PIC18F25K80 4 > 20 returns true. How is this possible?

Question

I'm using such code on PIC18F25K80 compiled with xc8 compiler:

unsigned long kline_kw1281_byte_time = 0;

void sendKline(void) {
  klineWrite(0x9);
  while (klineBusy());
  kline_kw1281_byte_time = millis();
  while (!klineAvailable()) {
      if (kline_kw1281_byte_time_out()) {
          mStatus = DISCONNECTED;
          while(Busy1USART());
          Write1USART(0x00);
          return;
      }
  }
}


bit kline_kw1281_byte_time_out(void) {
    unsigned long time_out = millis() - kline_kw1281_byte_time;

    if (time_out > 20){
        while(Busy1USART());
        Write1USART(time_out);
        return 1;
    } else
        return 0;
}

and I have strange issue. In kline_kw1281_byte_time_out() function I have true statement for time_out > 20 but Write1USART(time_out) prints me lower number than 20. This happens random. The function most of the time works correctly.

this is millis() code:

#include <p18cxxx.h>
#include "timer0.h"

volatile unsigned long runtime_millis;

void initialize_timer0() {
    T0CON = 0b11000011;
    INTCONbits.TMR0IE = 1;
    INTCON2bits.TMR0IP = 1; //set timer0 interrupt priority high
    INTCONbits.TMR0IF = 0; //reset timer0 interrupt flag
}

void timer0(void) {
    runtime_millis++;
    TMR0L = 8;
    INTCONbits.TMR0IF = 0;
}

unsigned long millis(void) {
    return runtime_millis;
}

void delay(unsigned long ms) {
    unsigned long cTime = runtime_millis;
    while (runtime_millis - cTime < ms);
}

Answer 1

if (time_out > 20){
    while(Busy1USART());
    Write1USART(time_out);

This writes time_out as ASCII character to the USART module. If you see '4' over the USART, that means the ASCII character '4', real value 0x34, is sent. And 0x34, 52 decimal, is greater than 20.

If you want to print the decimal value as a string to the USART, you can do this:

#define putch Write1USART 

printf("%ul", time_out);

The printf() function formats time_out as an unsigned long (hence %ul) and writes it byte by byte to putch(). putch() should therefore have the signature:

void putch(unsigned char);

The #define in the example above makes sure Write1USART() is used for putch().