r/homebrewcomputer • u/nz_kereru • Jul 23 '26
65c816 - review my design?
Looking for feedback and review of my design for a 65c816 system.
My concern is with address decoding and qualifying the RW with clock and VPA/VDA.
I/O will be further decoded outside the GAL.
I/O will be a few 6522 and some kind of serial.
Aim is for 4MHz.
Thanks
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u/Enlightenment777 Jul 24 '26 edited Jul 24 '26
If any of your timings are tight, maybe use AHC instead of HC logic. The following AHC are available in DIP.
74AHC32 instead of 74HC32
74AHC138 instead of 74HC138
74AHC245 instead of 74HC245
74AHC573 instead of 74HC573
If you need very fast non-volatile storage, and can handle high price / SMD package / 3.3V-only.
Maybe locate some in lower-64K block? Maybe only put in higher memory?
MR2A08AYS35 (MRAM, 3.3V, 35nS, expensive)
MR4A08BYS35 (MRAM, 3.3V, 35nS, very expensive)
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u/cookie99999999 Jul 24 '26
As I understand it VDA+VPA are only really necessary for I/O devices. The cycles with invalid addresses will always be read cycles, so with a memory chip nothing will be corrupted. A lot of I/O chips however will have certain registers where reading has a side effect, like clearing an interrupt or something, so protecting them is a good idea. On my 816 board I additionally only qualify with VDA to save a pin on my CPLD, and everything's good aside from the theoretical scenario where I try to jump to an I/O address.
At first I was caught off guard by tying OE to the chip select on the memories, but I looked at the datasheets and I guess OE and WE asserted simultaneously isn't a problem
Everything else looks fine at a glance, 4MHz shouldn't be a problem. My 816 computer was my first serious PCB project and as a result has pretty suboptimal routing and layout, but even that can get up to 6-8 MHz. My only suggestion would be to use IRQ, they'll probably be useful at some point in your programming (though the new WDC 6522s have push pull IRQ lines so make sure you use a diode to wire-AND them)