Well i'm I2C embarrased! :o)
After weeks of downloading and printing all the
Microchip appnotes, and the F873 and 24LC256
datasheets, with all been read at least once and
handwritten notes through them, and I2C timing
diagrams up on my wall, and downloading all the
piclist.com I2C source code, and thinking I knew
just about all the secrets to bit-banging I2C to a
serial eeprom, well... It doesn't work.

Yes i've checked appnote AN709 and used their
safety initialisation example. I've allowed around
2uS between any change of state of the SDA or SCL
pins, to keep rise/setup/hold times all safely
well within spec. I disable GIE (with the safe loop)
before doing any I2C stuff.

Hardware? I have 3 indicator leds, one flashes
the data bytes, others to confirm the write or read
control buttons. It is on a "protoboard" that I use
for projects that is kitted with it's own ICSP.
I have a 1k pullup resistor on SDA. Speed is 16MHz,
or 4MHz/inst.

Everything is designed in small simple code modules
that I have checked carefully.

Here's my problem:
* about 10% of the time the ack bit (from eeprom)
fails, ie my code detects ack was high after sending
8 bits. Any read or write to the 24LC256 eeprom
must send at least 3 bytes and receive back 3 acks,
for control, address_hi and address_lo.
* If it manages a read or write without failing an ack
the eeprom read comes back as all zeros. I'm assuming
that no valid write (and possibly no valid read) is
ever taking place.

Any suggestions? This is embarassing, after all the
I2C problems posted here I was determined to do my
homework first and get it right... :o)
-Roman

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