Robostix modifications
From GumstixDocsWiki
This page collects together some of the hardware mods for the Robostix and Roboaudiostix. These mods aren't necessarily required, and each section describes when you may want to use it.
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Tweener Mod
If you plan on using a tweener to provide the console with a gumstix/robostix, then you should cut the trace indicated by the left red mark on the Tweener modifications page.
Ground Wire Fix (robostix revisions prior to 1131)
On the Robostix, pin 2 of the UARTs connector is supposed to connect to ground. This photo shows the problem:
The light green are traces on the board. The darker green is the ground plane. You can see the ground plane attached to the corner pin doesn't quite connect to the rest of the ground plane. Due to a slightly different layout, the Roboaudiostix doesn't suffer from this particular problem.
The blue wire in the Robostix R341 photo below connects the corner pin to a nearby ground pin.
This fix is only required if you want pin 2 on the UARTs connector to be connected to ground.
This particular problem has been addrressed in the 1131 revision of the robostix.
Pullup Resistor (applies to all revisions - upto and including 1131)
If there is nothing driving the RxD line (like a serial adapter connected to the GUMSTIX connector), then it will float, which can cause u-boot to falsely think that a key has been pressed (technially, u-boot is detecting the break condition - which is a low which is longer than a character time) during bootup when the pin is unconnected.
If you want to boot your gumstix with no console connected, you should add a 10k pullup resistor between The FFUART-RxD line (pin 4 on the GUMSTIX connector) and a +5. Pins 3, 6, 11, and 14 from the UARTs connector are an "always on" +5. Pin 3 on the GUMSTIX connector is a switched +5, and is typically switched off by u-boot. However, it seems to be on long enough, and has some leak through voltage present, that it make u-boot not false detect. I've always been able to use a resistor between pins 3 & 4 and it seems to work fine. If you're using a regular through hole resistor, then picking pin 11 from the UARTs port seems like a sensible choice instead.
If you perform the "getting the console to work" modification described below, then you will be connecting V_BATT to pin 3 of the GUMSTIX port (same +5 as what's present on the UARTs port).
Looking at the Robostix R341 photo below (bottom of board), the four pins at the bottom right of the photo are the gumstix pins, with pin 4 being in the bottom right corner of the photo. I used an 0603 surface mount resistor, which fits nicely between the pins. The 103 code means 10k.
This fix is only required if you plan on booting a gumstix/robostix with no console present. If you use a Tweener board, then this fix is also not required.
Getting the console to work during boot (General Description)
The console (FFUART) comes from the gumstix as 3.3v signals. It is then voltage converted to/from 5v through the two 74LCV4245 chips. These chips get their 3.3v from VCC, and get their 5v from V_BATT, which are both enabled all the time. There is also an enable line which is active low. Fortunately, the enable line is configured as an output and driven low at boot time by u-boot. After being voltage converted to 5v, the console signals are presented on the 4 pin GUMSTIX connector. Unfortunately, the GUMSTIX connector provides VCC5, which is output from a voltage regulator which is off by default at boot time. Since V_BATT is powered all the time, it woud be better to have V_BATT rather than VCC5 on the GUMSTIX header.
This fix is only required if you need to provide power to an RS-232 apdapter during booting (so you can break into u-boot). If you use a bus powered USB serial adapter then this fix may not be required and depends on what logic level high voltage is generated. The signals on the GUMSTIX header are 5v logic.
An alternative to this fix is to just connect the power wire for the serial adapter up to one of the V_BATT pins on the UARTS connector (pins 3, 6, 11, or 14), instead of using pin 3on the GUMSTIX connector.
IMPORTANT: If your particular revision of robostix or roboaudiostix board isn't mentioned then these modifications may be slightly different or may not be applicable.
Getting the console to work (Robostix R1131)
The GUMSTIX connector now has V_BATT, which comes from the "always on" regulator. The 10k pullup on the RxD line is still required if you want to boot the gumstix with no Tweener and no serial adapter.
Getting the console to work during boot (Robostix R341 & R790)
This particular modification only applies to R341 and R790.
This shows the mods on the top side:
There is a trace (VCC5) which runs between the ISP connector and the gumstix connector. I cut this trace (just below the G in GUMSTIX). This isolates the gumstix connector from VCC5. Unfortunately, it also isloates a bunch of other VCC5 circuitry (like the power pins from Port C). The red wire at the top of the photo (I used 26 AWG solid wire) connects from the right side of the capacitor between the ATMega128 and the pin headers, to the bottom side of the ferrite bead located near the gap in the headers. This red wire reconnects V_BATT to the circuitry that was cut off. For those not comfortable soldering to the surface mount components, you could also run a wire on the bottom of the board from pin 2 of the ISP connector over to one of the center pins on Port C (this also has the side effect of bypassing the ferrite bead). I used heavier wire to ensure that enough current could be delivered to Port C.
This photo shows the mods on the bottom side:
Cutting the trace on the bottom (shown above) completes the isolation of pin 3 from VCC5. The red wire shown on the bottom side is 30 gauge wire wrap wire. It connects pin 14 of the UARTs port to pin 3 of the gumstix port. This provides V_BATT to the now isolated pin 3 of the GUMSTIX port.
Getting the console to work (Robostix R294)
This version of robostix is shipped with a patch wire (mine was blue) running from UARTS pin 5 to pin 9 on one of the 245s. The patches required on the bottom of the board are the same as for the R341. On the top of the board, there are 3 traces running between the gumstix connector and the ISP connector. Make sure you only cut the big one, closest to the edge of the board. You'll also notice that FB2 isn't present near the gap bwteeen the headers. Instead, the patch wire needs to be run from the capacitor all the way over to pin 5 of the 9515 chip (which is the only circuitry cut off when the bottom trace was cut). A fine wire, like 30 gauge wire wrap wire is all that's required in this case.
Getting the console to work (Roboaudiostix R431)
This photo shows the equivalent mods for the roboaudiostix.
The cuts are harder to see, but the traces coming from pin 3 of the GUMSTIX connector is cut between the LED and pin 4, as well as being cut through the U in GUMSTIX. The veritcal red wire runs from the top side of one of the LEDs (all are VCC5) to the cap below the G in GUMSTIX. This fixes up the two cuts that were made. The horizontal red wire runs from pin 3 over to the output cap on the V_BATT regulator. You could also run a wire on the bottom of the board from GUMSTIX pin 3 over to one of the V pins on the UARTs port (pins 3, 6, 11, or 14).
I added the 10k pullup resistor as pictured above in the Robostix R341 photo.
