From Gumstix User Wiki
I2C on Verdex Pro and earlier COMs
|I2C_SDA||I²C Data (Serial data/address bus)|
|I2C_SCL||I²C Clock (Serial clock)|
- Logic level (3.3V) signals
- 400 kbps Fast mode
- 100 kbps Standard mode
The I²C bus uses 3 signals, I2C_SDA, I2C_SCL, and GND (ground), all of which are present on the 60-pin Hirose, 120-pin MOLEX (Verdex COMs only) and 24-pin flex ribbon (Verdex Pro only) connectors. Some daughter cards allow the I²C signals to be accessed directly.
Note that some older versions of the Breakout board have pins labeled incorrectly. See the breakout page for more details.
On the robostix board, pin 12 on the UARTS connector is SDA, and pin 16 is SCL. These are both 5v signals and have pullup resistors.
On the Tweener, pin 19 on the hirose connector is SDA, and pin 20 is SCL. The JTAG connector page has a photo of the hirose connector. The TDO pin shown in the photo is pin 22. The via to the left under the T is pin 21, the one under the O is pin 20, and the one under the D is 19. These are all 3.3v signals and don't have any pullup resistors.
On the back of the waysmall board, amongst the set of 20 pads, the pad closest to the edge of the board and the power connector is the SDA signal, and the one next to it (also by the edge of the board) is the SCL signal. These are both 3.3v signals and don't have any pullup resistors. The waysmall board has a 3.3v regulator on it, just to the left of the power connector. It has LW33 stamped on it. The pin below the 3 (in LW33) is the 3.3v output (see the JTAG connector page which identifies the Vcc pin on the same type of regulator used on the Tweener).
The I2C bus requires pullup resistors on the I2C_SDA and I2C_SCL lines. The I2C specification gives some charts for min and max values (see page 39 and 40). For a 5v or 3.3v bus, the minimum pullup resistance should be around 2k ohms. A typical value to use would be 4.7k ohms.
All of the current daughter cards (except the robostix), need to have pullup resistors added, which pullup the SDA and SCL lines to 3.3v levels.
If you use a robostix, the 3.3v pullup resistors are already included. The SDA and SCL lines on the UARTS connector are at 5v levels, and also have pullup resistors on the robostix board.
Related pages on the old wiki: