I2C Speed Problem

I am using:

Pic Basic Pro 2.50 with patch 250b installed and all working OK.

My MCU chip is a PIC18F8722 and I need to read/write to an I2C EEPROM (24LC32) with a 400KHz clock frequency.

1. Using DEFINE OSC 10 i.e. run at 10 MHz - all is OK :) and the EEPROM happily reads and writes.

2. Using DEFINE OSC 40 ie. run at 40 MHz and programming with HSPLL- lots of garbled data :(.

3. Using DEFINE I2C_SLOW does not work either :(?

4. I have PAUSE 10 after all read/writes and I'm only doing one read/write at a time.

HELP! Any ideas?

Regards Bill Legge

What value pull-up's have you got on the SCL & SDA lines?

Try dropping them to say 1K.

BTW... you don't need 10mS pauses after I2CREADs, just I2CWRITEs.

and make sure your clock is lower than 400KHz & multiplexed stuff disabled as well.

As usual, without code, config fuse, it's impossible to say the exact solution... have you ever tried to find a needle in a field with your eye closed?

Melanie,

Thanks very much - your advice worked.

Original pull-up resistors were 10k.
Changed to 5k and all works OK - without DEFINE I2C_SLOW.

I sould have taken note of the PBP handbook that advises 4.7k!

Regards Bill Legge

Bill, if speed is an issue, you might also consider reading the eeprom datasheet section on polling the chip. On a recent one I worked on, the spec was something like 5ms write time max, but there was an option to poll to see if it was done. I put that in with a 50uS delay between polls, and it ended up only taking three polls to finish!