[sdiy] 8254, was:Roland DCOs

[Ingo Debus]

Am 28.02.2008 um 14:44 schrieb karl dalen:

> Just as a tiny side note. Thats Intels original, we have no idea if
> the silicon where redesigned coming from other wendors and there  
> are many
> to chose from.

We tried Intel, AMD and another manufacturer (forget which) whose '54  
were even spec'd for an input frequency of 12,5MHz. We also tried the  
non-CMOS version (8254 instead of 82C54). They all showed this  
behaviour. The 8253 worked flawlessly however, but is only spec'd for  
a much lower input frequency.

> Also there are plenty of clones under different labeling,
> altought 8253, 8254 was the system timer chip in 8088 286, 386 PC's  
> for
> quite some time. Inside those old PC's there are loads of these chips
> in them particularly 8088-86.

Yeah, that puzzled me a lot. How come that chip never caused problems  
in PCs?? I don't know for sure, but I suspect in a PC the chip was  
not often accessed from the processor side. In my design the '54 ran  
smoothly forever as long as the processor didn't read from or write  
to it. That also made me suspect that tWC and tWG timing was  
critical. Ah, memories come up again...

Ingo