How it works?
This I2C-bus analyzer capture all transmissions via I2C/TWI bus, decode it and send to PC via virtual RS-232 port. Received data can be displayed by any terminal program on PC computer. Main part of analyzer is ATTiny2313 microcontroller with 20MHz crystal. Link to PC are realized by FTDI FT245RL on UM245R module.

Example of transmission captured by I2C bus sniffer (read one byte from M24C64 I2C EEPROM) :

Download
  Schematic
  PCB bottom layer
  PCB top layer (wire jumpers)
  PCB top overlay
  Firmware

Parts list
  U1 : ATTiny2313-20PU
  U2 : UM245R DIP module
  X1 : 20MHz crystal
  C1,C2 : 22pF 0805
  C3 : 100nF 0805
  C4 : 10uF electrolytic
  R1 : 330Ohm
  R2 : 4,7kOhm
  LED : 3mm green LED

There are three SMD jumpers : JP1, JP2 and JP3. For I2C bus analyze JP1 must be short, JP2 and JP3 must be open. I2C bus (SCL, SDA and GND lines) must be connected to INT0, INT1 and GND pin. Fusebits must be set for external crystal oscillator > 8MHz. FTDI VCP drivers must be installed before use this sniffer. Data from sniffer can be displayed by any terminal program on the PC (Hyper terminal, Bray Terminal etc).

Assembled module :

See also : 1-wire sniffer (based on this same hardware, only firmware is different).