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Code is written in MPLABX v4.00 with XC8 compiler.

I am new to writing firmware for 8-bit PICs and could use some help with my code. I am using a PIC16F1829 for an LED module which gets RX commands. I am just trying to get the basics setup like turn on LEDs when a certain value is received on RX pin, but can't even get that.

Would like to get UART working via interrupts but can't even get it working with polling in main loop. My interrupt vector is commented out in the code below.

RX pin: RC5

TX pin: RB7

Pin to toggle LEDs on and off: RA5

Pin RA5 works fine to toggle LEDs on and off. TX pin is working, though I have not confirmed if interrupt TXIF is also not working like RCIF is not working.

I have tried reading RCIF and PIR1bits.RCIF. Both of them compiled. Neither one worked. I have tried this on two different PICs on 2 different LED modules. They turn on, but reading RX pin didn't work on either.

Variable RXIN is initially defined as 3 and thus due to the RXIN-- loop within the main loop the lights flash 3 times at startup so I know it is entering the main loop. But as far as I can tell the RCIF interrupt is not firing upon reception at RX pin.

I have confirmed on oscilloscope that the signal into RX and out of TX pins using same baud, so I think baud rate is configured correctly (300 baud, 8N1.) I have also confirmed on oscilloscope RX pin receiving strong and clean 5V signal. Neither polling RCIF or using an interrupt service routing has worked thus far. If anyone can see the issues with my code that I am not seeing, your help would be greatly appreciated.

My code:

