There is an UART on the Nexus 4

Posted on Feb 4, 2014

I chose the Nexus 4 as these devices are traditionally very open, as you can see in case of the Nexus 4 e. g. by the availability of Ubuntu Touch images. I didn`t expect however for it to be that hackable as I was about to find out on Saturday. I had just completed packaging the Mako kernel on Mer OBS in my home project and the package is now avail

There is an UART on the Nexus 4
Click here to download the full size of the above Circuit.

able (it however doesn`t boot properly yet). So the memories from me hacking on the Archos 5 IT with the openAOS project came back and I thought, wouldn`t it be nice to have an UART available for debugging A few queries to the Internet search engine of my least distrust later I had a first lead in the form of an XDA post. This however didn`t help me much yet, which logic levels which pins really the headphone socket I quickly checked and didn`t notice anything coming out of the port when piping out /dev/urandom to /dev/ttyHSL0. The circuit is basically a simple comparator with a reference Voltage of 2. 8V. If the input voltage on the other pin exceeds this, then it switches its output to VCC (logic 1/true). This apparently triggers a switch that turns off audio and puts a UART on R/RX, L/TX. I then proceeded to attach the bench power supply to the MIC and GND pins. (Set at 2. 9V and lowest possible current limit of about 10mA). Note that it`s connected so, that GND†’+ and MIC†’-, so -2, 9V. So, there you go, a nice and clean output. The RX port seems to suffer a bit from crosstalk though (but note the different V/div settings! for both channels!) Sadly I then connected some cables wrongly during my further experiments and it seems like I have now fried the switch. Audio out works fine, I just can`t seem to be able to enable the UART anymore. I`m a sad panda, but at least now others can benefit from this. Update: I...

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