I am currently working on an I2C EEPROM project using bit-banging to drive the SDA and SCL lines.
My read function works fine but whenever I write any byte with a leading "1", I always read FF back; even if the byte has been programmed with something else before. Leading "0" is perfect. It is not my read routine; as I can see on the scope it returns FF.
I am looking for suggestions on why this might be. Is there any obvious I could miss which could cause the problem? [I cannot post the code – company confidential… :(]
Every waveform I look at meets the spec exactly. I am decoupling the EEPROM. My pull ups are 2.2k so within spec. I'm clocking at about 500 Hz in this prototype. The chip is sending ACKs to each of my bytes so it recognises them. But it just doesn't work…
I am using a Microchip 24LC256.
Simplified writing algorithm for one byte:
wait SDA low SCL low wait for each bit if bit is set: SDA high if bit is unset: SDA low wait SCL high wait wait SCL low wait wait SDA high SCL high wait wait check ACK status SDA low SCL low wait return ACK status
Simplified reading algorithm for one byte:
wait SCL low SDA high for each bit (8 bits) SCL high wait wait SCL low wait check and store received bit wait do a NACK or ACK depending on if it is the last byte
You're reading the data after clock is low again. You'll have to do that between making the clock high and making it low. After the clock is low the slave is allowed to change the data line, not while it's high.
So reading should be like this: