R65C02 data bus sharing

phvic · Post by **phvic** » Sat Nov 22, 2014 7:12 pm

Hi! I'm building my first 6502 computer and I've got a question. In my design the 6502 shares address and data busses with a co-processor (actually an AVR MCU). I have a Roswell 65C02 and since it can't tristate the address bus I'm using two 74HC541s to detach the 6502 from the address bus while the co-processor is active.

My question is about the data bus. If I hold the 6502 in reset or halt the CPU using RDY, do I need a buffer on the data bus? I'm assuming the 6502 only drives the data lines when storing to memory, right? But, e.g. what happens in the case the CPU is executing a store instruction and the CPU is halted by pulling RDY low? Does the 6502 release it's grip on the data bus in this case?

EDIT: btw. a great forum you have here!

GARTHWILSON · Post by **GARTHWILSON** » Sat Nov 22, 2014 7:23 pm

Unlike the NMOS 6502, the 65c02 will stop even on writes when RDY is negated (which, since it is positive logic, means pulled down). When you're ReaDY to get going again, pull RDY up.

Modern WDC 65c02's do have a BE (bus-enable) pin (pin 36 if it's the DIP), so you can omit the 74xx541's.

I have a 6502 primer at http://wilsonminesco.com/6502primer/index.html in 22 sections covering many aspects of building a 6502 computer.

phvic · Post by **phvic** » Sat Nov 22, 2014 7:46 pm

Thank you Garth! I've been indeed reading your 6502 primer, excellent articles!

Ok, it should be ok to use the data bus while the MPU is halted by pulling RDY down. Does the same apply to reset? I.e. if I pull reset down, data bus gets released?

GARTHWILSON · Post by **GARTHWILSON** » Sat Nov 22, 2014 7:51 pm

I'm sure the processor releases the data bus on reset, but I never had any reason to test it. Note that pulling RDY low is just slow memory telling it, "Hold on-- I'm not ready yet. I need more time. Please hold the current state longer." So if you're writing, the processor will keep asserting the data bus while RDY is low.

phvic · Post by **phvic** » Sat Nov 22, 2014 8:02 pm

Ok, I'll test it this weekend. Many thanks!

EDIT: does someone happen to know how to get a W65C02 or two shipped to Finland in a reasonable amount of time without outrageous shipping costs…?

GARTHWILSON · Post by **GARTHWILSON** » Sat Nov 22, 2014 8:08 pm

Check the 65xx parts sources sticky topic at viewtopic.php?f=1&t=1953 which we try to keep updated.

Dr Jefyll · Post by **Dr Jefyll** » Sat Nov 22, 2014 9:06 pm

phvic wrote:

does someone happen to know how to get a W65C02 or two shipped to Finland in a reasonable amount of time without outrageous shipping costs…?

If you haven't already bought a CPU, perhaps you should consider getting one that has the /BE (Bus Enable) input Garth mentioned. Then you won't require the 74hc541's. WDC is one manufacturer whose chips have /BE, and there are others (although I forget most of them). In order to exactly match the pinout of the original 6502, the R65c02 doesn't have /BE. But the Rockwell R65c102 and R65c112 do have /BE, so watch for those when you're shopping.

BTW as an alternative to using the RDY input to start & stop the 'C02, you could arrange for the AVR to generate the signal that serves as the 'C02's clock input (on pin 37). (In other words, don't use an oscillator). It's just something to consider -- I don't know what your priorities are; perhaps there's no advantage.

If you go this route, you can ignore R/W. Read or write, the CPU never drives the data bus during Phase 1. So when you stop the clock, just stop it in the low state (Phase 1).

Good luck, welcome, and keep us posted on your progress!

Jeff
ps- Rockwell datasheet attached. Note the 'c112 variant can only be used with an external clock, such as from the AVR.

GARTHWILSON · Post by **GARTHWILSON** » Sat Nov 22, 2014 9:09 pm

Only WDC's can be stopped during phase 1 though. Rockwell's can't.

Dr Jefyll · Post by **Dr Jefyll** » Sat Nov 22, 2014 9:22 pm

GARTHWILSON wrote:

Only WDC's can be stopped during phase 1 though. Rockwell's can't.

Oops, right; thanks. 5 microseconds is what Rockwell lists for the maximum stoppage during Phase 1. But if you go the RDY route you can still use a 'c102 or 'c112 and have the benefit of using /BE.

phvic · Post by **phvic** » Sat Nov 22, 2014 9:31 pm

Yeah, I only have a Rockwell 65C02 without the BE pin at the moment. I'm probably going to use the WDC version, if I can obtain one, in the final version to save PCB space. I'm using Eagle so it could be a tight fit to get all components and traces on one homemade single-sided PCB.

I've just finishing wiring the thing up on a breadboard and I'm writing a test firmware for the AVR. We should get some results soon

EDIT: Roswell -> Rockwell

phvic · Post by **phvic** » Sat Nov 22, 2014 9:32 pm

Dr Jefyll wrote:

Good luck, welcome, and keep us posted on your progress!

Thank you, I will!

Aslak3 · Post by **Aslak3** » Sat Nov 22, 2014 9:50 pm

phvic wrote:

I've just finishing wiring the thing up on a breadboard and I'm writing a test firmware for the AVR. We should get some results soon

And don't forget, schematics and PCB designs are like candy to the folk here. Everyone loves a good PCB design. I'm impressed you are making these yourself, and single sided too! I hope you dont have too many jumps onto the component side. Curious to see pictures when it is made.

I've done simple single sided PCBs myself, but nothing as complex as a 8 bit computer. A few around here have though, eg. lordbubsy's MARC-1 computer: viewtopic.php?f=4&t=2756

PS. Welcome!

phvic · Post by **phvic** » Sat Nov 22, 2014 10:51 pm

Ok, small update: proof of concept works! AVR pulls reset down, acquires bus access, updates SRAM, releases bus and starts generating clock signal for 6502. Next I'll try to use RDY to interleave SRAM accesses from AVR and 6502.

Gotta get some sleep now, later!

phvic · Post by **phvic** » Sat Nov 22, 2014 10:53 pm

Aslak3 wrote:

phvic wrote:

I've just finishing wiring the thing up on a breadboard and I'm writing a test firmware for the AVR. We should get some results soon

And don't forget, schematics and PCB designs are like candy to the folk here. Everyone loves a good PCB design. I'm impressed you are making these yourself, and single sided too! I hope you dont have too many jumps onto the component side. Curious to see pictures when it is made.

I've done simple single sided PCBs myself, but nothing as complex as a 8 bit computer. A few around here have though, eg. lordbubsy's MARC-1 computer: viewtopic.php?f=4&t=2756

PS. Welcome!

Thanks! Sure, I'll post some pics but it's going to take some time before I'll get to the PCB stage. Breadboards for now..

phvic · Post by **phvic** » Sun Nov 23, 2014 10:09 am

Just to make sure I'm not doing anything wrong: is it ok to hardwire SRAM's OE to ground? From what I've gathered, it seems that SRAMs don't care about OE during writes.

phi2 clock would be connected to SRAM's CE through an inverter to prevent accidental writes when phi2 is low.

Did I get this right?

R65C02 data bus sharing

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