#include <stdio.h> #include <stdlib.h> #include <xc.h> // This is for 300 baud rate #define _BAUD_PRESCALER_LOW_ 0x2A #define _BAUD_PRESCALER_HIGH_ 0x68 #define _XTAL_FREQ 32000000 #pragma config FOSC = INTOSC // Oscillator Selection->INTOSC oscillator: I/O function on CLKIN pin #pragma config WDTE = OFF // Watchdog Timer Enable->WDT enabled #pragma config PWRTE = OFF // Power-up Timer Enable->PWRT disabled #pragma config MCLRE = OFF // MCLR Pin Function Select->MCLR/VPP pin function is digital input #pragma config CP = OFF // Flash Program Memory Code Protection->Program memory code protection is disabled #pragma config CPD = OFF // Data Memory Code Protection->Data memory code protection is disabled #pragma config BOREN = ON // Brown-out Reset Enable->Brown-out Reset enabled #pragma config CLKOUTEN = OFF // Clock Out Enable->CLKOUT function is disabled. I/O or oscillator function on the CLKOUT pin #pragma config IESO = OFF // Internal/External Switchover->Internal/External Switchover mode is disabled #pragma config FCMEN = OFF // Fail-Safe Clock Monitor Enable->Fail-Safe Clock Monitor is disabled // CONFIG2 #pragma config WRT = OFF // Flash Memory Self-Write Protection->Write protection off #pragma config PLLEN = ON // PLL Enable->4x PLL enabled #pragma config STVREN = ON // Stack Overflow/Underflow Reset Enable->Stack Overflow or Underflow will cause a Reset #pragma config BORV = LO // Brown-out Reset Voltage Selection->Brown-out Reset Voltage (Vbor), low trip point selected. #pragma config LVP = OFF int flagRXFramingError = 0; int flagRXOverrunError = 0; volatile unsigned char RXIN = 3; unsigned char UARTRead(){ return RCREG; } void writeRXIN(unsigned char a){ RXIN = a; } void TX(unsigned char a){ while(!TXIF){} TXREG = a; } int main(int argc, char** argv) { // SCS FOSC; SPLLEN disabled; IRCF 8MHz_HF; OSCCON = 0xF0; // TUN 0; OSCTUNE = 0x00; // Set the secondary oscillator // Wait for PLL to stabilize while(PLLR == 0) { } // WDTPS 1:65536; SWDTEN OFF; WDTCON = 0x16; __delay_ms(5); GIE = 1; // Global interrupts enabled __delay_ms(5); PEIE = 1; // Active peripheral interrupts enabled __delay_ms(5); RCIE = 1; // Enable USART Receive interrupt __delay_ms(5); TXIE = 1; // Enable USART Transmitter interrupt __delay_ms(5); ADIE = 1; // Enable ADC interrupts __delay_ms(5); RXDTSEL = 0; // RX is on RC5 pin __delay_ms(5); TXCKSEL = 0; // TX is on RB7 pin __delay_ms(5); TRISC5 = 1; // RX pin set as input __delay_ms(5); SPEN = 1; // Serial Port Enabled __delay_ms(5); SYNC = 0; // Asynchronous mode __delay_ms(5); RX9 = 0; // 8 bit reception __delay_ms(5); TX9 = 0; // 8-bit transmission __delay_ms(5); CREN = 1; // Receiver enabled __delay_ms(5); TXEN = 1; // Transmitter enabled __delay_ms(5); BRG16 = 1; // 16-bit baud generation __delay_ms(5); BRGH = 1; // High baud rate enabled __delay_ms(5); ABDEN = 0; // Auto baud detect disabled __delay_ms(5); // Baud prescaler n = [Fosc/(D*BR)] - 1 SPBRGH = _BAUD_PRESCALER_HIGH_; __delay_ms(5); SPBRGL = _BAUD_PRESCALER_LOW_; __delay_ms(5); TRISC6 = 0; // IadjPWM pin configured as output __delay_ms(5); ANSC6 = 0; // IadjPWM pin not analog input __delay_ms(5); TRISA5 = 0; // DimPWM pin configured as output __delay_ms(5); LATC6 = 1; // Max current for now until PWM written __delay_ms(5); while(1){ // Inline assembly code to clear watchdog timer //asm("CLRWDT"); /*if(RXIN == 5){ RA5 = 1; } else{ RA5 = 0; }*/ if(PIR1bits.RCIF){ writeRXIN(UARTRead()); //RA5 = 0; TX(RXIN); } // end if RCIF while(RXIN > 0){ RA5 = 1; __delay_ms(100); RA5 = 0; __delay_ms(100); RXIN--; } } // infinite loop // never leave this loop RA5 = 1; return (EXIT_SUCCESS); } // end main /*void interrupt ISR(void){ if(RCIF){// if USART Receive interrupt flag RA5 = 1; if(FERR){ flagRXFramingError = 1; SPEN = 0; SPEN = 1; } if(OERR){ flagRXOverrunError = 1; CREN = 0; CREN = 1; } while(RCIF){ // RCIF high as long as there is data in FIFO register. Read RCREG to clear RCIF flag writeRXIN(UARTRead()); } RA5 = 0; } if (TXIF){// if USART Transmit interrupt TXIF = 0; // Clear interrupt flag } } // end ISRs*/
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  • \$\begingroup\$ What's with all of the 5ms delays? Surely most or all of them shouldn't be necessary. Also, do you have the paid or free version of MPLAB? The free version won't optimize down those one-line functions. You're better off declaring them as macros. \$\endgroup\$ Commented Sep 5, 2017 at 0:15
  • \$\begingroup\$ Your reference to PIR1bits shouldn't be necessary either. \$\endgroup\$ Commented Sep 5, 2017 at 0:17
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    \$\begingroup\$ A quick glance at your code without deep diving into it. You've enabled interrupts with GIE, PEIE, RCIE, and TXIE, but your interrupt function is commented out. So if serial data is arriving on the RX pin, the processor is jumping to the designated interrupt memory location, but there's no instructions there. That is fatal. You shouldn't be dabbling in interrupts anyway if you're new to this MCU. \$\endgroup\$ Commented Sep 5, 2017 at 3:52
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    \$\begingroup\$ Also, you're adding extra complexity with your functions that read the serial data, write it back, then blink an LED. Since you're new to this, simplify it. Have it just turn on the LED if the receive flag (RCIF) goes high. If you can get that to work, then add one more later of complexity, and so on. Get rid of the functions for now until you can prove you can get the basic stuff working. \$\endgroup\$ Commented Sep 5, 2017 at 4:15
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    \$\begingroup\$ Please post your solution as an answer (and accept it) instead of editing the question. That way it is properly marked as answered. \$\endgroup\$ Commented Sep 10, 2017 at 4:12

1 Answer 1

Reset to default
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SOLVED THE PROBLEM

I am not sure what exactly solved the problem but I will share the major changes I made and the new code.

  1. I had TXIE enabled. TXIF is almost always high, so it generates continuous interrupts. I don't see a reason to enable TX interrupts, though there may be a good one. If you want to TX wait until TXIF is not zero and transmit, otherwise why use the flag?

  2. I had the interrupts enabling in the wrong order. I should have enabled the peripherals, then their individual interrupts if necessary, then PEIE, and finally GIE.

  3. I wasn't handling FERR and OERR in my interrupt, though they may be firing and causing interrupts.

Also I had RXDTSEL set wrong in my original code. Here is the new, working code. Right now all it does is echo the RX signal and blink the LEDs the number of times that is transmitted. 

