openAOS

The serial console is an extremely powerful tool while working on embedded systems on an low level. Rewriting the init process to enable the openAOS Boot-Menu would have been virtually impossible without this.
So, due to some requests for information on how to access the serial console on Archos generation 6 and 7 devices I put together this posting. Thanks also to those who contributed the Pictures to make this post possible!
What will you need?

• A soldering iron with a fine tip (capable of soldering SMD parts)
• Some thin, insulated wire
• A level converter 1,8V ⇄ RS232 (this is very important, more on that later)
• An Archos DVR station for gen6/7

Once you have everything prepared on a clean surface:

1. First open the DVR-station. The screws on the back are not important, the ones hidden below the pads on the bottom are.
2. You should now see something like this:
   

   Overview of the DVR-station PCB. Click image for larger version

   Notice how some parts are highlighted in red. Those are convenient spots to grab the serial TX, RX signals and an nearby ground pad.

3. To have an interference free serial interface you will have to sacrifice the IR receiver and transmitter. Or to fit a switch; though this goes beyond the scope of this guide so that design and implementation are left to the discretion of the interested reader.The following picture shows the two resistors that need to be removed while hilighting the appropriate solder pad to attach an wire. To the Right you also can see an easily accessible large ground pad which needs no further preparation.
   

   Closeup of the DVR-station PCB, with resistors still in place. Click image for larger version

4. After unsoldering the resistors you can proceed to soldering on the three wires.
   

   Closeup of the DVR-station PCB, with wires soldered on and resistors removed. Click image for larger version

   Notice how the TX and RX wire are soldered to the lower of the two pads where once the SMD resistor was located.

5. Apply some tape so there is no mechanical stress on the solder joints
6. Find a way to route the wires outside the case
7. Please notice that TX and RX is respective to each device, this means you need to wire: TX → RX and RX → TX!
8. Attach your 1,8V ⇄ RS232 converter to the three wires!I will emphasize this once again:
   This converter is a MUST! If you connect your Device directly to an RS-232 port you are going to cause irreversible damage to it!

   Alternatively you could use one of these USB converters. I heard that you can get away without modifying VCCIO for 1,8V and just leaving it at the 3,3V default.
   

9. Congratulations! You can now directly debug your Archos running an custom kernel!

Some further notes on this topic:

• This does not work with the original Archos firmware!
• Advanced users can connect an USB-to-serial converter chip that is capable of 1,8V directly to the port. This is not something you can buy off the shelf. Regular converters use RS232 levels → irreversible damage!
• If you are connecting an cheap USB-to-serial converter to the level converter use a “moderate” baud rate! Cheap converters usually contain an prolific-2303 chip. It works ok for 115k2 but won’t work reliable at 1M. I can recommend FTDI converters, they work just fine at 1M.
• A similar modification should be possible using the HDMI-Dock as I’ve been told it has two docking connectors (TX is on the left, RX on the right).

I will add some more notes here later on.

And once again. NEVER connect RS232 directly to the bare wires! You have been warned.

Tags: console, debug, gen6, gen7, hack, Linux kernel, RS-232, RS232, sde, serial, UART

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