I2C random access and streaming

Here's some code showing how to read/write to an eeprom using byte-addressing and Oshonsoft Basic. It was written for an 18f2455 running at 20Mhz and a 24C256 eeprom using I2C communication:

Dim addr As Word
Dim writebyte As Byte

Define I2CCLOCK_STRETCH = 9
Symbol sda = PORTB.3
Symbol scl = PORTB.4

addr=32
writebyte=100
Gosub writeI2C

End

---

writei2c:
  I2CWrite sda, scl, 0xa0, addr, writebyte
  addr = addr + 1
  WaitMs 5
Return

The key thing to note here is the "stretch" command and the 5ms delay after each write instruction. According to the datasheet, a delay of 5ms is needed to allow the chip to finish writing its data to the addressed memory location. In practice, this is a little bit long - you can get away with 4ms and some people have reported delays of 3ms or less. Others have also said it's possible to overclock these eeprom chips and have them running at 1Mhz or more.

I found that 4ms delay works, but to be extra-sure, I've gone for a 5ms delay after writing data. When reading data back from the eeprom, the delay is not necessary.

To use the code above, simply put a 2-byte address into the variable "addr".
Put a value into the variable "writebyte"
Call the sub-routine "writeI2C" using the gosub command.

Equally as important are the connections to your eeprom chip.
Leaving lines floating (ie. not tied to either power or ground) means really erratic behaviour with some read/write operations working and others failing. Here are the pin connections I used:

pin 1-3 (A0-A2): ground
pin 4: Vss : ground
pin 5: Vcc : 5V
pin 6: WP : 0v (write protect)
pin 7: SCL: to PIC pin (RB4)
pin 8: SDA: to PIC pin (RB3)