powerctrl.c

/*
 * This file is part of the MicroPython project, http://micropython.org/
 *
 * The MIT License (MIT)
 *
 * Copyright (c) 2013-2018 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.
 */

#include "py/mperrno.h"
#include "py/mphal.h"
#include "powerctrl.h"
#include "rtc.h"
#include "genhdr/pllfreqtable.h"

#if defined(STM32H7)
#define RCC_SR          RSR
#define RCC_SR_SFTRSTF  RCC_RSR_SFTRSTF
#define RCC_SR_RMVF     RCC_RSR_RMVF
#else
#define RCC_SR          CSR
#define RCC_SR_SFTRSTF  RCC_CSR_SFTRSTF
#define RCC_SR_RMVF     RCC_CSR_RMVF
#endif

// Location in RAM of bootloader state (just after the top of the stack)
extern uint32_t _estack[];
#define BL_STATE ((uint32_t *)&_estack)

static inline void powerctrl_disable_hsi_if_unused(void) {
    #if !MICROPY_HW_CLK_USE_HSI && (defined(STM32F4) || defined(STM32F7) || defined(STM32H7))
    // Disable HSI if it's not used to save a little bit of power
    __HAL_RCC_HSI_DISABLE();
    #endif
}

NORETURN void powerctrl_mcu_reset(void) {
    BL_STATE[1] = 1; // invalidate bootloader address
    #if __DCACHE_PRESENT == 1
    SCB_CleanDCache();
    #endif
    NVIC_SystemReset();
}

NORETURN void powerctrl_enter_bootloader(uint32_t r0, uint32_t bl_addr) {
    BL_STATE[0] = r0;
    BL_STATE[1] = bl_addr;
    #if __DCACHE_PRESENT == 1
    SCB_CleanDCache();
    #endif
    NVIC_SystemReset();
}

static __attribute__((naked)) void branch_to_bootloader(uint32_t r0, uint32_t bl_addr) {
    __asm volatile (
        "ldr r2, [r1, #0]\n"    // get address of stack pointer
        "msr msp, r2\n"         // get stack pointer
        "ldr r2, [r1, #4]\n"    // get address of destination
        "bx r2\n"               // branch to bootloader
        );
}

void powerctrl_check_enter_bootloader(void) {
    uint32_t bl_addr = BL_STATE[1];
    BL_STATE[1] = 1; // invalidate bootloader address
    if ((bl_addr & 0xfff) == 0 && (RCC->RCC_SR & RCC_SR_SFTRSTF)) {
        // Reset by NVIC_SystemReset with bootloader data set -> branch to bootloader
        RCC->RCC_SR = RCC_SR_RMVF;
        #if defined(STM32F0) || defined(STM32F4) || defined(STM32L0) || defined(STM32L4) || defined(STM32WB)
        __HAL_SYSCFG_REMAPMEMORY_SYSTEMFLASH();
        #endif
        uint32_t r0 = BL_STATE[0];
        branch_to_bootloader(r0, bl_addr);
    }
}

#if !defined(STM32F0) && !defined(STM32L0) && !defined(STM32WB)

typedef struct _sysclk_scaling_table_entry_t {
    uint16_t mhz;
    uint16_t value;
} sysclk_scaling_table_entry_t;

#if defined(STM32F7)
STATIC const sysclk_scaling_table_entry_t volt_scale_table[] = {
    { 151, PWR_REGULATOR_VOLTAGE_SCALE3 },
    { 180, PWR_REGULATOR_VOLTAGE_SCALE2 },
    // Above 180MHz uses default PWR_REGULATOR_VOLTAGE_SCALE1
};
#elif defined(STM32H7)
STATIC const sysclk_scaling_table_entry_t volt_scale_table[] = {
    // See table 55 "Kernel clock distribution overview" of RM0433.
    {200, PWR_REGULATOR_VOLTAGE_SCALE3},
    {300, PWR_REGULATOR_VOLTAGE_SCALE2},
    // Above 300MHz uses default PWR_REGULATOR_VOLTAGE_SCALE1
    // (above 400MHz needs special handling for overdrive, currently unsupported)
};
#endif

