I am really struggling with an UART communication with my PIC32MX110F016. It is the first time I am trying to implement an UART communication with PIC32MX family but I find it difficult and I don't know why it's not working properly. So I decided to post all my code in a tutorial way (step-by-step) in order to get some help from you guys. I hope it helps in the future for other people who face the same problem.

So firstly I imported the libraries and declared this microcontoller configs:

#pragma config   FNOSC       = FRCPLL
#pragma config   FPLLIDIV    = DIV_2
#pragma config   FPLLMUL     = MUL_20
#pragma config   FPLLODIV    = DIV_2
#pragma config   FPBDIV      = DIV_1
#pragma config   ICESEL      = ICS_PGx2
#pragma config   WDTPS       = PS16384

#define GetSystemClock()        (40000000ul)
#define GetPeripheralClock()    (GetSystemClock())
#define BaudRate   115200

Then I implemented my uartconfig() function:

void uartconfig(void){
PPSUnLock;                        // Allow PIN Mapping for BLE
  PPSOutput(1, RPA0, U1TX);     // MAP Tx to RA0 set to digital out
  PPSInput (3, U1RX, RPA2);     // MAP Rx to RA2 set to digital in
  PPSOutput(4, RPA3, U1RTS);
  PPSInput (2, U1CTS,RPA1);
PPSLock;                         // Prevent Accidental Mapping

#define UART1TX TRISAbits.TRISA0
#define UART1RX TRISAbits.TRISA2

#define CMD TRISBbits.TRISB5

UART1TX = 0;//output
UART1RX = 1;//input


UARTSetDataRate(UART1, GetPeripheralClock(), BaudRate);


// configure for multi-vectored mode

Then I have decided to create a command_print function to send data through UART:

void command_print(char *buffer){ 
while(*buffer != (char)0)
  UARTSendDataByte(UART1, *buffer++);
UARTSendDataByte(UART1, '\r');

I have decided to create a struct in order to create a buffer for the TX/RX pin (from now I am just using the inputBuffer):

typedef volatile struct UARTBuffer {
char outputBuffer[ UART_INPUT_BUFFER_SIZE ];
char inputBuffer[ UART_OUTPUT_BUFFER_SIZE ];
unsigned short int inputReaderPos;
unsigned short int inputWriterPos;
unsigned short int outputReaderPos;
unsigned short int outputWriterPos;
} UARTBuffer;

UARTBuffer UARTBuffer1;

In my interrupt service routine ISR I decided to ignore the U1TX part and clear the flag always. For the U1RX I defined the as:

void __ISR(_UART_1_VECTOR, ipl2)UART1HANDLER(void){
char c;
if ( INTGetFlag(INT_U1TX) )

if ( INTGetFlag(INT_U1RX) )

if( (U1STAbits.OERR == 1) || (U1STAbits.PERR == 1) || (U1STAbits.FERR == 1) ){
    // Ignore
    U1STAbits.OERR = 0;//clears if the Receive buffer has overflowed
    U1STAbits.PERR = 0;//parity error
    U1STAbits.FERR = 0;//framing error
else {
    // Check if the buffer is all readed. If so, clear the buffer;
    if( UARTBuffer1.inputWriterPos == UARTBuffer1.inputReaderPos ) {
        UARTBuffer1.inputWriterPos = 0;
        UARTBuffer1.inputReaderPos = 0;
    if (UARTBuffer1.inputWriterPos >= UART_INPUT_BUFFER_SIZE){
        // Buffer overflow
        UARTBuffer1.inputWriterPos = 0;
        UARTBuffer1.inputReaderPos = 0;
    c = U1RXREG;
    UARTBuffer1.inputBuffer[ UARTBuffer1.inputWriterPos++ ] = c;

Finally my main(void) is defined as:

int32_t main(void) {
UARTBuffer1.inputWriterPos = 0;

INTEnableSystemMultiVectoredInt(); //enable interrupts


while (1);

Actually my PIC is running normally and my interrupt is being triggered. What I can not understand is the reason why my code never get more than one character in the RXReg Part (namely U1RXREG) inside the ISR. I am clearing the flag by the end of the ISR and I think everything is well configured. Let me know what should I do to get my frame properly and not only the first character. I know the UART is well designed and if I will try to send another command than "SF,1" the RX register gives me a different char.

EDIT: I am not achieving what I want. I am losing characters in the interrupt so I connect the CTS + RTS but unfortunately I am not getting better results. Is anybody available to help me with the CTS + RTS configuration for the PIC32mx family? I know I have to cross the CTS with the RTS and the RTS with the CTS from the microcontroller to the "other" component. My problem is more related with the functions/configs do I have to do.

  • \$\begingroup\$ You are reading one char at a time off the U1RXREG, I think you can get away with saving all of U1RXREG to your variable in one go. \$\endgroup\$
    – Puffafish
    Commented Mar 15, 2016 at 17:30
  • \$\begingroup\$ I am not sure about it. I already took a look into U1RXREG and it just has 0x00000041 as a value and It's not an acceptable result. \$\endgroup\$
    – scuba
    Commented Mar 15, 2016 at 17:33
  • \$\begingroup\$ 0x41 is 'A'. What were you expecting to see as your 2nd character? \$\endgroup\$
    – brhans
    Commented Mar 15, 2016 at 19:35
  • \$\begingroup\$ 'AOK\r\n' is what I am trying to get. \$\endgroup\$
    – scuba
    Commented Mar 15, 2016 at 19:49
  • 1
    \$\begingroup\$ Should definitely make that UartBuffer1 struct 'volatile'. \$\endgroup\$
    – justing
    Commented Mar 16, 2016 at 23:28

1 Answer 1


At a baudrate of 115200,using no parity and 1 start & stop bit(total of 10 bits), it takes 86 microseconds for data to arrive at your RX buffer from the other end.

Looking at your ISR, i see that you are processing a lot of instructions in your ISR.

The scenario that might be happening is this: Your first character arrives(i.e "A"),

Interrupt is triggered,

no error has occurred(overflow,parity etc),

now the rest of the characters also arrive("OK\r\n"),

overflow flag gets set,

you clear RX interrupt(But you dont enter ISR again cause no more characters will be received).

Debug solutions: 1. Use flow control method(ACK for every byte)

  1. Check the size of the Rx FIFO of your uC.(if its 16 bytes then you dont need flow control.Else if no FIFO, you need flow control)

3.Avoid function calls in the ISR.

4.You can enable interrupt for overflow errors such that you dont explicitly have to write a routine for the interrupt. If overflow error occurs,just check the status of the Interrupt flag and clear the flag and discard the Rx data.This will help you reduce processing of such errors in the ISR as they can be checked in the main loop itself. Just copy the received data into a buffer and increment a count.This might help.

  1. Reduce your baud rate and check using your same code
  • \$\begingroup\$ The CPU clock rate is 40MHz and the baud rate is 115200bps. Each character has 8 data bits, a stop bit and a start bit, for 10 bits total per character. Therefore you have 40MHz / 115200bps * 10 bits per character = 3,472 CPU cycles to process each character before the next one arrives. Therefore it is unlikely that your ISR is too slow to process the chars as they come in, so unless you are disabling interrupts somewhere else you are not likely to overflow. \$\endgroup\$
    – user4574
    Commented Apr 6, 2016 at 18:00
  • \$\begingroup\$ Already tried with the baudrate at 19200 and no better results. \$\endgroup\$
    – scuba
    Commented Apr 6, 2016 at 23:55

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