#include <stdio.h> #include <stdlib.h> #include <xc.h> // This is for 300 baud rate #define _BAUD_PRESCALER_LOW_ 0x2A #define _BAUD_PRESCALER_HIGH_ 0x68 #define _XTAL_FREQ 32000000 #define _PIN_DIMPWMPIN_ RA5 #pragma config FOSC = INTOSC // Oscillator Selection->INTOSC oscillator: I/O function on CLKIN pin #pragma config WDTE = OFF // Watchdog Timer Enable->WDT enabled #pragma config PWRTE = OFF // Power-up Timer Enable->PWRT disabled #pragma config MCLRE = OFF // MCLR Pin Function Select->MCLR/VPP pin function is digital input #pragma config CP = OFF // Flash Program Memory Code Protection->Program memory code protection is disabled #pragma config CPD = OFF // Data Memory Code Protection->Data memory code protection is disabled #pragma config BOREN = ON // Brown-out Reset Enable->Brown-out Reset enabled #pragma config CLKOUTEN = OFF // Clock Out Enable->CLKOUT function is disabled. I/O or oscillator function on the CLKOUT pin #pragma config IESO = OFF // Internal/External Switchover->Internal/External Switchover mode is disabled #pragma config FCMEN = OFF // Fail-Safe Clock Monitor Enable->Fail-Safe Clock Monitor is disabled // CONFIG2 #pragma config WRT = OFF // Flash Memory Self-Write Protection->Write protection off #pragma config PLLEN = ON // PLL Enable->4x PLL enabled #pragma config STVREN = ON // Stack Overflow/Underflow Reset Enable->Stack Overflow or Underflow will cause a Reset #pragma config BORV = LO // Brown-out Reset Voltage Selection->Brown-out Reset Voltage (Vbor), low trip point selected. #pragma config LVP = OFF int flagRXFramingError = 0; int flagRXOverrunError = 0; volatile unsigned char RXIN = 3; unsigned char RX(){ return RCREG; } void writeRXIN(volatile unsigned char a){ RXIN = a; } void TX(unsigned char a){ while(!PIR1bits.TXIF); // TXIF is usually 1, only 0 when busy transmitting TXREG = a; } int main(int argc, char** argv) { // SCS FOSC; SPLLEN disabled; IRCF 8MHz_HF; OSCCON = 0xF0; // TUN 0; OSCTUNE = 0x00; // Set the secondary oscillator // Wait for PLL to stabilize while(OSCSTATbits.PLLR == 0){} ADCON0bits.ADON = 0; ANSELA = 0x00; ANSELB = 0x00; ANSELC = 0x00; PIE1bits.ADIE = 0; // Disable ADC interrupts TRISCbits.TRISC5 = 1; // RX pin set to input TRISCbits.TRISC6 = 0; // IadjPWM pin configured as output TRISAbits.TRISA5 = 0; // DimPWM pin configured as output LATCbits.LATC6 = 1; // Max current for now until PWM written //UART Init BAUDCONbits.BRG16 = 1; // 16-bit baud generation TXSTAbits.BRGH = 1; // High baud rate enabled BAUDCONbits.ABDEN = 0; // Auto baud detect disabled // Baud prescaler n = [Fosc/(D*BR)] - 1 SPBRGH = _BAUD_PRESCALER_HIGH_; __delay_ms(1); SPBRGL = _BAUD_PRESCALER_LOW_; __delay_ms(1); APFCON0bits.RXDTSEL = 1; // RX is on RC5 pin APFCON0bits.TXCKSEL = 0; // TX is on RB7 pin TXSTAbits.SYNC = 0; // Asynchronous mode RCSTAbits.SPEN = 1; // Serial Port Enabled RCSTAbits.RX9 = 0; // 8 bit reception TXSTAbits.TX9 = 0; // 8-bit transmission RCSTAbits.CREN = 1; // Receiver enabled TXSTAbits.TXEN = 1; // Transmitter enabled PIE1bits.TXIE = 0; // Enable USART Transmitter interrupt PIE1bits.RCIE = 1; // Enable USART Receive interrupt while(PIR1bits.RCIF){ writeRXIN(RX()); } INTCONbits.PEIE = 1; // Enable peripheral interrupts INTCONbits.GIE = 1; // Enable global interrupts while(1){ while(RXIN > 0){ TX(RXIN); _PIN_DIMPWMPIN_ = 1; __delay_ms(100); _PIN_DIMPWMPIN_ = 0; __delay_ms(100); RXIN--; } } // infinite loop // never leave this loop return (EXIT_SUCCESS); } // end main void interrupt ISR(void){ if(PIE1bits.RCIE && PIR1bits.RCIF){ // handle RX pin interrupts while(PIR1bits.RCIF){ writeRXIN(RX()); } if(RCSTAbits.FERR){ flagRXFramingError = 1; SPEN = 0; SPEN = 1; } if(RCSTAbits.OERR){ flagRXOverrunError = 1; CREN = 0; CREN = 1; } } // end RX pin interrupt handlers } // end ISRs*/
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  • \$\begingroup\$ To your point about TX interrupts: only enable it after starting to send a byte so the next byte can be sent from the ISR. If you aren't sending anything, disable the interrupt. \$\endgroup\$ Commented Sep 10, 2017 at 7:08

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