STATIC int powerctrl_config_vos(uint32_t sysclk_mhz) {
    #if defined(STM32F7) || defined(STM32H7)
    uint32_t volt_scale = PWR_REGULATOR_VOLTAGE_SCALE1;
    for (int i = 0; i < MP_ARRAY_SIZE(volt_scale_table); ++i) {
        if (sysclk_mhz <= volt_scale_table[i].mhz) {
            volt_scale = volt_scale_table[i].value;
            break;
        }
    }
    if (HAL_PWREx_ControlVoltageScaling(volt_scale) != HAL_OK) {
        return -MP_EIO;
    }
    #endif
    return 0;
}

// Assumes that PLL is used as the SYSCLK source
int powerctrl_rcc_clock_config_pll(RCC_ClkInitTypeDef *rcc_init, uint32_t sysclk_mhz, bool need_pllsai) {
    uint32_t flash_latency;

    #if defined(STM32F7)
    if (need_pllsai) {
        // Configure PLLSAI at 48MHz for those peripherals that need this freq
        // (calculation assumes it can get an integral value of PLLSAIN)
        const uint32_t pllm = (RCC->PLLCFGR >> RCC_PLLCFGR_PLLM_Pos) & 0x3f;
        const uint32_t pllsaip = 4;
        const uint32_t pllsaiq = 2;
        const uint32_t pllsain = 48 * pllsaip * pllm / (HSE_VALUE / 1000000);
        RCC->PLLSAICFGR = pllsaiq << RCC_PLLSAICFGR_PLLSAIQ_Pos
            | (pllsaip / 2 - 1) << RCC_PLLSAICFGR_PLLSAIP_Pos
            | pllsain << RCC_PLLSAICFGR_PLLSAIN_Pos;
        RCC->CR |= RCC_CR_PLLSAION;
        uint32_t ticks = mp_hal_ticks_ms();
        while (!(RCC->CR & RCC_CR_PLLSAIRDY)) {
            if (mp_hal_ticks_ms() - ticks > 200) {
                return -MP_ETIMEDOUT;
            }
        }
        RCC->DCKCFGR2 |= RCC_DCKCFGR2_CK48MSEL;
    }
    #endif

    // If possible, scale down the internal voltage regulator to save power
    int ret = powerctrl_config_vos(sysclk_mhz);
    if (ret) {
        return ret;
    }

    #if defined(STM32F7)

    // These flash_latency values assume a supply voltage between 2.7V and 3.6V
    if (sysclk_mhz <= 30) {
        flash_latency = FLASH_LATENCY_0;
    } else if (sysclk_mhz <= 60) {
        flash_latency = FLASH_LATENCY_1;
    } else if (sysclk_mhz <= 90) {
        flash_latency = FLASH_LATENCY_2;
    } else if (sysclk_mhz <= 120) {
        flash_latency = FLASH_LATENCY_3;
    } else if (sysclk_mhz <= 150) {
        flash_latency = FLASH_LATENCY_4;
    } else if (sysclk_mhz <= 180) {
        flash_latency = FLASH_LATENCY_5;
    } else if (sysclk_mhz <= 210) {
        flash_latency = FLASH_LATENCY_6;
    } else {
        flash_latency = FLASH_LATENCY_7;
    }

    #elif defined(MICROPY_HW_FLASH_LATENCY)
    flash_latency = MICROPY_HW_FLASH_LATENCY;
    #else
    flash_latency = FLASH_LATENCY_5;
    #endif

    rcc_init->SYSCLKSource = RCC_SYSCLKSOURCE_PLLCLK;
    if (HAL_RCC_ClockConfig(rcc_init, flash_latency) != HAL_OK) {
        return -MP_EIO;
    }

    powerctrl_disable_hsi_if_unused();

    return 0;
}

#endif

#if !defined(STM32F0) && !defined(STM32L0) && !defined(STM32L4) && !defined(STM32WB)

