I2C

Parameters for the operation of the VFAT board are downloaded to the chip using the I2C protocol.
A good overview of I2C is available on wikipedia

Components

The gumstix motherboard used is the verdex XL6P. It has a 600MHz Marvell XScale processor, 128MB of RAM, and 32MB of flash.
The 120 pin expansion socket has a netCF-vx network interface card for TCP/IP networking. This card also has a Compact Flash slot should additional storage be required.
The 60 pin expansion socket has a breakout-vx card to allow access to the I2C signals, serial ports, and various other GPIO possibilities.

Electrical Characteristics

The I2C specification limits the I2C bus capacitance to 400pF. The cat5e wire specification limits wire capacitance to 50pF/m, so we should be able to push I2C at least 8m. Testing is needed, but by lowering the bus clock and the pull-up resistors a much greater length should be possible.

Let's go for 150 ft signal transmission lengths

VFAT I2C Addressing

With the test rig the addressing wires are as follows:

• Wh/Green - Least Significant Bit (1)
• Wh/Orange - Middle Bit (2)
• Wh/Blue - Most Significant Bit (4)

Unlike most I2C devices the VFAT responds to a range of 16 addresses instead of just one. These addresses are in the range _0 to _F where the _ value is determined by the sum of the addressing bits, ie green & orange high is an address range of 30-3F. For initial development we'll assume blue high, orange and green low for a range of 40-4F

Task List

1. Basic I2C working from Gumstix (completed)
2. VFAT powered on and responding to I2C (completed)
3. VFAT read/write sqecific registers (completed)
4. I2C controlled from a C program on Gumstix (completed)
5. I2C transmission line testing
6. Interface Gumstix to controller computer (Web interface tested)

Stuff to buy

• Power Supply
• Gumstix (x2)
  • Motherboard
  • Network card
  • Breakout Board
  • Power Plug
  • CF Card
• I2C expanders
• I2C eproms (for feedback testing)
• header pins
• header pin connectors
• LED's
• TTL to RS232 serial converters
• 1U box for gumstix
• DB9 headers

Go backVFAT_readout_electronics