_callback_obj, pyb_timer_callback);

STATIC const mp_map_elem_t pyb_timer_locals_dict_table[] = {
    // instance methods
    { MP_OBJ_NEW_QSTR(MP_QSTR_init), (mp_obj_t)&pyb_timer_init_obj },
    { MP_OBJ_NEW_QSTR(MP_QSTR_deinit), (mp_obj_t)&pyb_timer_deinit_obj },
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    { MP_OBJ_NEW_QSTR(MP_QSTR_channel), (mp_obj_t)&pyb_timer_channel_obj },
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    { MP_OBJ_NEW_QSTR(MP_QSTR_counter), (mp_obj_t)&pyb_timer_counter_obj },
    { MP_OBJ_NEW_QSTR(MP_QSTR_prescaler), (mp_obj_t)&pyb_timer_prescaler_obj },
    { MP_OBJ_NEW_QSTR(MP_QSTR_period), (mp_obj_t)&pyb_timer_period_obj },
    { MP_OBJ_NEW_QSTR(MP_QSTR_callback), (mp_obj_t)&pyb_timer_callback_obj },
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    { MP_OBJ_NEW_QSTR(MP_QSTR_UP),                  MP_OBJ_NEW_SMALL_INT(TIM_COUNTERMODE_UP) },
    { MP_OBJ_NEW_QSTR(MP_QSTR_DOWN),                MP_OBJ_NEW_SMALL_INT(TIM_COUNTERMODE_DOWN) },
    { MP_OBJ_NEW_QSTR(MP_QSTR_CENTER),              MP_OBJ_NEW_SMALL_INT(TIM_COUNTERMODE_CENTERALIGNED1) },
    { MP_OBJ_NEW_QSTR(MP_QSTR_PWM),                 MP_OBJ_NEW_SMALL_INT(CHANNEL_MODE_PWM_NORMAL) },
    { MP_OBJ_NEW_QSTR(MP_QSTR_PWM_INVERTED),        MP_OBJ_NEW_SMALL_INT(CHANNEL_MODE_PWM_INVERTED) },
    { MP_OBJ_NEW_QSTR(MP_QSTR_OC_TIMING),           MP_OBJ_NEW_SMALL_INT(CHANNEL_MODE_OC_TIMING) },
    { MP_OBJ_NEW_QSTR(MP_QSTR_OC_ACTIVE),           MP_OBJ_NEW_SMALL_INT(CHANNEL_MODE_OC_ACTIVE) },
    { MP_OBJ_NEW_QSTR(MP_QSTR_OC_INACTIVE),         MP_OBJ_NEW_SMALL_INT(CHANNEL_MODE_OC_INACTIVE) },
    { MP_OBJ_NEW_QSTR(MP_QSTR_OC_TOGGLE),           MP_OBJ_NEW_SMALL_INT(CHANNEL_MODE_OC_TOGGLE) },
    { MP_OBJ_NEW_QSTR(MP_QSTR_OC_FORCED_ACTIVE),    MP_OBJ_NEW_SMALL_INT(CHANNEL_MODE_OC_FORCED_ACTIVE) },
    { MP_OBJ_NEW_QSTR(MP_QSTR_OC_FORCED_INACTIVE),  MP_OBJ_NEW_SMALL_INT(CHANNEL_MODE_OC_FORCED_INACTIVE) },
    { MP_OBJ_NEW_QSTR(MP_QSTR_IC),                  MP_OBJ_NEW_SMALL_INT(CHANNEL_MODE_IC) },
    { MP_OBJ_NEW_QSTR(MP_QSTR_HIGH),                MP_OBJ_NEW_SMALL_INT(TIM_OCPOLARITY_HIGH) },
    { MP_OBJ_NEW_QSTR(MP_QSTR_LOW),                 MP_OBJ_NEW_SMALL_INT(TIM_OCPOLARITY_LOW) },
    { MP_OBJ_NEW_QSTR(MP_QSTR_RISING),              MP_OBJ_NEW_SMALL_INT(TIM_ICPOLARITY_RISING) },
    { MP_OBJ_NEW_QSTR(MP_QSTR_FALLING),             MP_OBJ_NEW_SMALL_INT(TIM_ICPOLARITY_FALLING) },
    { MP_OBJ_NEW_QSTR(MP_QSTR_BOTH),                MP_OBJ_NEW_SMALL_INT(TIM_ICPOLARITY_BOTHEDGE) },
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};
STATIC MP_DEFINE_CONST_DICT(pyb_timer_locals_dict, pyb_timer_locals_dict_table);

const mp_obj_type_t pyb_timer_type = {
    { &mp_type_type },
    .name = MP_QSTR_Timer,
    .print = pyb_timer_print,
    .make_new = pyb_timer_make_new,
    .locals_dict = (mp_obj_t)&pyb_timer_locals_dict,
};

