Let's break down why your TMR0IF flag never triggers and fix the issue step by step. Your core problem is that the timer isn't running or configured properly, so it never overflows to set the flag. Here's what's missing or misconfigured in your code:

1. You forgot to enable the TMR0 timer #

Your code sets up TMR0's mode and clock source, but you never turn the timer on. The T0CON0bits.T0EN bit is the critical enable switch—without setting this to 1, the timer won't count at all, so TMR0IF will never get set.

Add this line right after your T0CON configurations:

T0CON0bits.T0EN = 1; // Enable the TMR0 timer

2. Verify your clock source configuration #

You set T0CON1bits.T0CS = 0b010—for most modern 8-bit PIC MCUs (like the PIC16F18xx series that uses PIR0), this selects FOSC/4 as the clock source. Since you defined _XTAL_FREQ 64000000 (64MHz), FOSC/4 equals 16MHz. That's valid, but double-check:

• Your MCU's oscillator is actually running at 64MHz (confirm oscillator config bits like CONFIG1 for HS/PLL modes)
• If you intended a different clock source (e.g., external clock, low-power internal oscillator), adjust T0CS to match the datasheet's values.

3. Clear the TMR0IF flag before polling #

Before entering your while loop, manually clear the flag to avoid false triggers from reset or unintended events:

PIR0bits.TMR0IF = 0; // Reset the flag before starting the timer

4. Optional: Set an initial TMR0 value (for specific timeouts) #

If you want a precise overflow time, pre-load the 16-bit TMR0 registers. For example, to get a ~1ms timeout with 16MHz clock:

// Load initial value for ~1ms overflow (65536 - 16000 = 49536 = 0xF830)
TMR0H = 0xF8;
TMR0L = 0x30;

Without an initial value, TMR0 starts at 0 and counts to 0xFFFF—with 16MHz clock, that's about 4ms of wait time.

Fixed Example Code #

Here's your code with all critical fixes applied:

#define _XTAL_FREQ 64000000
#define ACM_STEP_TRIS TRISAbits.TRISA4
#define ACM_STEP LATAbits.LATA4
#define ACM_ENABLE_TRIS TRISAbits.TRISA5
#define ACM_nENABLE LATAbits.LATA5

void main(void) {
    ACM_STEP_TRIS = 0; // Set RA4 as output

    // TMR0 Configuration
    T0CON0bits.T0OUT = 0; // Disable TMR0 output driver
    T0CON0bits.T016BIT = 1; // Enable 16-bit timer mode
    T0CON0bits.T0OUTPS = 0; // No postscaler (1:1 ratio)
    T0CON1bits.T0CS = 0b010; // Clock source = FOSC/4 (16MHz)
    
    PIR0bits.TMR0IF = 0; // Clear flag before starting
    T0CON0bits.T0EN = 1; // Enable TMR0 timer
    
    // Wait for TMR0 overflow
    while(PIR0bits.TMR0IF == 0);
    
    // Action to take when flag is set
    ACM_STEP = 1; // Example: Toggle step pin
}

Additional Checks #

• Confirm your MCU's datasheet matches your register names (some older PICs use INTCON instead of PIR0 for TMR0IF, but your code uses the correct modern register)
• Make sure no other code is accidentally clearing TMR0IF before your loop
• If you plan to use interrupts later, configure PIE0bits.TMR0IE (not required for polling, though)

内容的提问来源于stack exchange，提问作者nancy