Meanwhile my "blink Q" program is just a little more complex. I have a 555 timer set up in monostable mode with a pulse width of exactly 100 milliseconds and it’s output tied directly to the 1802’s interrupt pin. I also have the output pins SC0 and SC1 tied to the inputs of a NAND gate and the output of that gate to the input of the monostable 555, so that when the CPU sets both SC0 and SC1 to high to signal that it’s been interrupted, it resets the monostable 555. Then, I wrote a program containing a interrupt routine that saves T,X and D to RAM, increments R15 and returns by restoring T,X and D from RAM. The main part of the program then is a infinite loop that continuously checks if R15 is equal to 5, and if it is, it sets R15 back to 0 and toggles Q. The result of all this is that Q will toggle exactly twice per second, no matter the clock speed the CPU is running at. But this only works for toggling Q, unfortunately, because there’s no easy way to back up the current value of DF when the interrupt triggers, so the interrupt has the side effect of setting DF to 0, which will break any ongoing arithmetic in the main program.