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/// \moduleref pyb
/// \class TimerChannel - setup a channel for a timer.
///
/// Timer channels are used to generate/capture a signal using a timer.
///
/// TimerChannel objects are created using the Timer.channel() method.
STATIC void pyb_timer_channel_print(void (*print)(void *env, const char *fmt, ...), void *env, mp_obj_t self_in, mp_print_kind_t kind) {
    pyb_timer_channel_obj_t *self = self_in;

    print(env, "TimerChannel(timer=%u, channel=%u, mode=%s)",
          self->timer->tim_id,
          self->channel,
          qstr_str(gChannelMode[self->mode].name));
}

/// \method capture([value])
/// Get or set the capture value associated with a channel.
/// capture, compare, and pulse_width are all aliases for the same function.
/// capture is the logical name to use when the channel is in input capture mode.

/// \method compare([value])
/// Get or set the compare value associated with a channel.
/// capture, compare, and pulse_width are all aliases for the same function.
/// compare is the logical name to use when the channel is in output compare mode.

/// \method pulse_width([value])
/// Get or set the pulse width value associated with a channel.
/// capture, compare, and pulse_width are all aliases for the same function.
/// pulse_width is the logical name to use when the channel is in PWM mode.
STATIC mp_obj_t pyb_timer_channel_capture_compare(uint n_args, const mp_obj_t *args) {
    pyb_timer_channel_obj_t *self = args[0];
    if (self->channel == 0) {
        nlr_raise(mp_obj_new_exception_msg_varg(&mp_type_ValueError, "Timer %d no channel specified", self->timer->tim_id));
    }
    if (n_args == 1) {
        // get
        return mp_obj_new_int(__HAL_TIM_GetCompare(&self->timer->tim, TIMER_CHANNEL(self)) & TIMER_CNT_MASK(self->timer));
    } else {
        // set
        __HAL_TIM_SetCompare(&self->timer->tim, TIMER_CHANNEL(self), mp_obj_get_int(args[1]) & TIMER_CNT_MASK(self->timer));
        return mp_const_none;
    }
}
STATIC MP_DEFINE_CONST_FUN_OBJ_VAR_BETWEEN(pyb_timer_channel_capture_compare_obj, 1, 2, pyb_timer_channel_capture_compare);

/// \method callback(fun)
/// Set the function to be called when the timer channel triggers.
/// `fun` is passed 1 argument, the timer object.
/// If `fun` is `None` then the callback will be disabled.
STATIC mp_obj_t pyb_timer_channel_callback(mp_obj_t self_in, mp_obj_t callback) {
    pyb_timer_channel_obj_t *self = self_in;
    if (callback == mp_const_none) {
        // stop interrupt (but not timer)
        __HAL_TIM_DISABLE_IT(&self->timer->tim, TIMER_IRQ_MASK(self->channel));
        self->callback = mp_const_none;
    } else if (mp_obj_is_callable(callback)) {
        self->callback = callback;
        HAL_NVIC_EnableIRQ(self->timer->irqn);
        // start timer, so that it interrupts on overflow
        switch (self->mode) {
            case CHANNEL_MODE_PWM_NORMAL:
            case CHANNEL_MODE_PWM_INVERTED:
                HAL_TIM_PWM_Start_IT(&self->timer->tim, TIMER_CHANNEL(self));
                break;
            case CHANNEL_MODE_OC_TIMING:
            case CHANNEL_MODE_OC_ACTIVE:
            case CHANNEL_MODE_OC_INACTIVE:
            case CHANNEL_MODE_OC_TOGGLE:
            case CHANNEL_MODE_OC_FORCED_ACTIVE:
            case CHANNEL_MODE_OC_FORCED_INACTIVE:
                HAL_TIM_OC_Start_IT(&self->timer->tim, TIMER_CHANNEL(self));
                break;
            case CHANNEL_MODE_IC:
                HAL_TIM_IC_Start_IT(&self->timer->tim, TIMER_CHANNEL(self));
                break;
        }
    } else {
        nlr_raise(mp_obj_new_exception_msg(&mp_type_ValueError, "callback must be None or a callable object"));
    }
    return mp_const_none;
}
STATIC MP_DEFINE_CONST_FUN_OBJ_2(pyb_timer_channel_callback_obj, pyb_timer_channel_callback);

