Proxy wrote:
My personal record at 3.3V is around 20MHz and it was achieved by connecting the 65c816 directly to an FPGA which did all the demuxing, memory decoding, etc.
so there is still more memory latency than your design due to the FPGA being inbetween the CPU and memory.
but even with that bit of extra delay i feel like it should be possible to reach 25MHz at 5V.
I tried it on my 65c816 SBC a few times, which already uses pretty damn fast components (10ns CPLD, 10ns SRAM), but it's just not stable. so maybe you got better luck!
In the video I get about 30MHz without issues and that's at 3.9V.
In previous testing I can get 39.5Mhz but I cannot cross the 40MHz boundary*. However that's also at 3.9V to 4.1V. At higher voltages my clock signal becomes so bad that it rings all the way down to about 2.5V and I can't tell if the main problem is the
'816 or my board's signals.
(I'm pretty certain it's the board)
This is a big part of why I want to do a V2 board and clean those signals up (seriously I've done better on strip-board - but I had magically thinking and assumed because it was on a PCB it would just work).
Further in testing: up to 36Mhz does not glitch (unless I crank the voltage up). But. Sometimes higher frequencies do glitch and sometimes they do not - and it will run until I turn the board off.
(All the components that gotta go fast are LVC or ALVC, which really helps as their propagation times usually tend to the minimums in their datasheets).
kernelthread wrote:
According to this thread:
viewtopic.php?p=85784#p85784 it can go up to 40MHz if you don't need access to A16-A23. If you do, then it seems to max around 25MHz.
Plasmo's work is big part of inspiration behind this. Initially reading what he'd achieved caused me to redesign my
'816 based computer on a 20Mhz clock rather than the 10Mhz I was using. I decided I should probably test it first and just take a peek at what the signals coming from the
'816 actually really look like before I go too far. And little project became this. The
816 Bodge Board.
*
It's SO frustrating!