Nuts and Volts - May 2010

By Ron Hildreth

Reading and Writing an EEPROM

Using the Arduino

My brother-in-law has five out-of-production CNC machines he uses in his business, three of which had stopped working. By process of elimination, he found the culprit to be a Xicor X88C64 EEPROM that translated user commands to servo controls. Unable to buy programmed replacement chips and finding commercial programmers to duplicate one of his remaining working EEPROM chips prohibitively expensive, he asked me if I could help find a solution. I had gotten an Arduino Duemilanove for Christmas and had experimented with it enough to think it might provide a solution to his problem.

Signal
Address 12
Address 11
Address 10
Address 9
Address 8
Address/Data 7
Address/Data 6
Address/Data 5
Address/Data 4
Address/Data 3
Address/Data 2
Address/Data 1
Address/Data 0
ALE
RD
WR
CE Master
CE Slave
WC
Register
C
C
C
C
C
A
A
A
A
A
A
A
A
L
L
L
L
L
L
Register Bit
4
3
2
1
0
7
6
5
4
3
2
1
0
0
1
2
3
4
5

My first task was to Google the X88C64 EEPROM to locate the datasheet for the part (see Links). I looked for any operation that required completion faster than would be possible for the Arduino. The only instance I found was that every write during a Page Write operation must be completed in less than 100 µs. Since the Arduino ( ) which would allow plenty of instructions to meet the timing requirements.

In looking at the chip specification, I noted that I would need 13 digital pins for the address/data bus plus several control leads. This would be a problem for my existing Arduino, however, since it only supports 13 digital pins (and two of those are needed for serial I/O). Not wanting to add external circuitry, I decided on an Arduino Mega which has 54 digital pins. (I’ll use any excuse to get some new gear!)

Another interesting aspect of the X88C64 was that the eight data bits and the lower eight address bits shared the same pins using the Address Latch Enable (ALE) control to designate when the pins are addresses. Since the Mega pins can be easily changed from output to input, I didn’t think this would be much of a problem.

Another issue I would need to address was that the X88C64 had 8K bytes of EEPROM. My original thought was to load the contents of a working EEPROM into an array, and then replace the chip with a blank EEPROM and write the array contents into the new chip. However, since the Mega only has 8K bytes of RAM that wouldn’t