In SX Microcontrollers, SX/B Compiler and SX-Key Tool, bean wrote:

Okay so now your checking a variable instead of RTCC. How are you going to make sure the variable has reached zero ?

Maybe like this:

  TEST counter
  JNZ $-1

Okay so let's say the counter is 1, and the interrupt occurs right after the TEST instruction. It's going to be another 7 cycles until you get out of the loop.

Now let's say the counter is 1 and the interrupt occurs right before the TEST instruction. It's going to be another 3 cycles until your out of the loop. So there will still be 4 cycles of "play".  Plus you have to figure in the cycles used for the interrupt call and return. 

The problem is NOT setting a variable at the proper time. The problem is in the looping to wait for that variable.

You could just use RTCC and set it for -159 and wait for RTCC to reach zero the same way as your interrupt routine.

There may be a way around this, but I think an interrupt driven video generation method is best. 
Interrupts are completely deterministic on the SX.

Bean.

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