UART on dsPIC33EP512MU810

Question

I want to continuously transmit the data on the UART2 of my dsPIC33EP512MU810

. UART2 Tx pin i am configuring it to Pin 51 of the controller which is RP99.

Now the UART output is being converted to RS485 using ST4485EB on board IC. 3 number pin of ST4485EB IC is connected with controller's 55 number pin which is RA2. I am making RA2 pin high to activate the IC.

My board also has a on board crstal TXC 4.00 MFC which is a 4 MHZ crystal.

Trying to configure the UART with baudrate of 38400 8 data bits, 1 stop bit, none parity and none flow control. Using the following code but not getting the desired result. Please help me out with this.

/******************************************************************************/
/* Files to Include                                                           */
/******************************************************************************/

/* Device header file */
#if defined(__XC16__)
    #include <xc.h>
#elif defined(__C30__)
    #if defined(__dsPIC33E__)
        #include <p33Exxxx.h>
    #elif defined(__dsPIC33F__)
        #include <p33Fxxxx.h>
    #endif
#endif
#define FP 40000000
#define BAUDRATE 9600
#define BRGVAL ((FP/BAUDRATE)/16)-1
unsigned int i;
#define DELAY_105uS asm volatile ("REPEAT, #4201"); Nop(); // 105uS delay

#include <stdint.h>        /* Includes uint16_t definition                    */
#include <stdbool.h>       /* Includes true/false definition                  */

unsigned int i;
#define DELAY_105uS asm volatile ("REPEAT, #4201"); Nop(); // 105uS delay

#include <stdio.h>
#include <stdlib.h>
#include <string.h>
#include <math.h>
#include <xc.h>
#include <float.h>
#include <libpic30.h>
#include <p33Exxxx.h>
#include "system.h"        /* System funct/params, like osc/peripheral config */
#include "user.h"          /* User funct/params, such as InitApp              */

/******************************************************************************/
/* Global Variable Declaration                                                */
/******************************************************************************/

/* i.e. uint16_t <variable_name>; */

/******************************************************************************/
/* Main Program                                                               */
/******************************************************************************/
int16_t main(void)
{

    TRISAbits.TRISA2=0;
    PORTAbits.RA2 = 1;

    RPOR8bits.RP99R = 3; // Assign U2TX peripheral to pin RP84
    RPINR19bits.U2RXR = 19; // Assign pin RPI16 to U2RX peripheral  

    U2MODEbits.STSEL = 0; // 1-Stop bit
    U2MODEbits.PDSEL = 0; // No Parity, 8-Data bits
    U2MODEbits.ABAUD = 0; // Auto-Baud disabled
    U2MODEbits.BRGH = 0; // Standard-Speed mode
    U2BRG = BRGVAL; // Baud Rate setting for 9600
    U2STAbits.UTXISEL0 = 0; // Interrupt after one TX character is transmitted
    U2STAbits.UTXISEL1 = 0;
    IEC1bits.U2TXIE = 1; // Enable UART TX interrupt
    U2MODEbits.UARTEN = 1; // Enable UART
    U2STAbits.UTXEN = 1; // Enable UART TX
    /* Wait at least 105 microseconds (1/9600) before sending first char */
    DELAY_105uS
    U2TXREG = 'a'; // Transmit one character
    while(1)
    {
    }
}

void __attribute__((__interrupt__)) _U2TXInterrupt(void)
{
IFS1bits.U2TXIF = 0; // Clear TX Interrupt flag
U2TXREG = 'a'; // Transmit one character//comment this once testing is done and UART is working.
}

I can see the data on the PC terminal but its not 'a'. Tried different baud rate but none of them is giving me the result 'a' which I am sending.

Answer 1

This is most likely a baud rate/clock error. Try capturing the TX line on a scope and measure the baud rate you actually see, compared to what you've programmed. From that you should be able to tell that you're off by a specific ratio, and with any luck that will make it easier to track down the bug.

Since you've tried various baud rates and none look right, you may be unintentionally creating a "non-standard" baud rate. You're using FP = 40000000 (40MHz). Can you confirm the chip is running at 40MHz? I don't see your PLL settings, so I can't say for sure.

Answer 2

I see that you copied most of the code from the User Manual.

It defines FP as 4Mhz and it appears in the calculations of BRGVAL as "4,000,000" whereas yours is 40MHz, "40,000,000".

[Reason of comma usage was to make reading easier, do not c/p them.]

Answer 3

You are sending UART data without any idle time. This can cause the receiver to "lock" onto a wrong bit as start bit (any "zero" data bit could be misinterpreted as start bit).

Try sending 0x00 or 0xFF once in a while - these values allow the receiver to re-sync to the sender.

Alternatively you can introduce some idle time (more than 8 bits) - this would also allow correct syncing.