Re: 2764 -> 2364/68764 adapter

On 13/07/2016 8:48 PM, Gerrit Heitsch wrote:
> On 07/13/2016 01:40 PM, smf wrote:
>> On 12/07/2016 17:59, Jim Brain wrote:
>>>
>>> I think I tied OE to the 2364 select line, so if your use case uses
>>> CE, you'll have to cut a trace, and if it uses both, then you're into
>>> an OR gate, which my design does not support.
>>>
>>
>> I'd have thought 2364 pin 20 (_CE) would make more sense to be connected
>> to 2764 pin 20 (_CE). This would cover the case for _OE being grounded
>> as well as when _CE & _OE are tied together.
>
> In man circuits they control the EPROM through _OE and tie _CE to GND 
> since that allows you to get away with slower EPROMs. This way the 
> EPROM is always aktive and decoding addresses, just the output buffers 
> are disabled. So you can hide quite a bit of the access time inside 
> the delay for the decoder that selects the EPROM. Downside: higher 
> power consumption with the old NMOS types. Doesn't really matter with 
> CMOS.
>
>
>
>  If _CE & _OE are
>> independently controlled then you probably ought to add a hex buffer
>> with _OE1 and _OE2 and feed _CE and _OE to it.
>
> A simple OR gate (74LS32 for example) will tie _OE and _CE together 
> and produce the correct signal for a 2364.
>

I usually construct a simple OR gate using diodes and a resistor. Hasn't 
failed me yet.



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