After acquiring a large ( 26cm x 13cm) 4 digit 7 segment display, from George (M1GEO) and not wanting to unpack I set about making it work under linux.

The board contains a SAA1064 - an I2c 4 digit 7 segement display driver chip. Ideal for connecting to my bus pirate.

After much stumbling around and trial and error I managed to get it to light each segment. The notable points(errors) were:


  1. I2C is an open collector bus, meaning each device only pulls the data lines low. This meant that I had to turn on the Bus pirates on board pull up resistors, as for some reason the ones I thought were doing it on the board weren’t.
  2. When using the bus pirates pullup resistors, the Vpu(V pull up) lead should be connected to +5V.
  3. The board requires a large voltage inorder to get the Led’s to light, my initial attempts used the bus pirate to power it, but this was both too low current 125mA and too low in voltage for anything to light. I then connected it to an external power supply and found it lights best at about 12V although the chip does get a little warm :S (The data sheet claims it’s good till 18V)
  4. The digits are counted from the right hand side(with the chip and sockets at the bottom).

The Digits are are as follows:

0 - 0xfd
1 - 0x0c
2 - 0xda
3 - 0x9e
4 - 0x2e
5 - 0xb6
6 - 0xf6
7 - 0x22
8 - 0xff
9 - 0xfd

Bus pirate:

I once it's connected the bus pirate need to be set to I2c mode i found the default 5khz speed worked fine for me. The pullups and on board power supplies are turned on by sending 'Pn' and 'Wn' respectively. Then to write to all the digits at once I used the following command:
[ 0x76 0x00 0xf2 d1 d2 d3 d4 ]
Where 0x76 is the device address (determined by a potential divider connected to pin 1 of the chip). 0x00 Tells it i want to start writing at the control register. 0xf2 Tells it to refresh all digits and I want them at maximum brightness. then d1,d2,d3,d4 represent the digits in hex.