gertk
Experienced Member
Hi,
My video ULA recently died in the BBC B here, no clock signals whatsoever, tried it in another BBC and no startup either.
I tried replacing it for testing with a GAL 16V8 generating the 4 clock signals(8/4/2/1 MHz), the CRT clock and the throughput/blanking of the SAA5050 RGB signals.
So far I managed to get the machine running albeit very unstable. The screen is somewhat glitchy and my cursor is one position off (to be expected).
But I do not understand why the computer is not running stable, it crashes after a few keystrokes for example.
The clock signals look fine on the oscilloscope, I tried altering the phase of CRTCLK (which in my GAL logic is synchronous and fixed to the 1 MHz clock signal output).
Even tried with an extra 16 MHz oscillator as input on the GAL but no diference. My understanding is that all clock stretching and such is done after the generation by the ULA.
Is there a detailed description of the ULA timing(s) including for example the phase between the clock signals available somewhere?
For now it would be ok just running in MODE 7 as that seemed the easiest mode to get going at first.
My video ULA recently died in the BBC B here, no clock signals whatsoever, tried it in another BBC and no startup either.
I tried replacing it for testing with a GAL 16V8 generating the 4 clock signals(8/4/2/1 MHz), the CRT clock and the throughput/blanking of the SAA5050 RGB signals.
So far I managed to get the machine running albeit very unstable. The screen is somewhat glitchy and my cursor is one position off (to be expected).
But I do not understand why the computer is not running stable, it crashes after a few keystrokes for example.
The clock signals look fine on the oscilloscope, I tried altering the phase of CRTCLK (which in my GAL logic is synchronous and fixed to the 1 MHz clock signal output).
Even tried with an extra 16 MHz oscillator as input on the GAL but no diference. My understanding is that all clock stretching and such is done after the generation by the ULA.
Is there a detailed description of the ULA timing(s) including for example the phase between the clock signals available somewhere?
For now it would be ok just running in MODE 7 as that seemed the easiest mode to get going at first.