Also, the time spend in the ISR code between stop T1 and start T1 varies, depending on the

if (++multiplier == 20)
 

comparison.
A better way should be to take care of timer T1 first and then make any actions, so to avoid jitter every time the comparison comes true.

/*--------------------------------------------
    This occurs every 50ms
---------------------------------------------*/
void timer_1_interr(void) interrupt 3
{
    static unsigned char data multiplier = 0;
    TR1 = 0;
    TF1 = 0;
    TH1 = SOFT_RTC_TH1_RELOAD;    //reload timer 1
    TL1 = SOFT_RTC_TL1_RELOAD;
    TR1 = 1;

    if (++multiplier == 20) {
       multiplier = 0;
       soft_RTC_ticks++; 
    }

    if (multitap_timeout)    //dec if not already 0
       multitap_timeout--;            

}

 

and combined with the zero comparison by Per,

/*--------------------------------------------
    This occurs every 50ms
---------------------------------------------*/
void timer_1_interr(void) interrupt 3
{
    static unsigned char data multiplier = 20;
    TR1 = 0;
    TF1 = 0;
    TH1 = SOFT_RTC_TH1_RELOAD;    //reload timer 1
    TL1 = SOFT_RTC_TL1_RELOAD;
    TR1 = 1;

    if (--multiplier == 0) {
       multiplier = 20;
       soft_RTC_ticks++;          
    }

    if (multitap_timeout)         //dec if not already 0
       multitap_timeout--;            

}

 

K.L.Angelis