STATIC uint32_t calc_ahb_div(uint32_t wanted_div) {
    #if defined(STM32H7)
    if (wanted_div <= 1) {
        return RCC_HCLK_DIV1;
    } else if (wanted_div <= 2) {
        return RCC_HCLK_DIV2;
    } else if (wanted_div <= 4) {
        return RCC_HCLK_DIV4;
    } else if (wanted_div <= 8) {
        return RCC_HCLK_DIV8;
    } else if (wanted_div <= 16) {
        return RCC_HCLK_DIV16;
    } else if (wanted_div <= 64) {
        return RCC_HCLK_DIV64;
    } else if (wanted_div <= 128) {
        return RCC_HCLK_DIV128;
    } else if (wanted_div <= 256) {
        return RCC_HCLK_DIV256;
    } else {
        return RCC_HCLK_DIV512;
    }
    #else
    if (wanted_div <= 1) {
        return RCC_SYSCLK_DIV1;
    } else if (wanted_div <= 2) {
        return RCC_SYSCLK_DIV2;
    } else if (wanted_div <= 4) {
        return RCC_SYSCLK_DIV4;
    } else if (wanted_div <= 8) {
        return RCC_SYSCLK_DIV8;
    } else if (wanted_div <= 16) {
        return RCC_SYSCLK_DIV16;
    } else if (wanted_div <= 64) {
        return RCC_SYSCLK_DIV64;
    } else if (wanted_div <= 128) {
        return RCC_SYSCLK_DIV128;
    } else if (wanted_div <= 256) {
        return RCC_SYSCLK_DIV256;
    } else {
        return RCC_SYSCLK_DIV512;
    }
    #endif
}

STATIC uint32_t calc_apb1_div(uint32_t wanted_div) {
    #if defined(STM32H7)
    if (wanted_div <= 1) {
        return RCC_APB1_DIV1;
    } else if (wanted_div <= 2) {
        return RCC_APB1_DIV2;
    } else if (wanted_div <= 4) {
        return RCC_APB1_DIV4;
    } else if (wanted_div <= 8) {
        return RCC_APB1_DIV8;
    } else {
        return RCC_APB1_DIV16;
    }
    #else
    if (wanted_div <= 1) {
        return RCC_HCLK_DIV1;
    } else if (wanted_div <= 2) {
        return RCC_HCLK_DIV2;
    } else if (wanted_div <= 4) {
        return RCC_HCLK_DIV4;
    } else if (wanted_div <= 8) {
        return RCC_HCLK_DIV8;
    } else {
        return RCC_HCLK_DIV16;
    }
    #endif
}

STATIC uint32_t calc_apb2_div(uint32_t wanted_div) {
    #if defined(STM32H7)
    if (wanted_div <= 1) {
        return RCC_APB2_DIV1;
    } else if (wanted_div <= 2) {
        return RCC_APB2_DIV2;
    } else if (wanted_div <= 4) {
        return RCC_APB2_DIV4;
    } else if (wanted_div <= 8) {
        return RCC_APB2_DIV8;
    } else {
        return RCC_APB2_DIV16;
    }
    #else
    return calc_apb1_div(wanted_div);
    #endif
}

int powerctrl_set_sysclk(uint32_t sysclk, uint32_t ahb, uint32_t apb1, uint32_t apb2) {
    // Return straightaway if the clocks are already at the desired frequency
    if (sysclk == HAL_RCC_GetSysClockFreq()
        && ahb == HAL_RCC_GetHCLKFreq()
        && apb1 == HAL_RCC_GetPCLK1Freq()
        && apb2 == HAL_RCC_GetPCLK2Freq()) {
        return 0;
    }

    // Default PLL parameters that give 48MHz on PLL48CK
    uint32_t m = MICROPY_HW_CLK_VALUE / 1000000, n = 336, p = 2, q = 7;
    uint32_t sysclk_source;
    bool need_pllsai = false;

