USART Internal Helpers

Functions
static void	usart_set_pclk (USART_REGDEF_ts const *instance, EN_STATUS_te en_status)
	Enables or disables the peripheral clock for a USART instance.
static void	usart_set_baud_rate (USART_CFG_ts *usart_cfg)
	Computes and writes the BRR register for the configured baud rate.

Detailed Description

Function Documentation

usart_set_pclk()

void usart_set_pclk ( USART_REGDEF_ts const * instance, EN_STATUS_te en_status )

static

Enables or disables the peripheral clock for a USART instance.

Routes to the appropriate RCC APB1 or APB2 clock enable/disable call:

• USART1 and USART6 are on APB2.
• USART2 is on APB1.

Called by usart_init and usart_deinit.

Parameters

[in]	instance	Pointer to the USART peripheral instance.
[in]	en_status	ENABLE to enable the clock, DISABLE to disable it.

Definition at line 183 of file stm32f401re_usart.c.

                                                                                    {
    if(instance == USART1) {
        rcc_set_pclk_apb2(RCC_APB2ENR_USART1EN, en_status);
    }
    else if(instance == USART2) {
        rcc_set_pclk_apb1(RCC_APB1ENR_USART2EN, en_status);
    }
    else if(instance == USART6) {
        rcc_set_pclk_apb2(RCC_APB2ENR_USART6EN, en_status);
    }
}

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usart_set_baud_rate()

void usart_set_baud_rate ( USART_CFG_ts * usart_cfg )

static

Computes and writes the BRR register for the configured baud rate.

Uses the fixed-point scaling method from the STM32 reference manual:

• USARTDIV × 100 = (25 × PCLK) / (multiplier × baud_rate), where multiplier is 2 for OVER8 and 4 for OVER16.
• The mantissa is USARTDIV / 100.
• The fraction is rounded and masked to 3 bits (OVER8) or 4 bits (OVER16).

USART1 and USART6 use the APB2 clock; USART2 uses the APB1 clock.

Parameters

[in]

usart_cfg

Pointer to the USART configuration structure.

Definition at line 209 of file stm32f401re_usart.c.

                                                         {
    usart_cfg->instance->USART_BRR &= ~(0xFFFF);
    uint32_t usart_pclk = 0;
    uint32_t temp_reg = 0;
    uint32_t usartdiv = 0;
    uint32_t usartdiv_mantissa;
    uint32_t usartdiv_fraction;
 
    if(usart_cfg->instance == USART1 || usart_cfg->instance == USART6) {
        usart_pclk = rcc_get_apb2_clk();
    }
    else if(usart_cfg->instance == USART2) {
        usart_pclk = rcc_get_apb1_clk();
    }
 
    if(usart_cfg->oversampling == USART_OVERSAMPLING_8) {
        usartdiv = ((25 * usart_pclk) / (2 * usart_cfg->baud_rate));
    }
    else if(usart_cfg->oversampling == USART_OVERSAMPLING_16) {
        usartdiv = ((25 * usart_pclk) / (4 * usart_cfg->baud_rate));
    }
 
    usartdiv_mantissa = usartdiv / 100;
    temp_reg |= usartdiv_mantissa << 4;
 
    usartdiv_fraction = (usartdiv - (usartdiv_mantissa * 100));
 
    if(usart_cfg->oversampling == USART_OVERSAMPLING_8) {
        // Round fraction to 3 bits for OVER8
        usartdiv_fraction = (((usartdiv_fraction * 8) + 50) / 100) & ((uint8_t)0x07);
    }
    else if(usart_cfg->oversampling == USART_OVERSAMPLING_16) {
        // Round fraction to 4 bits for OVER16
        usartdiv_fraction = (((usartdiv_fraction * 16) + 50) / 100) & ((uint8_t)0x0F);
    }
 
    temp_reg |= usartdiv_fraction;
    usart_cfg->instance->USART_BRR = temp_reg;
}

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