EEPROM Upgrade

4th January 2008



The programme for the ECU is held in a 27C256 EPROM IC. This device requires a UV light source to erase it and can only be erased and reprogrammed a limited number of times. To assist development and to give greater reliabilty I have replaced this device with an X28HC256. This device can be electrically erased and can be reprogrammed many more times then the 27C256. The PCB needs a couple of mods to fit this alternative device. Needless to say, the programme must be copied from the original memory IC to the new one.

Here is a step by step guide to how I did it, naturally I used an IC socket to allow easy replacement of the chip.




1. Cut track near via

2. Links to pin 1

3. Cut track near via

4. Link pins 27 & 28

Pin 27 is the Write Enable pin, this is tied High to 5V (pin 28) to hold the chip in Read mode. At some point in the future it may be possible to tie the Write Enable to a spare processor pin to enable reprogramming down the serial link. For now the chip will have to be removed and placed in a programmer, to facilitate this a socket is fitted. On the top side of the PCB the track from pin 1 is cut close to the IC. Note that the socket used has turned pins and is not the less reliable formed pins.



The pic below shows the EEPROM installed. The two pin header fitted to the RW (Read/~WRite) and E (bus clock) tracks allows a Logic Analyser to be connected here.



Now to start some serious hacking. The main adaptor for Logic Analyser is made from a piece of strip board and fits between the socket and the IC. It is possible to buy clips that go over the IC but I'm a cheapskate. The adaptor will also stay in place if I end up driving round with the LA attached.