    // Search for a valid PLL configuration that keeps USB at 48MHz
    uint32_t sysclk_mhz = sysclk / 1000000;
    for (const pll_freq_table_t *pll = &pll_freq_table[MP_ARRAY_SIZE(pll_freq_table) - 1]; pll >= &pll_freq_table[0]; --pll) {
        uint32_t sys = PLL_FREQ_TABLE_SYS(*pll);
        if (sys <= sysclk_mhz) {
            m = PLL_FREQ_TABLE_M(*pll);
            p = PLL_FREQ_TABLE_P(*pll);
            if (m == 0) {
                // special entry for using HSI directly
                sysclk_source = RCC_SYSCLKSOURCE_HSI;
            } else if (m == 1) {
                // special entry for using HSE directly
                sysclk_source = RCC_SYSCLKSOURCE_HSE;
            } else {
                // use PLL
                sysclk_source = RCC_SYSCLKSOURCE_PLLCLK;
                uint32_t vco_out = sys * p;
                n = vco_out * m / (MICROPY_HW_CLK_VALUE / 1000000);
                q = vco_out / 48;
                #if defined(STM32F7)
                need_pllsai = vco_out % 48 != 0;
                #endif
            }
            goto set_clk;
        }
    }
    return -MP_EINVAL;

set_clk:
    // Let the USB CDC have a chance to process before we change the clock
    mp_hal_delay_ms(5);

    // Desired system clock source is in sysclk_source
    RCC_ClkInitTypeDef RCC_ClkInitStruct;
    RCC_ClkInitStruct.ClockType = (RCC_CLOCKTYPE_SYSCLK | RCC_CLOCKTYPE_HCLK | RCC_CLOCKTYPE_PCLK1 | RCC_CLOCKTYPE_PCLK2);
    if (sysclk_source == RCC_SYSCLKSOURCE_PLLCLK) {
        // Set HSE as system clock source to allow modification of the PLL configuration
        // We then change to PLL after re-configuring PLL
        #if MICROPY_HW_CLK_USE_HSI
        RCC_ClkInitStruct.SYSCLKSource = RCC_SYSCLKSOURCE_HSI;
        #else
        RCC_ClkInitStruct.SYSCLKSource = RCC_SYSCLKSOURCE_HSE;
        #endif
    } else {
        // Directly set the system clock source as desired
        RCC_ClkInitStruct.SYSCLKSource = sysclk_source;
    }

    // Determine the bus clock dividers
    // Note: AHB freq required to be >= 14.2MHz for USB operation
    RCC_ClkInitStruct.AHBCLKDivider = calc_ahb_div(sysclk / ahb);
    #if !defined(STM32H7)
    ahb = sysclk >> AHBPrescTable[RCC_ClkInitStruct.AHBCLKDivider >> RCC_CFGR_HPRE_Pos];
    #endif
    RCC_ClkInitStruct.APB1CLKDivider = calc_apb1_div(ahb / apb1);
    RCC_ClkInitStruct.APB2CLKDivider = calc_apb2_div(ahb / apb2);
    #if defined(STM32H7)
    RCC_ClkInitStruct.SYSCLKDivider = RCC_SYSCLK_DIV1;
    RCC_ClkInitStruct.APB3CLKDivider = RCC_APB3_DIV2;
    RCC_ClkInitStruct.APB4CLKDivider = RCC_APB4_DIV2;
    #endif

    #if MICROPY_HW_CLK_LAST_FREQ
    // Save the bus dividers for use later
    uint32_t h = RCC_ClkInitStruct.AHBCLKDivider >> 4;
    uint32_t b1 = RCC_ClkInitStruct.APB1CLKDivider >> 10;
    uint32_t b2 = RCC_ClkInitStruct.APB2CLKDivider >> 10;
    #endif

