The "executive summary" pictures:
I want to decode the serial signal coming out of my tablet's headphone jack. This is a somewhat weird "hack" that exists in a few phones and tablets: basically, if you feed 3.3V in the microphone input of your TRRS plug, the left and right channels become serial TX/RX.
I used a Raspberry PI TRRS-to-TV cable (as you can see in the 2nd picture) to get access to the 4 places I needed: GND, MIC, L, R. The cable is not supposed to do anything other than expose the 3 signals (MIC,L,R - paired with GND) in the three corresponding cables (red, white, yellow).
I used my BitScope's probes to probe between the TX (tip of white cable in 2nd picture) and the common GND (brown probe at the bottom of the 2nd picture). I also used two probes (red and blue one) to "feed" 3.3V from my USB/TTL chip (a PL2303HX plugged in my laptop) to the MIC (red) tip.
Upon rebooting the tablet, I indeed saw what is unmistakably a serial signal at 115200 (peak-to-peak of 8 to 9us), but with lots of capacitance (video).
So, my question - before I go online and order a TRRS plug, cables and a soldering iron - is the capacitance I am seeing due to...
- the 1 meter long TRRS-to-TV cable, or the use of probes instead of soldered cables
- the probes and cable in fact cannot account for this much capacitance, and the reason I am seeing this is that the tablet's headphone jack simply wasn't designed to emit this signal (i.e. what I am seeing is indeed what comes out of the jack).
As you can probably guess, I am very new to this sort of thing; I am a software guy, bought my BitScope a week ago, and would love to access my tablet's serial for "fun and profit" (hacking bootloader stuff, compiling Cyanogenmod for it, etc).
I'd appreciate an estimated guess on whether this is a lost cause (i.e. the cables can't explain this much capacitance) or not.
Thanks in advance for any help/suggestions.