STM32F10x Std. Periph. Lib: USART baud rate bug?

Question

Posted on August 18, 2016 at 10:28Hello, I disovered some weird behaviour with STM32F10x standard peripheral lib V.3.5.0: When I try to set a UART baud rate of 215,000 B/s via USART_Init() function, it results in a baud rate of 250,000 B/s instead (BRR = 0x60 at 24MHz). When I choose 214,000 or 216,000 everything works fine.Is that a known bug? #stm32-stdperiph_lib

Tesla DeLorean · Answer

Posted on August 18, 2016 at 23:27 Is that a known bug?The math is definitely broken, and unnecessarily complicated, there have been other complaints C:\MSVC\STM32>usartf1 96 60 250000112 70 214285111 6F 216216 #include #include typedef unsigned long uint32_t; typedef unsigned char uint8_t; void baud(uint32_t apbclock, uint32_t baudrate) { uint32_t tmpreg = 0x00; uint32_t integerdivider = 0x00; uint32_t fractionaldivider = 0x00; int over8 = 0; /* Determine the integer part */ if (over8) { /* Integer part computing in case Oversampling mode is 8 Samples */ integerdivider = ((25 * apbclock) / (2 * baudrate)); } else /* if ((USARTx->CR1 & CR1_OVER8_Set) == 0) */ { /* Integer part computing in case Oversampling mode is 16 Samples */ integerdivider = ((25 * apbclock) / (4 * baudrate)); } tmpreg = (integerdivider / 100) << 4; /* Determine the fractional part */ fractionaldivider = integerdivider - (100 * (tmpreg >> 4)); /* Implement the fractional part in the register */ if (over8) { tmpreg |= ((((fractionaldivider * 8) + 50) / 100)) & ((uint8_t)0x07); } else /* if ((USARTx->CR1 & CR1_OVER8_Set) == 0) */ { tmpreg |= ((((fractionaldivider * 16) + 50) / 100)) & ((uint8_t)0x0F); } /* Write to USART BRR */ // USARTx->BRR = (uint16_t)tmpreg; printf('%3d %02X %10d', tmpreg, tmpreg, apbclock / tmpreg); } int main(int argc, char **argv) { baud(24000000, 215000); baud(24000000, 214000); baud(24000000, 216000); return(1); }

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