Help: hardware pause button weirdly disables interrupt

By inv

Supporter (4)

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!

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