My question has two parts:

  1. Does MPLABX have a UART simulator?

  2. I need help debugging this code:

 * Created on December 30, 2014, 11:15 AM

// PIC18F4550 Configuration Bit Settings

// 'C' source line config statements

#include <xc.h>

// #pragma config statements should precede project file includes.
// Use project enums instead of #define for ON and OFF.

#pragma config PLLDIV = 5       // PLL Prescaler Selection bits (Divide by 5 (20 MHz oscillator input))
#pragma config CPUDIV = OSC4_PLL6// System Clock Postscaler Selection bits ([Primary Oscillator Src: /4][96 MHz PLL Src: /6])
#pragma config USBDIV = 1       // USB Clock Selection bit (used in Full-Speed USB mode only; UCFG:FSEN = 1) (USB clock source comes directly from the primary oscillator block with no postscale)

#pragma config FOSC = HSPLL_HS  // Oscillator Selection bits (HS oscillator, PLL enabled (HSPLL))
#pragma config FCMEN = OFF      // Fail-Safe Clock Monitor Enable bit (Fail-Safe Clock Monitor disabled)
#pragma config IESO = OFF       // Internal/External Oscillator Switchover bit (Oscillator Switchover mode disabled)

#pragma config PWRT = OFF       // Power-up Timer Enable bit (PWRT disabled)
#pragma config BOR = ON         // Brown-out Reset Enable bits (Brown-out Reset enabled in hardware only (SBOREN is disabled))
#pragma config BORV = 3         // Brown-out Reset Voltage bits (Minimum setting)
#pragma config VREGEN = OFF     // USB Voltage Regulator Enable bit (USB voltage regulator disabled)

#pragma config WDT = OFF        // Watchdog Timer Enable bit (WDT disabled (control is placed on the SWDTEN bit))
#pragma config WDTPS = 32768    // Watchdog Timer Postscale Select bits (1:32768)

#pragma config CCP2MX = ON      // CCP2 MUX bit (CCP2 input/output is multiplexed with RC1)
#pragma config PBADEN = OFF     // PORTB A/D Enable bit (PORTB<4:0> pins are configured as digital I/O on Reset)
#pragma config LPT1OSC = OFF    // Low-Power Timer 1 Oscillator Enable bit (Timer1 configured for higher power operation)
#pragma config MCLRE = ON       // MCLR Pin Enable bit (MCLR pin enabled; RE3 input pin disabled)

#pragma config STVREN = ON      // Stack Full/Underflow Reset Enable bit (Stack full/underflow will cause Reset)
#pragma config LVP = ON         // Single-Supply ICSP Enable bit (Single-Supply ICSP enabled)
#pragma config ICPRT = OFF      // Dedicated In-Circuit Debug/Programming Port (ICPORT) Enable bit (ICPORT disabled)
#pragma config XINST = OFF      // Extended Instruction Set Enable bit (Instruction set extension and Indexed Addressing mode disabled (Legacy mode))

#pragma config CP0 = OFF        // Code Protection bit (Block 0 (000800-001FFFh) is not code-protected)
#pragma config CP1 = OFF        // Code Protection bit (Block 1 (002000-003FFFh) is not code-protected)
#pragma config CP2 = OFF        // Code Protection bit (Block 2 (004000-005FFFh) is not code-protected)
#pragma config CP3 = OFF        // Code Protection bit (Block 3 (006000-007FFFh) is not code-protected)

#pragma config CPB = OFF        // Boot Block Code Protection bit (Boot block (000000-0007FFh) is not code-protected)
#pragma config CPD = OFF        // Data EEPROM Code Protection bit (Data EEPROM is not code-protected)

#pragma config WRT0 = OFF       // Write Protection bit (Block 0 (000800-001FFFh) is not write-protected)
#pragma config WRT1 = OFF       // Write Protection bit (Block 1 (002000-003FFFh) is not write-protected)
#pragma config WRT2 = OFF       // Write Protection bit (Block 2 (004000-005FFFh) is not write-protected)
#pragma config WRT3 = OFF       // Write Protection bit (Block 3 (006000-007FFFh) is not write-protected)

#pragma config WRTC = OFF       // Configuration Register Write Protection bit (Configuration registers (300000-3000FFh) are not write-protected)
#pragma config WRTB = OFF       // Boot Block Write Protection bit (Boot block (000000-0007FFh) is not write-protected)
#pragma config WRTD = OFF       // Data EEPROM Write Protection bit (Data EEPROM is not write-protected)

#pragma config EBTR0 = OFF      // Table Read Protection bit (Block 0 (000800-001FFFh) is not protected from table reads executed in other blocks)
#pragma config EBTR1 = OFF      // Table Read Protection bit (Block 1 (002000-003FFFh) is not protected from table reads executed in other blocks)
#pragma config EBTR2 = OFF      // Table Read Protection bit (Block 2 (004000-005FFFh) is not protected from table reads executed in other blocks)
#pragma config EBTR3 = OFF      // Table Read Protection bit (Block 3 (006000-007FFFh) is not protected from table reads executed in other blocks)

#pragma config EBTRB = OFF      // Boot Block Table Read Protection bit (Boot block (000000-0007FFh) is not protected from table reads executed in other blocks)

void init_uart()
    TRISCbits.TRISC7 = 1;
    TRISCbits.TRISC6 = 1;
    TXSTA = 0b00100000;
    RCSTA = 0b10010000;
    BAUDCON = 0b00000000;
    SPBRG = 12;

void uart_send(char ch)
    TXREG = ch;

int main() {



It works enough that TXSTAbits.TRMT is shown full and then empty. But on Proteus Virtual Terminal no output is shown.

  • \$\begingroup\$ And what chip would that be for? Ah, the PIC18F4550 is mentioned in the config bits header. Is that right? \$\endgroup\$
    – Majenko
    Dec 30, 2014 at 17:18
  • \$\begingroup\$ MPLAB SIM supports the UART and will allow the output to be redirected either to a file or to a MPLABX window. \$\endgroup\$ Dec 30, 2014 at 17:20
  • 6
    \$\begingroup\$ Tip: Don't assign whole values to registers. Use the XXXbits.yyy for everything. Yes, it's more work, yes it's not as efficient to run, but it's a whole lot easier to understand when you're debugging. \$\endgroup\$
    – Majenko
    Dec 30, 2014 at 17:23
  • \$\begingroup\$ Your function uart_send() takes a char. So you should be giving it the char 'V', not the string "V". \$\endgroup\$ Dec 31, 2014 at 0:46
  • \$\begingroup\$ @Majenko yes it is PIC18f4550 and i will keep your tip in mind for the future. \$\endgroup\$ Jan 1, 2015 at 10:24

1 Answer 1

  1. Yes, MPLABX does simulate the UART in a PIC24F4550.

To enable it go to File/Project Properties, click on Simulator and select UART I/O Options from the Option Categories dropdown box. If you choose 'window' for output then transmitted characters will be shown in the UART1 Output window.

However this may not help much, because it only shows what characters the UART module transmits internally, not what happens at the TX pin (RC6, pin 25).

  1. The line TRISCbits.TRISC6 = 1; puts RC6 into input mode. To enable output on the pin you must set the TRISC6 bit to 0.

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