Receiving bad character through the USART

[mnemxys]

Posted on July 10, 2015 at 17:36

Hi, I am working in a wired point to point communication system but I have a problem receiving characters.

I have two electronics communicating each other through USART. I want to send a char periodically from the transmitter (TX) to the receiver (RX) but RX does not read the same char sent from TX. TX is the development kit STM32F4 Discovery. I have no problems sending chars through USART1 in this kit. For my tests I just send the same char periodically.  RX is my own design where I use the microcontroller STM32F405. I use USART2 to receive the chars from TX. The configuration of the USART is the same in TX (USART1) and RX (USART2). It is as follows: void USART1_Config(){     GPIO_InitTypeDef GPIO_InitStructure;     USART_InitTypeDef USART_InitStructure;     RCC_APB2PeriphClockCmd(RCC_APB2Periph_USART1, ENABLE);     // Configure AF (Alternate Function) of pins PA0, PA1     GPIO_PinAFConfig(GPIOA, GPIO_PinSource9, GPIO_AF_USART1);     GPIO_PinAFConfig(GPIOA, GPIO_PinSource10, GPIO_AF_USART1);     USART_InitStructure.USART_BaudRate = 2400;     USART_InitStructure.USART_WordLength = USART_WordLength_8b;     USART_InitStructure.USART_StopBits = USART_StopBits_1;     USART_InitStructure.USART_Parity = USART_Parity_No;     USART_InitStructure.USART_HardwareFlowControl = USART_HardwareFlowControl_None;     USART_InitStructure.USART_Mode = USART_Mode_Rx | USART_Mode_Tx;     GPIO_InitStructure.GPIO_OType = GPIO_OType_PP;     GPIO_InitStructure.GPIO_PuPd = GPIO_PuPd_UP;     GPIO_InitStructure.GPIO_Mode = GPIO_Mode_AF;     GPIO_InitStructure.GPIO_Speed = GPIO_Speed_2MHz;     // Configure USART Tx     GPIO_InitStructure.GPIO_Pin = GPIO_Pin_9;     GPIO_Init(GPIOA, &GPIO_InitStructure);     // Configure USART Rx     GPIO_InitStructure.GPIO_Pin = GPIO_Pin_10;     GPIO_Init(GPIOA, &GPIO_InitStructure);     USART_Init(USART1, &USART_InitStructure);     USART_Cmd(USART1, ENABLE);     // Config IT     NVIC_InitTypeDef NVIC_InitStructure;     // Enable the USARTx Interrupt RX     NVIC_InitStructure.NVIC_IRQChannel = USART1_IRQn;     NVIC_InitStructure.NVIC_IRQChannelPreemptionPriority = 0;     NVIC_InitStructure.NVIC_IRQChannelSubPriority = 0;     NVIC_InitStructure.NVIC_IRQChannelCmd = ENABLE;     NVIC_Init(&NVIC_InitStructure);     // Config RX IT USARTx     USART_ITConfig(USART1, USART_IT_RXNE, ENABLE); } As I want to check all the characters received in RX I have my interrupt routine as it is indicated next: void USART2_IRQHandler(void){     if(USART_GetITStatus(USART2, USART_IT_RXNE) != RESET)       {         /* Read one byte from the receive data register */         char rxchar = USART_ReceiveData(USART2);         ...       } } The problem is that the variable rxchar does not contain the same character sent from TX. I have checked the signal from RX and TX with an oscilloscope and they are the same (as it was expected since the system is wired point-to-point). At this point I suppose that the problem is related to the USART configuration but both have the same. I am completely getting stuck on this and I don't know what can I do. Can anybody help me with this? Thank you very much in advance for your help. Greetings! mnem

[Tesla DeLorean]

Posted on July 10, 2015 at 18:33

PA9 is not viable for USART1_TX on the STM32F4-DISCO, C49 represents a huge problem.

Got the grounds connected between the boards? Show enough code to actually be able to demonstrate the problem.

[mnemxys]

Posted on July 13, 2015 at 10:12

I'm sorry, I forgot to say that in STM32F4-DISCO I use PB6 as USART1_TX and PA10 as USART1_RX.

Regarding to the grounds, yes, they are connected between the boards. And the code is simply like that, in TX I just configure the USART1 and then in the main loop I send all the characters. int main(void){     int i=0;     USART1_Config();     for(i=0; i< 2500; i++); //delay     while (1){         USART_SendData(USART1, 68); // char D         while (!USART_GetFlagStatus(USART1, USART_FLAG_TXE));     }/* Infinite loop */ }  Then, in the main() on RX I configure the USART2 and in the interrupt routine in my previous post, I just read the character so with the debug tool I can check the value of the variable containing the expected character but it never agrees on the one sent from RX.  mnem

[mnemxys]

Posted on July 13, 2015 at 16:27

Ok, I made some progress but still does not work... I made a test with another USART. I was using that USART to communicate with the PC through a bluetooth module. This USART receives commands from the PC and it was working perfectly. I have changed de code to see if that USART receives correctly the character from TX and I found that it works the same way, the cahracter is wrong. The only change I made was that I have changed the baudrate from 19200 to 2400 so I think that the USART is not working well at 2400 bauds in rx.

Maybe the values used by the libraries I use to generate the baud rate are not good. Can anybody show me a piece of code to program the USART at 2400 bauds changing the registers? I suppose I need to specify the good values for the baud rate. Thank you very much for your help in advanced. mnem

[Tesla DeLorean]

Posted on July 13, 2015 at 16:45

Are you using HSI or HSE base clocks?

The 2400 should have a pretty wide tolerance for error, would suggest you review bit timing with scope to ensure it's not wildly off. There is code in stm32f4xx_usart.c to set the rate. The method is explained/documented in the reference manual.   The basic math is USARTx->BRR = APBxClock / Baud As I recall USART1 and 6 are on APB2 (the fast bus), the rest on APB1 ie USART1->BRR = 84000000 / 2400; You could check the current setting, and print computed baud rate as a float, the compute % error in setting. Other than that you're going to need to look a fundamental problems with you hardware, because the USART should be pretty solid.