I2C Hardware

Here is the level shifter I’m using for the Router (3v) I2C interface (5v on Pic and other chips I have)

                 5v
                |
                |
                R
               4k7
                R
                |
SCL ---+--R220--+-----> 5v SCL
       |
     =====  Use the Same
      ZD\   For the Data
     /3.3\  Line
       |
       |
0v-----+---------------

The above has so far worked happily with an old philips 2048bit (256 bytes!) eeprom.

Pin Header (for breadboard)

1: 0v

2: 3v

3: 5v

4: KEY NC

5: TX (I/O Bus -> project)

6: RX (project <- I/O Bus)

7: LED3 GPIO8 I2C SDA

8: LED5 GPIO11 I2C SCL

I2C Software Settings

In the sunspot.co.uk Midge rom, the default I2C port uses different gpio lines.  To use gpio 8 and 11, the file “/etc/init.d/S99start_i2c” file needs changing:-

The line containing “insmod i2c-adm5128″ needs to read “insmod i2c-adm5120 scl=11 sda=8″

I have been using the 5 port ‘scart socket’ I/O Bus simply as a passive backplane.  All of the lines are bussed together and using it simply involves hooking up the correct pins of a scart type plug to the corresponding pins on a circuit.

The I/O bus started life as a cheap breakout box for the Sweex/Edimax Router so that I could hook up different circuits to it without risking physical damage to the router itself – at least I won’t be able to break off a component whilst connecting a new circuit.

The Bus simply carries the router led, power (3v and 5v) and serial lines. I have been using a transistor based level converter to connect the router to a PC for uploading new ROM images.

Now that I’m going to add a PIC Processor and try a bit of communication between the PIC and the Router, it occured to me that it would be useful if I cut the serial lines somewhere on the bus so that I could hook up more than one serial device & converter.

However, there is just room for my level converter or one based on a MAX232 IC (a better idea if you have one handy) where the original scart lead exited the case.

I added the circuit, connecting it to the nearest socket and cut the traces between the serial pins and the rest of the I/O Bus.

Now, if I need to boot the router and upload a new ROM Image or work with the serial port, I plug the router into the first socket.  If I move the router to another socket, I can plug a PIC or whatever into the first socket and upload software to that.