Reconfigure timer during timer interrupt interrupt 8051

Question

I'm using SDCC compiler.

What I'm trying to achieve is to reconfigure Timer0 in Mode 2 auto-reload, during its own interrupt handler. Here is the C code:

void reconf(void)  __interrupt(1){
    TR0=0;
    TH0=0xC0;
    TL0=0xC0;
    TR0=1;
}

Here are the questions:

1. Is it possible to reconfigure TL0 and TH0 for Timer0 in Mode 2 auto-reload mode during its own interrupt handler execution?

2. Is it necessary to stop the Timer0 during reconfiguration or not because it is not running during the interruption?

3. Are the TH0 and TL0 values PUSHed into the stack before the interrupt routine starts? If these values are PUSHed into the stack, and then during the routine handler execution I reconfigure these values, will these values be overridden by the POPed values from the stack upon exiting from interrupt?

Answer 1

Before addressing your questions, I think it's worth pointing out what TR0, TH0, and TL0 are. They are not variables local to the function or sitting on the stack (assuming you have a call stack, which most 8051 applications do not -- look up memory overlays). These are Special Function Registers, usually abbreviated SFR. You can read more about SFRs, but for the purposes of your question you can treat them as global variables from a scope perspective.

1. You may modify the TL0 and TH0 registers at any time, including within the Timer 0 peripheral's interrupt handler.

2. It is not necessary to stop the the timer to modify its value, but be aware that it will continue counting while you're doing that. This could be a problem if you are writing just as the lower byte is rolling over where you might end up with a timer value different than you intended.

   <previous code>  // Timer increments to 0x12fe
   TH0 = 0xff;      // Timer is now 0xffff
                    // Timer increments to 0x0000
   TL0 = 0x52;      // Timer is now 0x0052
                    // Timer increments to 0x0053
   

   You tried to set the timer to 0xff52 but you ended up with 0x0052. This is an extreme example, but the risk is there. You can be less risky by writing TL0 first followed by TH0, but turning off the timer is the simplest solution.

3. Since you now know that TL0 and TH0 are SFRs with global scope, you don't have to worry about the stack or any other function argument passing mechanism interfering with them.