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Installing a Serial Port on your Gdium

This is a quick and dirty guide to installing a debugging serial port on the Gdium's motherboard. This is a fairly advanced process, and if you've not soldered QFPs before, I suggest forgetting about this process entirely.

Note that how to solder is beyond the scope of this document – there are many good tutorials on the web on how to solder QFPs and such!

You will need to install a special version of PMON to enable the serial port! This is the revision that was shipped with Gdium development kits: sst39vf040-pmon-serial-tested.bin

The alternative PMON image can be flashed directly with PMON, or the SST39VF040 Flash chip can be extracted and programmed externally. Be sure to back up your original flash image if you chose to do it the latter way!

Prerequisites

  • Your Gdium, taken apart, with the motherboard free to be worked on
  • A good quality soldering iron - I use a Weller WESD51 for this purpose
  • Solder, desoldering braid and flux paste (really important – you can't get away without it)
  • A MAX3232CUE (RS232 voltage level shifter), a 7x5mm 3.6864MHz clock crystal (I used a CW512CT from Conner-Winfield), a TL16C550CPT (48 LQFP package) UART
  • a 3-pin, 0.100” header
  • Optionally: An ohmmeter, useful for checking for a short, if you suspect there is one, a lit magnifying glass to check the quality of a solder connection.
  • Patience, courage and steady hands. Practice is also good, since this is all being done in a tight area.

1. Installing the oscillator

The first part to install is the oscillator, on the unpopulated OSC1 footprint (this is named unusually from what oscillators are usually named; the rest of the oscillators on the board are prefixed by a Y). The oscillator is Cover the unpopulated pads with flux. Make sure that pin 1 (marked by a dot on the package) is lined up and solder it in place. The UART and Oscillator installed on the board

2. Installing the UART

The UART is next to be installed, in the unpopulated U5 footprint. Again, cover the unpopulated solder pads with flux. Make sure the dot on the LQFP package is lined up with pin one on the board, and solder it in place. I recommend soldering the two kiddy-corner pins (i.e. pin 1 and pin 24) then soldering the rest of the package in place. This is fairly common practice for soldering QFPs in place.

3. Installing the RS232 Level Shifter

The RS232 level shifter must then be installed, on the unpopulated U6 footprint. This footprint is slightly under the heatpipe, and is a bit difficult to get to – a long enough soldering iron tip helps with this process, however, and you shouldn't have a problem if this is the case. Again, cover the unpopulated solder pads with flux paste. Line up the dot on the TSSOP16 package with pin 1 (the top-left hand pin on the board, if the GKey USB port is facing you). Solder the chip in place. The RS232 Voltage Level shifter and the pin header, installed on the Gdium motherboard

4. Installing the pin header

Finally, in J1, install the pin header. This is a through-hole part, so you will need to flip the board over to solder it in place, after placing the header into the holes. This is easy enough to do, however.

Pinout

Once the header is in place, the pinout is as follows: 

Pin 1:Tx
Pin 2:GND
Pin 3:Rx

If you don't have a serial cable with flyoff connectors for each signal, you can easily make such a cable out of a Cisco Router cable (just cut off the RJ45 end and crimp on some female terminators).


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