    // Configure clock
    if (HAL_RCC_ClockConfig(&RCC_ClkInitStruct, FLASH_LATENCY_1) != HAL_OK) {
        return -MP_EIO;
    }

    #if defined(STM32F7)
    // Deselect PLLSAI as 48MHz source if we were using it
    RCC->DCKCFGR2 &= ~RCC_DCKCFGR2_CK48MSEL;
    // Turn PLLSAI off because we are changing PLLM (which drives PLLSAI)
    RCC->CR &= ~RCC_CR_PLLSAION;
    #endif

    // Re-configure PLL
    // Even if we don't use the PLL for the system clock, we still need it for USB, RNG and SDIO
    RCC_OscInitTypeDef RCC_OscInitStruct;
    RCC_OscInitStruct.OscillatorType = MICROPY_HW_RCC_OSCILLATOR_TYPE;
    RCC_OscInitStruct.HSEState = MICROPY_HW_RCC_HSE_STATE;
    RCC_OscInitStruct.HSIState = MICROPY_HW_RCC_HSI_STATE;
    RCC_OscInitStruct.HSICalibrationValue = RCC_HSICALIBRATION_DEFAULT;
    RCC_OscInitStruct.PLL.PLLState = RCC_PLL_ON;
    RCC_OscInitStruct.PLL.PLLSource = MICROPY_HW_RCC_PLL_SRC;
    RCC_OscInitStruct.PLL.PLLM = m;
    RCC_OscInitStruct.PLL.PLLN = n;
    RCC_OscInitStruct.PLL.PLLP = p;
    RCC_OscInitStruct.PLL.PLLQ = q;

    #if defined(STM32H7)
    RCC_OscInitStruct.PLL.PLLR = 0;
    if (MICROPY_HW_CLK_VALUE / 1000000 <= 2 * m) {
        RCC_OscInitStruct.PLL.PLLRGE = RCC_PLL1VCIRANGE_0; // 1-2MHz
    } else if (MICROPY_HW_CLK_VALUE / 1000000 <= 4 * m) {
        RCC_OscInitStruct.PLL.PLLRGE = RCC_PLL1VCIRANGE_1; // 2-4MHz
    } else if (MICROPY_HW_CLK_VALUE / 1000000 <= 8 * m) {
        RCC_OscInitStruct.PLL.PLLRGE = RCC_PLL1VCIRANGE_2; // 4-8MHz
    } else {
        RCC_OscInitStruct.PLL.PLLRGE = RCC_PLL1VCIRANGE_3; // 8-16MHz
    }
    if (MICROPY_HW_CLK_VALUE / 1000000 * n <= 420 * m) {
        RCC_OscInitStruct.PLL.PLLVCOSEL = RCC_PLL1VCOMEDIUM; // 150-420MHz
    } else {
        RCC_OscInitStruct.PLL.PLLVCOSEL = RCC_PLL1VCOWIDE; // 192-960MHz
    }
    RCC_OscInitStruct.PLL.PLLFRACN = 0;
    #endif

    if (HAL_RCC_OscConfig(&RCC_OscInitStruct) != HAL_OK) {
        return -MP_EIO;
    }

    // Set PLL as system clock source if wanted
    if (sysclk_source == RCC_SYSCLKSOURCE_PLLCLK) {
        RCC_ClkInitStruct.ClockType = RCC_CLOCKTYPE_SYSCLK;
        int ret = powerctrl_rcc_clock_config_pll(&RCC_ClkInitStruct, sysclk_mhz, need_pllsai);
        if (ret != 0) {
            return ret;
        }
    }

