Johnny Billquist wrote:
On 2021-03-06 02:33, Paul Koning wrote:
The explanation I heard for the slow J-11 clock is that the original J-11
spec called for it to operate at 20 MHz. When Harris failed to deliver
and the max useable clock speed ended up to be 18 MHz, most designs had
no trouble. But the Pro support chips were designed to run synchronous
with the CPU clock and for various other reasons needed a clock frequency
that's a multiple of 10 MHz, so when 20 MHz was ruled out that left 10 MHz
as the only alternative.
I do think it sounds weird that the support chips would require a clock
that is a multiple of 10 MHz. But I wouldn't know for sure.
Somewhere else I read/heard that they didn't work reliable above 10 MHz,
but for the F11 that was ok. When the -380 came, they just reused those
support chips.
The 6502 CPU in the BBC Micro operates at 2 MHz but periperals such as the
6522 Versatile Interface Adapters contain timers which only run at their
expected speed when used with a 1 MHz clock. I wonder could it be that the
Pro support chips would run ok at 18 MHz but there would have been
difficulties with programming timers etc which are clocked at an odd speed?
(The BBC Micro got around this problem by running the CPU at 2 MHz and
stretching the clock cycle to 1 MHz when accessing the VIAs. The clock
circuit is a real birds nest...)
Regards,
Peter Coghlan.
