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Having trouble with USART6 RECEIVE

Question asked by David Harrison on Oct 19, 2017
Latest reply on Oct 20, 2017 by David Harrison

Hi, I am having trouble getting my USART6 receive on PORT PC7 working.


I have USART6 initialized at 115200 baud in TX and RX mode and can transmit fine on USART6 PORT PC6. In order to test USART6 reception I have a simple forever polled loop which echoes the USART6 received bytes onto my debug port which is UART7. UARt7 Transmit works fine. but I do not see any data coming in on USART6 which is coming from a GPS receiver, verified by monitoring the incoming data in another serial terminal window.

My echoing loop is as follows:

while (true)
      // Do we have a received character in USART6?
         // Then transmit it on the Debug port, UART7.
         uint8_t receivedByte = (uint8_t)(USART6->RDR & 0xFF); // This also clears the USART_ISR_RXNE bit.

         // Loop until transmit data register is empty
         while (!(UART7->ISR & USART_FLAG_TXE));

         UART7->TDR = (uint32_t)receivedByte;


My initialization code is here:

void InitGPSUARTTx(uint32_t baudRate)


// Enable USART6 clock


// Configure PC6 USART6 TX as alternate function I/O for USART6


USART_SetConfig(USART6, USART_MODE_TX_RX, baudRate);

// UART6 default clock is from PCLK1 which is initially set to 54MHz

// because of the initial half speed 108MHz AHB/PHB busses.

// Those bus speeds are later increased to 216MHz in InitSDCard()

// after the SD card initialization at the lower bus speed.

// Set GPS USART6 BRR register for 108MHz / 115200.

// this gives a better match to 115200 than the computed value from USART_SetConfig()

// This value has been determined experimentally by measuring the actual transmission rate.

USART6->BRR = 938;

// Enable USART6



The GPIOSetAltFunc() and USART_SetConfig() are my own functions that have been widely proven elsewhere in my application to simply set the AF modes for GPIO pins and also a simplified USART Config, that originally derived from the STD periph lib.

Am I missing something obvious here? Any help is appreciated, thanks.