STATIC const mp_map_elem_t pyb_timer_channel_locals_dict_table[] = {
    // instance methods
    { MP_OBJ_NEW_QSTR(MP_QSTR_callback), (mp_obj_t)&pyb_timer_channel_callback_obj },
    { MP_OBJ_NEW_QSTR(MP_QSTR_pulse_width), (mp_obj_t)&pyb_timer_channel_capture_compare_obj },
    { MP_OBJ_NEW_QSTR(MP_QSTR_capture), (mp_obj_t)&pyb_timer_channel_capture_compare_obj },
    { MP_OBJ_NEW_QSTR(MP_QSTR_compare), (mp_obj_t)&pyb_timer_channel_capture_compare_obj },
};
STATIC MP_DEFINE_CONST_DICT(pyb_timer_channel_locals_dict, pyb_timer_channel_locals_dict_table);

const mp_obj_type_t pyb_timer_channel_type = {
    { &mp_type_type },
    .name = MP_QSTR_TimerChannel,
    .print = pyb_timer_channel_print,
    .locals_dict = (mp_obj_t)&pyb_timer_channel_locals_dict,
};

void timer_handle_irq_channel(pyb_timer_obj_t *tim, uint8_t channel, mp_obj_t callback) {
    uint32_t irq_mask = TIMER_IRQ_MASK(channel);

    if (__HAL_TIM_GET_FLAG(&tim->tim, irq_mask) != RESET) {
        if (__HAL_TIM_GET_ITSTATUS(&tim->tim, irq_mask) != RESET) {
            // clear the interrupt
            __HAL_TIM_CLEAR_IT(&tim->tim, irq_mask);

            // execute callback if it's set
            if (callback != mp_const_none) {
                // When executing code within a handler we must lock the GC to prevent
                // any memory allocations.  We must also catch any exceptions.
                gc_lock();
                nlr_buf_t nlr;
                if (nlr_push(&nlr) == 0) {
                    mp_call_function_1(callback, tim);
                    nlr_pop();
                } else {
                    // Uncaught exception; disable the callback so it doesn't run again.
                    tim->callback = mp_const_none;
                    __HAL_TIM_DISABLE_IT(&tim->tim, irq_mask);
                    if (channel == 0) {
                        printf("Uncaught exception in Timer(" UINT_FMT
                               ") interrupt handler\n", tim->tim_id);
                    } else {
                        printf("Uncaught exception in Timer(" UINT_FMT ") channel "
                               UINT_FMT " interrupt handler\n", tim->tim_id, channel);
                    }
                    mp_obj_print_exception((mp_obj_t)nlr.ret_val);
                }
                gc_unlock();
            }
        }
    }
}

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void timer_irq_handler(uint tim_id) {
    if (tim_id - 1 < PYB_TIMER_OBJ_ALL_NUM) {
        // get the timer object
        pyb_timer_obj_t *tim = pyb_timer_obj_all[tim_id - 1];

        if (tim == NULL) {
            // timer object has not been set, so we can't do anything
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            printf("No timer object for id=%d\n", tim_id);
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            return;
        }
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        // Check for timer (versus timer channel) interrupt.
        timer_handle_irq_channel(tim, 0, tim->callback);
        uint32_t handled = TIMER_IRQ_MASK(0);

        // Check to see if a timer channel interrupt was pending
        pyb_timer_channel_obj_t *chan = tim->channel;
        while (chan != NULL) {
            timer_handle_irq_channel(tim, chan->channel, chan->callback);
            handled |= TIMER_IRQ_MASK(chan->channel);
            chan = chan->next;
        }

        // Finally, clear any remaining interrupt sources. Otherwise we'll
        // just get called continuously.
        uint32_t unhandled = __HAL_TIM_GET_ITSTATUS(&tim->tim, 0xff & ~handled);
        if (unhandled != 0) {
            __HAL_TIM_CLEAR_IT(&tim->tim, unhandled);
            printf("Unhandled interrupt SR=0x%02lx (now disabled)\n", unhandled);
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        }
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    }
}