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UV EPROM Defeats Ghetto Programmer

May 26, 2017

I was able to get the breadboard programmer to read(I think) the CY27C256 EPROMS but my attempts at programming failed. The essence of the programming is to apply a series of 200 uS low pulses on the chip select (pin 20) while holding VPP(pin 1) at 12 volts. I first built that into the ghetto programmer code but when it didn’t work I just made a standalone sketch that would do 30 pulses and changed all the other signal levels with jumpers.  I set it up with A14,13 & 12 at +V and the others at 0 so i would be programming location 7000; i pulled D0-D7 high, held /OE(pin 22) at +V; applied 12V from a wall wart to VPP, and pulsed pin 22. After 30 pulses I disconnected the jumpers and tried to read the same location but all the bits remain stubbornly 0.

I tried each of two chips with the same result so it’s more likely some error on my part than a faulty part.

The chips have a signature built in which you read out by holding all address lines low except A9 which you connect to 12V(!).  I was able to do that on the breadboard and read out the signature successfully so it’s even less likely that the chips are bad.  I have a friend in Toronto who can maybe program them for me so i’ll try that.  I was surprised not to have found an arduino shield or schematic for programming them but i see why now – it’s a lot of trouble.

The pics below show the eprom on the breadboard set up to read the signature and then to program it.  In each case the 12V VPP is being supplied from a wall wart through the alligator clips at the bottom.  There’s an arduino off to the right supplying 5V and, in the programming case, pulsing the /CE pin low.


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