Re: EEPROM Circuit?

[mjmahon@aol.com (Michael J. Mahon)]

Glenn Jones replied:

>"Michael J. Mahon" <mjmahon@aol.com> wrote in message
>20040409204614.01174.00000110@mb-m14.aol.com">news:20040409204614.01174.00000110@mb-m14.aol.com...
>> Glenn Jones replied:
>>
>> >> In general, byte-wide EEPROMs are much simpler to
>> >> interface to a byte-wide bus--in fact the Apple bus has
>> >> most of the decoding done for you.
>> >
>> >Thanks for the info  .. I was trying to keep the physical size
>> >of the chip to a minimum and serial eeproms tend to be very
>> >small compared to byte wde eeproms that require 8 data lines
>> >plus address etc
>> >
>> >given what you just said .. the combined size of the additional glue
>> >to access as serial eeprom might end up being equivilent or larger
>> >than a byte wide eeprom
>>
>> That is true, unless you happen to have some unused I/O
>> pins around, plus some firmware/software space to control them.
>>
>> Many microcontrollers include some EEPROM for program
>> and parameter storage--maybe the one on the ethernet module
>> is one of them.
>
>No nonvolatile storage or extra ports on this module.

Wierd.  An ethernet port could use some configuration data...

>The module does have an i2c interface but that is for the module interface
>if you choose that vs direct or indirect modes
>
>Apparently one can also use some spare ram on the module $300- $3FFF but it
>is volatile.  :O(
>
>Their reference example shows using the 24c02
>with a 8051 which has ports that can be used to bit bang ic2.
>Most new microcontrollers are coming with i2c embedded masters as well
>
>I just found this chip ..have to do more reading .. it might help
>
>http://www.semiconductors.philips.com/acrobat/datasheets/PCF8584_4.pdf

It certainly has the functionality--but it's a 20-pin chip, so it
"outweighs" the EEPROM by quite a bit.  ;-)  You might as well
just stick on a small parallel EEPROM.

>An other option might be to emulate the annunciator and push button
>circuitry in the apple.

That would be an addressable latch for the output(s) and a simple
gated tristate bus driver for the input.  These could be easily used
in the Device Select address space, but, again, their size will
dwarf the EEPROM and are (cumulatively) larger than a parallel
EEPROM.

>I guess i should decide on one vs two wire interface first ....

Either will work, and both present very similar problems.

The fact is that adding a little bit of EEPROM to a peripheral bus
card is something of a pain.  The industry has moved to serial
interfaces for small chips (of course) and the usual communication
schemes do not easily match up with a parallel address and data
bus.

The way some systems handle it is to use a combined real-time
clock and EEPROM/NVRAM solution.  The usual serial interface
is still a problem with a parallel bus, but at least the solution can
be leveraged over two functions.  ;-)

-michael

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