Potential facepalm moment - chip select line on EPROM

Thread Starter

metermannd

Joined Oct 25, 2020
219
When I was asking here some time ago on how to properly 'line up' two 2732s to replace them with a 2764, it was suggested that I tie the ~CE line to ground.

I just realized... wouldn't that cause bus contention issues?

Should I have tied that line to the master clock instead?

Every other major IC on the two logic boards has an enable line tied to the master clock.

Link to old thread
 
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Chip enable shouldn't be connected to clock. It would enable and disable with every change on the clock. If there is only one device on the bus tying CE to ground would normally be good. If more than one device you need to keep CE high while working with other devices and set CE low on the device you want to work with
 

KeithWalker

Joined Jul 10, 2017
1,946
To replace two smaller e-proms with one larger one, the two chip enable lines (CE) should be or-ed together to drive CE on the larger one. How this is done will depend on the output circuitry on the controlling device.
 
Chip enable shouldn't be connected to clock. It would enable and disable with every change on the clock. If there is only one device on the bus tying CE to ground would normally be good. If more than one device you need to keep CE high while working with other devices and set CE low on the device you want to work with
Nevermind any of that. I took another look when I came home and found I was thinking SPI where I should have been thinking parallel. I really need to stop using my phone for stuff like this... don't always get all the information right.
 

LesJones

Joined Jan 8, 2017
3,523
You will have one more address line on the 2764 than you have on the 2732. You connect address lines A0 to A11 to both 2732s. You connect A12 to Not CE on one of the 2732s and also pass it through an inverter to NOT CE on the other 2732.
This way A12 selects which 2732 is being used at the time.
EDIT. I miss interpreted your question. I thought that you wanted to use two 2732s in place of a 2764. But it is the other way round.
You will need to do what KeithWalker suggests in post #3 Also OR the two 2732 NOT OE together and connect to NOT OE on the 2764. One of the 2732 NOT CE lines needs to be connected to A12 on the 2764. (It does not matter which one but it would make sense to use the 2732 that was the lower one in the address range on the original configuration. This would mean the addressing of the two half's of the2764 would be the right way round.)
Les.
 
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Thread Starter

metermannd

Joined Oct 25, 2020
219
What I had been wondering about was running the clock to the _OE line and (inverted) A15 to the _CE line.

I did make a daughterboard adapter based on the information in the original thread, burned a 2764 with both 2732s' software in the correct order, put it in the original unit, and it does work fine.

However, the overall copy I made of the unit doesn't work, and I'm just trying to figure out if there were any incorrect design assumptions versus the rare CPU like the one in the original unit (a custom 6800 variant and which is now unobtainium) being dead.

Normally, I'd have the original and my copy side by side so I could compare measurements at critical points, but it is NOT safe to go poking around inside the original unit (even plugged in and powered down) because whoever designed it didn't have safety in mind - HOT CHASSIS! And yes, I've been nipped a couple times.
 

AnalogKid

Joined Aug 1, 2013
9,406
Without drilling down into the other thread, it is not clear what you mean by "clock". What did the clock do in the previous, 2-chip circuit? If it was something like a read strobe, then it probably should go to the output enable pin. Presumably you had high-order address bits going through a comparator to determine when to enable which eprom. For the new eprom, the lowest of those high-order address bits now goes to the highest address bit input on the new, double-sized chip. One more address line to the chip, one less address line to the chip select decoder.

ak
 
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