STM32F4 Std Periph Lib USART baud rate BRR bug?

[avik]

Posted on March 14, 2017 at 18:16

I've been starting to use an F446, and while the clock settings look OK (at least PWM frequencies match expected, but I can definitely test this more), I consistently get half the requested baud rate on USART3. Looking into the code, in USART_Init in the standard peripheral library,

/* Determine the integer part */

if ((USARTx->CR1 & USART_CR1_OVER8) != 0)

{

/* Integer part computing in case Oversampling mode is 8 Samples */

integerdivider = ((25 * apbclock) / (2 * (USART_InitStruct->USART_BaudRate)));

}

else /* if ((USARTx->CR1 & USART_CR1_OVER8) == 0) */

{

/* Integer part computing in case Oversampling mode is 16 Samples */

integerdivider = ((25 * apbclock) / (4 * (USART_InitStruct->USART_BaudRate)));

}

tmpreg = (integerdivider / 100) << 4;

i.e. the 'integer part' of the baud rate divider is = 25 * apbclock / ((2 * (2-OVER8) * BR * 100) = apbclock/(16 * (2-OVER8) * BR)

However, looking at the RM (both for F446, F405, and probably other F4 chips), e.g. in 25.4.4 in the F446 RM, 

Tx/Rx baud = apbclock / (8 * (2 - OVER8) * divider

Considering an example where I only need an integer divider, it seems like the SPL code is a factor of two off, right? Am I missing something?

EDIT: This is with the latest v1.8.0 of the STM32F4xx_StdPeriph_Driver

#uart-baud-rate #usart #stm32f4 #standard-peripheral

[Tesla DeLorean]

Posted on March 14, 2017 at 19:02

I've built things using USART on the F446 without any issues. I use OVER16 mode

What is the HSE clock speed on your system? Is that correctly reflected in the HSE_VALUE define?

The baud rate clock is going to be impacted by the APB it is situated on, the clock for that bus, and if you use OVER8 or OVER16 mode.

In x16 USARTx->BBR = APBCLK / baudrate;

You'll need to provide a more complete example project that clearly demonstrates your issue.

[avik]

Posted on March 15, 2017 at 04:36

Hi

‌ thanks for the answer. I'm including some info below, please let me know what else would be helpful (I'm traveling so I might provide more debug info on Saturday).

[Edit: I should add that I usually rely on the ST clock config tool Excel files, but alas, the F4 one hasn't been updated for the F4 So, I possibly could have mistakes in the system_stm32f4xx file.]

