The I2C Remote Control refused to work.  Reading the analog pin on the PIC 16F876a seemed to work a couple of times after startup but failed after that.  It looked like the power to the 5v rail was still too low.

As I had allowed myself a slightly longer cable, it seemed reasonable that a combination of this and 78L05 were causing the problems.

I ditched both the 78L05s and wired in a 5v 2A PlugPack adaptor that used to run a Wifi Bridge – plenty of power!

Still the 16F876a processor refused to talk consistently to the I2C.  I checked that the bus was indeed working by plugging in the little i2c eeprom module I tested the original I/O bus with.  That worked no problems.

Looking at the I2C details on the www.sunspot.co.uk site and others, I wondered if the I2C pull ups were anything to do with the problem.  I was using 4k7 and I saw others using values as low as 2k2.

Substituting a pair of 2k2 resistors solved the problem, however, the 5v line dropped below the 5v mark.  Increasing them to 3k2 brought the 5v rail back to just under 5v and the PIC continued to talk to the I2C bus.

Lesson:  be prepared to tinker with the I2C pullups etc. when using different line lengths and slave devices.

When I tried to open the case, I found that the wires to the Scart Plug had melted themselves into the plastic base of the router!

Router Case After the Wires 'Moulded' Themselves to it.

I thought ‘perhaps I had drawn too much power through the wires’, but the marks at the top of the picture are from the wires to the LEDs.

I guess the problem is a result of a chemical reaction between the soft plastic of the base and the rubbery style insulation used on the black wires from the ‘cheap’ scart splitter.

I notice that the scart to scart leads with coloured wires inside seem to be made with a different insulating material.  I’ll have to re-wire, (using one of these) as when I pulled the wires from the base, several of them left a good portion of insulation behind.

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″

The Scart Connector Pins

+==============================+
| 1  3  5  7  9  11 13 15 17 19|(21) - - shield
|                               \
|  2  4  6  8  10 12 14 16 18 20 |
+================================+

Bus Connections & LED I/O Line Use.

I added extra power lines as the wire used is quite fine. The extra power and 0v lines are connected at the router, they are not linked on the Bus I/O board to avoid any ground loop.

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.