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Ghetto Programmer Meets UV EPROM

May 22, 2017

The Cypress CY27C256T is a UV erasable EPROM.  Mostly those are the wide .6″ DIP’s  but this is available in a .3″ wide package which means it will fit under the non-volatile RAM on the membership card. That spot is meant for a RAM but the pinouts are similar enough that I should be able to make it work for at least the bottom part of the address space.  To program it I’m adapting my ghetto eeprom programmer which I last used for the Boyd Calculator reprogramming.  I don’t want to bollix it permanently so I’m trying to be careful with the changes.  The first try is a read test. For  this all I had to do was connect pin 1(VPP) to +V along with pins 27(MA14) and 26(MA13), pin 23(MA11) goes to A11. Probably if I were smarter I’d make an adapter socket to match up with the footprint for the AT28C64 that the programmer was originally made for.  For now, I’ll just have to be careful.

17-05-22 ghetto 27c256I fired it up and it does read as 0’s which is good. I was also able to measure the 5V current at 27MA(including the 74595) and, by switching the /CE to +V, put the chip in low power mode and see the current drop to 7MA. The 74HC595’s data sheet says the supply current is max .08MA so I’ll ignore that!

To actually program it I have to set VPP(pin 1) to 12V with /OE and /CE held high then pulse /CE low many times for 200us at a time until the programming takes.  Then do it twice as many times again.  Fortunately the VPP is static at 12V so if i can figure out how to source it I don’t have to pulse it.  The ghetto programmer doesn’t normally control /CE but there’s no /WE required so that signal is freed up to use(the arduino’s D2).

Below is stuff I’m gathering for the programming push:

17-05-22 WS27C256 fit to programmer

17-05-22 WS27C256 fit to U2

As regards putting the EPROM chip into the Membership card, I have to think about pins 1,20, and 27. As currently routed, pin 1(VPP) will fluctuate with A14 which theoretically is ok. pin 20(/CE) needs to be wired to some variant of A15 to divide the address space with the RAM and pin 27(A14) would fluctuate with /WE which again theoretically doesn’t matter for testing since I won’t write in the ROM. So, hell, I may just be able to leave it all alone except for whatever jumpers lee provides for addressing.  Aaand it looks like the default jumpers on the board address the ROM high and the RAM low.  Perfect for my first programming attempt.  I’ll write something in the high part of the chip and then see if i can read it with the 1802.

Programming Instructions for the CY27C256


membership card schematic

UPDATE: Poking around my pile of castoffs I have a 12V wall wart that actually delivers just a bit over 12V.  I may try to get a regulator tomorrow or i may just try it out.

Below is the EPROM in place under the RAM.  I actually have to solder in some pin sockets and a capacitor before i run it but the fit looks fine.

27-05-22 ROM under RAM


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