Help: hardware pause button weirdly disables interrupt

By inv

Supporter (1)

inv's picture

26-06-2019, 17:13

Hi all, this is a newbie who is trying to fix a Sony HB-F1XDJ which I recently obtained.
After spending some time, everything seems to work, except one very strange behavior, regarding the hardware pause key.

When I press the pause button, everything stops as expected, but when I press the pause button again, every keyboard key no longer works. Interestingly, though, the pause button pauses and resumes a running program correctly. So I made a simple test program:

1 cls
2 locate 0,0 : print i, time : i=i+1 : goto 2

Okay, when I run the program, both the values of i and time increase. By the pause button, both stop, as expected. But by pushing the pause button again, i starts increasing again, but time (jiffy) does not.

So, I guessed the (vdp) interrupt does not work after the first pause.

Because I have only a basic multimeter and don't have an oscilloscope, the only thing I could do to confirm my guess was to test the voltage of /INT (pin 25) of the VDP v9938 (relative to the ground).

When I first turn on the machine, /INT is about 4.8V, which makes sense: it must be high normally, and low only when the timer interrupt is given. So I assume this reflects the normal behavior.

After pressing the pause button once, /INT of v9938 stays at about 90mV, which is definitely low. Even after I press the pause button again, /INT still stays low. This is a weird behavior, I think!

I don't understand what kind of hardware problem might cause this. I believe this is not a software problem (for instance the IE0 bit of VDP(1) seems to be 1, which is correct). Any comments and thoughts will be greatly helpful please!

Login or register to post comments