    #if MICROPY_HW_CLK_LAST_FREQ
    // Save settings in RTC backup register to reconfigure clocks on hard-reset
    #if defined(STM32F7)
    #define FREQ_BKP BKP31R
    #else
    #define FREQ_BKP BKP19R
    #endif
    // qqqqqqqq pppppppp nnnnnnnn nnmmmmmm
    // qqqqQQQQ ppppppPP nNNNNNNN NNMMMMMM
    // 222111HH HHQQQQPP nNNNNNNN NNMMMMMM
    p = (p / 2) - 1;
    RTC->FREQ_BKP = m
        | (n << 6) | (p << 16) | (q << 18)
        | (h << 22)
        | (b1 << 26)
        | (b2 << 29);
    #endif

    return 0;
}

#endif

void powerctrl_enter_stop_mode(void) {
    // Disable IRQs so that the IRQ that wakes the device from stop mode is not
    // executed until after the clocks are reconfigured
    uint32_t irq_state = disable_irq();

    #if defined(MICROPY_BOARD_ENTER_STOP)
    MICROPY_BOARD_ENTER_STOP
    #endif

    #if defined(STM32L4)
    // Configure the MSI as the clock source after waking up
    __HAL_RCC_WAKEUPSTOP_CLK_CONFIG(RCC_STOP_WAKEUPCLOCK_MSI);
    #endif

    #if !defined(STM32F0) && !defined(STM32L0) && !defined(STM32L4) && !defined(STM32WB)
    // takes longer to wake but reduces stop current
    HAL_PWREx_EnableFlashPowerDown();
    #endif

    #if defined(STM32F7)
    HAL_PWR_EnterSTOPMode((PWR_CR1_LPDS | PWR_CR1_LPUDS | PWR_CR1_FPDS | PWR_CR1_UDEN), PWR_STOPENTRY_WFI);
    #else
    HAL_PWR_EnterSTOPMode(PWR_LOWPOWERREGULATOR_ON, PWR_STOPENTRY_WFI);
    #endif

    // reconfigure the system clock after waking up

    #if defined(STM32F0)

    // Enable HSI48
    __HAL_RCC_HSI48_ENABLE();
    while (!__HAL_RCC_GET_FLAG(RCC_FLAG_HSI48RDY)) {
    }

    // Select HSI48 as system clock source
    MODIFY_REG(RCC->CFGR, RCC_CFGR_SW, RCC_SYSCLKSOURCE_HSI48);
    while (__HAL_RCC_GET_SYSCLK_SOURCE() != RCC_CFGR_SWS_HSI48) {
    }

    #else

    #if !defined(STM32L4)
    // enable clock
    __HAL_RCC_HSE_CONFIG(MICROPY_HW_RCC_HSE_STATE);
    #if MICROPY_HW_CLK_USE_HSI
    __HAL_RCC_HSI_ENABLE();
    #endif
    while (!__HAL_RCC_GET_FLAG(MICROPY_HW_RCC_FLAG_HSxRDY)) {
    }
    #endif

    #if defined(STM32F7)
    // Enable overdrive to reach 216MHz (if needed)
    HAL_PWREx_EnableOverDrive();
    #endif

    // enable PLL
    __HAL_RCC_PLL_ENABLE();
    while (!__HAL_RCC_GET_FLAG(RCC_FLAG_PLLRDY)) {
    }

    // select PLL as system clock source
    MODIFY_REG(RCC->CFGR, RCC_CFGR_SW, RCC_SYSCLKSOURCE_PLLCLK);
    #if defined(STM32H7)
    while (__HAL_RCC_GET_SYSCLK_SOURCE() != RCC_CFGR_SWS_PLL1) {
    }
    #elif defined(STM32WB)
    while (__HAL_RCC_GET_SYSCLK_SOURCE() != RCC_SYSCLKSOURCE_STATUS_PLLCLK) {
    }
    #else
    while (__HAL_RCC_GET_SYSCLK_SOURCE() != RCC_CFGR_SWS_PLL) {
    }
    #endif

    powerctrl_disable_hsi_if_unused();

    #if defined(STM32F7)
    if (RCC->DCKCFGR2 & RCC_DCKCFGR2_CK48MSEL) {
        // Enable PLLSAI if it is selected as 48MHz source
        RCC->CR |= RCC_CR_PLLSAION;
        while (!(RCC->CR & RCC_CR_PLLSAIRDY)) {
        }
    }
    #endif

