Heres one for more recent initiates to microcontroller programming in C. More experienced programmers are welcome to assist me in improving this code, provided they are civil.

Pseudo timers may be useful when non-blocking program delays are required. Multiple pseudo timers may be implemented using just the one microcontroller hardware timer. The number of timing elements available to the programmer is then not restricted to microcontroller timer hardware alone.

The Timers_0.0 software module implements four, millisecond precision countdown timers, numbered 0 through 3. Concurrent, non-blocking delays between approximately 1ms to 65 seconds are possible. It is a simple matter to set a timer to expire after a given period, and then to reiteratively test for timer expiry during program execution.

A simple illustrative example that flashes an indicator LED at 0.5Hz follows:

#include "Timers.h"

sbit LED = P2^6;

void main(void)
{
    // Initialise the Timers module.
    timers_init();

    // Round Robin task management, reiterative execution loop.
    while(1)
    {
        // Flash an LED output continuoulsy at 0.5Hz, 50% duty cycle.
        if (timers_is_expired_timer(0))
        {
            // Invert LED output.
            LED = ~LED;

            // Set timer 0 to expire in 1000ms.
            timers_restart_timer(0, 1000);
        }

        // Additional Round Robin task managed tasks.
    }
}
 

Happily, it is no more difficult to implement multiple, concurrent delays using the pseudo timer module.

Here we use 2 timers to flash 2 indicator LEDs at differing frequencies and duty cycles:

#include "Timers.h"

sbit LED_1 = P2^6;
sbit LED_2 = P2^7;

void main(void)
{
    // Initialise the Timers module.
    timers_init();

    // Round Robin task management, reiterative execution loop.
    while(1)
    {
        // Flash an LED output continuoulsy at 0.5Hz, 50% duty cycle.
        if (timers_is_expired_timer(0))
        {
            // Invert LED output.
            LED_1 = ~LED_1;

            // Set timer 0 to expire in 1000ms.
            timers_restart_timer(0, 1000);
        }

        // Flash another LED continuously at 1.3Hz, 33% duty cycle.
        if (timers_is_expired_timer(1))
        {
            if (LED_2)
            {
                // Turn LED on.
                LED_2 = 0;

                // Set timer 1 to expire in 250ms.
                timers_restart_timer(1, 250);
            }
            else
            {
                // Turn LED off.
                LED_2 = 1;

                // Set timer 1 to expire in 500ms.
                timers_restart_timer(1, 500);
            }
        }

        // Additional Round Robin task managed tasks.
    }
}
 

The Timers_0.0 software module is written 8051/ 8052 variants in Keil C, and is available for free download from my homepage at http://members.iinet.net.au/~vanluynm/ Please link only to the root webpage as the module's URL will change with revisions. I hope you may find this software module useful.

Regards,
Murray R. Van Luyn.