Haveing just done the same things you did, I know the feeling.  Took me a
couple days to get all the timeing right, etc.  Only thing I think of is
the LED's themselves.  Are they buffered?  I don't know what the
capacitance is on a LED but could it be slowing down the bus?

Roman Black wrote:

> 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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