I have a

https://www.sitime.com/products/datasheets/sit8008/SiT8008-datasheet.pdf

, and here are (I think) therelevant parts of my system_stm32f4xx.c

#define PLL_M 4#define PLL_N 180#define PLL_P 2#define PLL_Q 2#define PLL_R 2//...#if defined(STM32F427_437xx) || defined(STM32F429_439xx) || defined(STM32F446xx) || defined(STM32F469_479xx) uint32_t SystemCoreClock = 180000000;#endif /* STM32F427_437x || STM32F429_439xx || STM32F446xx || STM32F469_479xx *///...__I uint8_t AHBPrescTable[16] = {0, 0, 0, 0, 0, 0, 0, 0, 1, 2, 3, 4, 6, 7, 8, 9};// ...void SystemInit(void){ /* FPU settings ------------------------------------------------------------*/ // #if (__FPU_PRESENT == 1) && (__FPU_USED == 1) SCB->CPACR |= ((3UL << 10*2)|(3UL << 11*2)); /* set CP10 and CP11 Full Access */ // #endif /* Reset the RCC clock configuration to the default reset state ------------*/ /* Set HSION bit */ RCC->CR |= (uint32_t)0x00000001; /* Reset CFGR register */ RCC->CFGR = 0x00000000; /* Reset HSEON, CSSON and PLLON bits */ RCC->CR &= (uint32_t)0xFEF6FFFF; /* Reset PLLCFGR register */ RCC->PLLCFGR = 0x24003010; /* Reset HSEBYP bit */ RCC->CR &= (uint32_t)0xFFFBFFFF; /* Disable all interrupts */ RCC->CIR = 0x00000000;#if defined(DATA_IN_ExtSRAM) || defined(DATA_IN_ExtSDRAM) SystemInit_ExtMemCtl();#endif /* DATA_IN_ExtSRAM || DATA_IN_ExtSDRAM */ /* Configure the System clock source, PLL Multiplier and Divider factors, AHB/APBx prescalers and Flash settings ----------------------------------*/ SetSysClock(); /* Configure the Vector Table location add offset address ------------------*/#ifdef VECT_TAB_SRAM SCB->VTOR = SRAM_BASE | VECT_TAB_OFFSET; /* Vector Table Relocation in Internal SRAM */#else SCB->VTOR = FLASH_BASE | VECT_TAB_OFFSET; /* Vector Table Relocation in Internal FLASH */#endif}//...void SystemCoreClockUpdate(void){ uint32_t tmp = 0, pllvco = 0, pllp = 2, pllsource = 0, pllm = 2; uint32_t pllr = 2; /* Get SYSCLK source -------------------------------------------------------*/ tmp = RCC->CFGR & RCC_CFGR_SWS; switch (tmp) { case 0x00: /* HSI used as system clock source */ SystemCoreClock = HSI_VALUE; break; case 0x04: /* HSE used as system clock source */ SystemCoreClock = HSE_VALUE; break; case 0x08: /* PLL P used as system clock source */ /* PLL_VCO = (HSE_VALUE or HSI_VALUE / PLL_M) * PLL_N SYSCLK = PLL_VCO / PLL_P */ pllsource = (RCC->PLLCFGR & RCC_PLLCFGR_PLLSRC) >> 22; pllm = RCC->PLLCFGR & RCC_PLLCFGR_PLLM; if (pllsource != 0) { /* HSE used as PLL clock source */ pllvco = (HSE_VALUE / pllm) * ((RCC->PLLCFGR & RCC_PLLCFGR_PLLN) >> 6); } else { /* HSI used as PLL clock source */ pllvco = (HSI_VALUE / pllm) * ((RCC->PLLCFGR & RCC_PLLCFGR_PLLN) >> 6); } pllp = (((RCC->PLLCFGR & RCC_PLLCFGR_PLLP) >>16) + 1 ) *2; SystemCoreClock = pllvco/pllp; break; case 0x0C: /* PLL R used as system clock source */ /* PLL_VCO = (HSE_VALUE or HSI_VALUE / PLL_M) * PLL_N SYSCLK = PLL_VCO / PLL_R */ pllsource = (RCC->PLLCFGR & RCC_PLLCFGR_PLLSRC) >> 22; pllm = RCC->PLLCFGR & RCC_PLLCFGR_PLLM; if (pllsource != 0) { /* HSE used as PLL clock source */ pllvco = (HSE_VALUE / pllm) * ((RCC->PLLCFGR & RCC_PLLCFGR_PLLN) >> 6); } else { /* HSI used as PLL clock source */ pllvco = (HSI_VALUE / pllm) * ((RCC->PLLCFGR & RCC_PLLCFGR_PLLN) >> 6); } pllr = (((RCC->PLLCFGR & RCC_PLLCFGR_PLLR) >>28) + 1 ) *2; SystemCoreClock = pllvco/pllr; break; default: SystemCoreClock = HSI_VALUE; break; } /* Compute HCLK frequency --------------------------------------------------*/ /* Get HCLK prescaler */ tmp = AHBPrescTable[((RCC->CFGR & RCC_CFGR_HPRE) >> 4)]; /* HCLK frequency */ SystemCoreClock >>= tmp;}//...static void SetSysClock(void){/******************************************************************************//* PLL (clocked by HSE) used as System clock source *//******************************************************************************/ __IO uint32_t StartUpCounter = 0, HSEStatus = 0; /* Enable HSE */ // RCC->CR |= ((uint32_t)RCC_CR_HSEON); RCC->CR |= ((uint32_t)RCC_CR_HSEON | RCC_CR_HSEBYP);// NOTE: I have a lock, so I enabled the bypass /* Wait till HSE is ready and if Time out is reached exit */ do { HSEStatus = RCC->CR & RCC_CR_HSERDY; StartUpCounter++; } while((HSEStatus == 0) && (StartUpCounter != HSE_STARTUP_TIMEOUT)); if ((RCC->CR & RCC_CR_HSERDY) != RESET) { HSEStatus = (uint32_t)0x01; } else { HSEStatus = (uint32_t)0x00; } if (HSEStatus == (uint32_t)0x01) { /* Select regulator voltage output Scale 1 mode */ RCC->APB1ENR |= RCC_APB1ENR_PWREN; PWR->CR |= PWR_CR_VOS; /* HCLK = SYSCLK / 1*/ RCC->CFGR |= RCC_CFGR_HPRE_DIV1; /* PCLK2 = HCLK / 2*/ RCC->CFGR |= RCC_CFGR_PPRE2_DIV2;//Want APB2 = 90 Mhz /* PCLK1 = HCLK / 4*/ RCC->CFGR |= RCC_CFGR_PPRE1_DIV4; /* Configure the main PLL */ RCC->PLLCFGR = PLL_M | (PLL_N << 6) | (((PLL_P >> 1) -1) << 16) | (RCC_PLLCFGR_PLLSRC_HSE) | (PLL_Q << 24) | (PLL_R << 28); /* Enable the main PLL */ RCC->CR |= RCC_CR_PLLON; /* Wait till the main PLL is ready */ while((RCC->CR & RCC_CR_PLLRDY) == 0) { } /* Enable the Over-drive to extend the clock frequency to 180 Mhz */ PWR->CR |= PWR_CR_ODEN; while((PWR->CSR & PWR_CSR_ODRDY) == 0) { } PWR->CR |= PWR_CR_ODSWEN; while((PWR->CSR & PWR_CSR_ODSWRDY) == 0) { } /* Configure Flash prefetch, Instruction cache, Data cache and wait state */ FLASH->ACR = FLASH_ACR_PRFTEN | FLASH_ACR_ICEN |FLASH_ACR_DCEN |FLASH_ACR_LATENCY_5WS; /* Select the main PLL as system clock source */ RCC->CFGR &= (uint32_t)((uint32_t)~(RCC_CFGR_SW)); RCC->CFGR |= RCC_CFGR_SW_PLL; /* Wait till the main PLL is used as system clock source */ while ((RCC->CFGR & (uint32_t)RCC_CFGR_SWS ) != RCC_CFGR_SWS_PLL); { } } else { /* If HSE fails to start-up, the application will have wrong clock configuration. 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The code I use to init the UART is like this:

 // Init USART USART_InitStructure.USART_BaudRate = baud;// I use 115200 to test USART_InitStructure.USART_WordLength = USART_WordLength_8b; USART_InitStructure.USART_Parity = USART_Parity_No; USART_InitStructure.USART_StopBits = USART_StopBits_1; USART_InitStructure.USART_HardwareFlowControl = USART_HardwareFlowControl_None; USART_InitStructure.USART_Mode = USART_Mode_Rx | USART_Mode_Tx; USART_Init(usartMap->USARTx, &USART_InitStructure);//usartMap->USARTx is USART3 USART_Cmd(usartMap->USARTx, ENABLE);�?�?�?�?�?�?�?�?�?�?�?�?�?�?�?�?�?�?�?�?�?