    #if defined(STM32H7)
    // Enable PLL3 for USB
    RCC->CR |= RCC_CR_PLL3ON;
    while (!(RCC->CR & RCC_CR_PLL3RDY)) {
    }
    #endif

    #if defined(STM32L4)
    // Enable PLLSAI1 for peripherals that use it
    RCC->CR |= RCC_CR_PLLSAI1ON;
    while (!(RCC->CR & RCC_CR_PLLSAI1RDY)) {
    }
    #endif

    #endif

    #if defined(MICROPY_BOARD_LEAVE_STOP)
    MICROPY_BOARD_LEAVE_STOP
    #endif

    // Enable IRQs now that all clocks are reconfigured
    enable_irq(irq_state);
}

void powerctrl_enter_standby_mode(void) {
    rtc_init_finalise();

    #if defined(MICROPY_BOARD_ENTER_STANDBY)
    MICROPY_BOARD_ENTER_STANDBY
    #endif

    // We need to clear the PWR wake-up-flag before entering standby, since
    // the flag may have been set by a previous wake-up event.  Furthermore,
    // we need to disable the wake-up sources while clearing this flag, so
    // that if a source is active it does actually wake the device.
    // See section 5.3.7 of RM0090.

    // Note: we only support RTC ALRA, ALRB, WUT and TS.
    // TODO support TAMP and WKUP (PA0 external pin).
    #if defined(STM32F0) || defined(STM32L0)
    #define CR_BITS (RTC_CR_ALRAIE | RTC_CR_WUTIE | RTC_CR_TSIE)
    #define ISR_BITS (RTC_ISR_ALRAF | RTC_ISR_WUTF | RTC_ISR_TSF)
    #else
    #define CR_BITS (RTC_CR_ALRAIE | RTC_CR_ALRBIE | RTC_CR_WUTIE | RTC_CR_TSIE)
    #define ISR_BITS (RTC_ISR_ALRAF | RTC_ISR_ALRBF | RTC_ISR_WUTF | RTC_ISR_TSF)
    #endif

    // save RTC interrupts
    uint32_t save_irq_bits = RTC->CR & CR_BITS;

    // disable RTC interrupts
    RTC->CR &= ~CR_BITS;

    // clear RTC wake-up flags
    RTC->ISR &= ~ISR_BITS;

    #if defined(STM32F7)
    // disable wake-up flags
    PWR->CSR2 &= ~(PWR_CSR2_EWUP6 | PWR_CSR2_EWUP5 | PWR_CSR2_EWUP4 | PWR_CSR2_EWUP3 | PWR_CSR2_EWUP2 | PWR_CSR2_EWUP1);
    // clear global wake-up flag
    PWR->CR2 |= PWR_CR2_CWUPF6 | PWR_CR2_CWUPF5 | PWR_CR2_CWUPF4 | PWR_CR2_CWUPF3 | PWR_CR2_CWUPF2 | PWR_CR2_CWUPF1;
    #elif defined(STM32H7)
    // TODO
    #elif defined(STM32L4) || defined(STM32WB)
    // clear all wake-up flags
    PWR->SCR |= PWR_SCR_CWUF5 | PWR_SCR_CWUF4 | PWR_SCR_CWUF3 | PWR_SCR_CWUF2 | PWR_SCR_CWUF1;
    // TODO
    #else
    // clear global wake-up flag
    PWR->CR |= PWR_CR_CWUF;
    #endif

    // enable previously-enabled RTC interrupts
    RTC->CR |= save_irq_bits;

    #if defined(STM32F7)
    // Enable the internal (eg RTC) wakeup sources
    // See Errata 2.2.2 "Wakeup from Standby mode when the back-up SRAM regulator is enabled"
    PWR->CSR1 |= PWR_CSR1_EIWUP;
    #endif

    // enter standby mode
    HAL_PWR_EnterSTANDBYMode();
    // we never return; MCU is reset on exit from standby
}