This post I am going to be demonstrating how to read an I2C device using a BusPirate. The BusPirate is a brilliant little tool that provides a command line interface to various communication buses. I am using the BusPirate V4, but the theory will be the same for the V3 and V3.5. The first device I am going to try and read is the ST24LC21. This ST part is a 1Kbit EEPROM that stored the TV’s VGA information that allows a PC to detect the capabilities of the display.
Attach your probes as follows: Vcc -> +5, GND -> GND, SCL -> CLK, SDA -> MOSI, WP -> AUX. Connect to the BusPirate using your favourite terminal program, and type the following “m 4 1 3″ to enter I2C mode. Next enable the power “P”, connect the pull-ups “e”, and enable the pull-ups “W”. Next you can search for any devices on the bus “(1)”. This should show a device on 0xA0 and 0xA1. This is the same device, but the write and read address. To read the data from the chip we set up the address “[0xA0 0]”, and send 128 read commands “[0xA1 r:128]”. Actually deciphering the numbers means delving into the EDID specification (or at least the Wikipedia article), but with that it is possible to determine manufacturing details of the TV, and its capabilities.
For example, byte7-8 (counting on from byte0) contain a compressed ASCII string. 0x58B3 is 0b0101100010110011, which in this scheme turns into 0bx 10110 00101 10011, or 22 5 19, or VES. Microsoft are responsible for allocating these values, and checking here reveals that the owner of VES is indeed Vestel Elektronik Sanayi ve Ticaret, the manufacturers of the control panel. Looking further on, byte17 (counting from byte0) is the year of manufacture minus 1990. In this instance the value is 0x0A, or 10 in decimal. This puts the manufacture year at 2000. I was expecting a much later year, but maybe the EEPROM image hasn’t been updated over all these years.
That’s enough for today, especially as I managed to break one of my BusPirate probe leads. Next time I will be looking through the main NVM chip to see if there is anything recognisable that I can modify. Wish me luck.