Finding the serial (UART) pinout on KF

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How to find the UART

Thanks to jamez70's, we now know where the serial pins are.

Here is where the UART pins sits on the back of the motherboard.

(Courtesy of ifixit photo and jamez70's illustration)

The pins from 1-4 are:
RX TX GND GND (where #4 is marked)

From probing, we can detect the 1V8 level signal and the baud rate of 115200.

(Courtesy of jamez70)

If you want to hook this up, I suggest you solder on some wires to the pads and run the wires to the front, making life easier to re-assemble the KF.

Details

I suggest you get something like the Breakout Board for FT232RL USB to Serial for hooking this up. Note that the KF is 1V8, so when you want to connect this unit (default 3V3), you may want to use a level shifter, or simply find the VCC on the KF and loop that back that through the VCCIO on the Sparkfun-converter. Sending 3V3 directly into the KF could possibly seriously hurt it!

If you want to construct a simple voltage divider, simply get two resistors and get your soldering gun out. According to Vout = Vin * Rb/(Ra+Rb)

Voltage divider circuit (courtesy of calculatoredge.com)

• Vin: 3V3
• Vout: 1V8
• Ra: 10 kOhm
• Rb: 12 kOhm

If you need more UART details, here are some tips:

• Baud: 115200
• Bits: 8
• Parity: None
• Stop Bits: 1

Background

In the work to get a booting 3.0 kernel on the KF we need some low level access to debug since the USB initializes way later in the kernel sequence. I did some research on the OMAP4430 (the CPU in our dear KF) to try and find out where the UART interface might be accessed.

Looking at the datasheet we get a feeling of all the ball points ("pins") on the CPU. (Observe that this is seen from under the CPU)


OMAP4430 ball bottom layout

By then looking at the UART table TX pin numbers I try to get a feel where they might be located on the chip.


UART 1, 2, 3, 4 table

Finally, I cross referenced the UART table with TX pin and physical position on the board. The colors represent UART 1 TX, UART 2 TX and UART 4 TX. UART 3 doesn't seem likely to be mapped.


UART 1, 2 and 4 TX pin guesses

Observe that these are all estimates, but should narrow the search down (unless they're mapped somewhere way outside of the CPU area that is). I haven't had a chance of testing any of my guesses yet, but I thought I'd at least post my thoughts to get the ball running.

I'd put money on the four solder pads near the edge on the back (lcd facing) side.

Originally Posted by pokey9000

I'd put money on the four solder pads near the edge on the back (lcd facing) side.

Where exactly? Could you describe more in detail? If you're referring to the ones on the right, I'm pretty sure they are for the two speakers.


Courtesy of iFixit.net

I opened my attempted to probe my KF one more time today. It was a close call where I tried finding the TX amongst the bigger solder pads and a spark went off. Luckily, an unplug/plug of the battery connector booted it right up again. My probes on my multimeter are waaay to big for all the small dots. Hopefully, somebody has some more fitting equipment for the task.

Originally Posted by csholmq

Where exactly? Could you describe more in detail.

I'd be willing to shave my pet poodle that he means the four solder pads in the middle on the right side of your image.

Originally Posted by Buffet_of_Lies

I'd be willing to shave my pet poodle that he means the four solder pads in the middle on the right side of your image.

Yeah, I just realised that I had been flipping it the wrong way in my head The pads on that side looks very promising! I'm not sure how I'm gonna test out the pads without disconnecting everything from the board. Our best bet is probably to solder some wires and test directly on them. Won't do that on my KF atm though. Too dependant on it for my lecture slides

Yeah. I don't blame you. I'm a big fan of circuit-bending but you usually use old junky electronics, not a brand new state-of-the-art device like this. Not really willing to go near this thing with my soldering iron at this point. But something else I noticed is that on a photo of the OTHER side of that board... there's a serialnumber/MAC address sticker right smack on top of where those four solder pads are. Totally screaming in an attempt to hide where the traces are going!

Something else of interest is the closeup photo of the wifi/bt chip:


What I've long suspected is that the bluetooth isn't wired onto the motherboard. Looking at the image you posted above it's on the upper left side and darned if I don't see some empty component spaces!

Originally Posted by Buffet_of_Lies

Yeah. I don't blame you. I'm a big fan of circuit-bending but you usually use old junky electronics, not a brand new state-of-the-art device like this. Not really willing to go near this thing with my soldering iron at this point. But something else I noticed is that on a photo of the OTHER side of that board... there's a serialnumber/MAC address sticker right smack on top of where those four solder pads are. Totally screaming in an attempt to hide where the traces are going!

Something else of interest is the closeup photo of the wifi/bt chip:


What I've long suspected is that the bluetooth isn't wired onto the motherboard. Looking at the image you posted above it's on the upper left side and darned if I don't see some empty component spaces!

I'm no kernel dev. But for those who are, it would certainly be worth looking under the sticker! Assuming the UART is TTL, I recommend getting Teensy USB dev board or something similar. Certainly helps with the debugging. I have one and it works great (it's compatible with the Arduino IDE).

Originally Posted by csholmq

I'm no kernel dev. But for those who are, it would certainly be worth looking under the sticker! Assuming the UART is TTL, I recommend getting Teensy USB dev board or something similar. Certainly helps with the debugging. I have one and it works great (it's compatible with the Arduino IDE).

The Teensy's great, and would probably work if you're running it at 3.3V. The serial port on the Fire should be 1.8V, and the ATMEGA32U4 at 3.3V should have a suitable input threshold. I recommend though getting an FT232R breakout board from Sparkfun if you're going to be hacking serial ports onto modern embedded gadgets. I've heard that the Bus Pirate works too, but I thought the buffer doesn't go down that low.

As for missing components and bluetooth, there are only a couple of no-pop discretes, and some that look like a pull-up/pull-down select. I wouldn't count BT out just yet.

There's a lot of busted lcd kindles on ebay for cheap. Probably a good place to start.

Originally Posted by pokey9000

The Teensy's great, and would probably work if you're running it at 3.3V. The serial port on the Fire should be 1.8V, and the ATMEGA32U4 at 3.3V should have a suitable input threshold. I recommend though getting an FT232R breakout board from Sparkfun if you're going to be hacking serial ports onto modern embedded gadgets. I've heard that the Bus Pirate works too, but I thought the buffer doesn't go down that low.

As for missing components and bluetooth, there are only a couple of no-pop discretes, and some that look like a pull-up/pull-down select. I wouldn't count BT out just yet.

Sure, but the Teensy has analog inputs aswell. So as long as it's below 5 V you should be able to use it to read serial. Even though the KF is just at 1.8 V.

Edit: Actually, it looks like it reads high signal in above 0.7*Vcc, which makes 3.3V a no go - as with the Teensy :/ At least with proper serial communication.