Quote:
Originally Posted by pepijndevos
In order to get USB OTG support on my Kindle, I had the debug the kernel and attach a serial circuit.
I used this Sparkfun breakout based on the excellent FT232RL chip.
What is great about this chip is that it has a VCIO pin on which you can supply any voltage you need. I combined this with a simple voltage divider for 1.65v.
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This works great for reading kernel output, but sending data only works when the Kindle is plugged in to USB. 
I'm not sure if this is related to speed or voltage. I admit my voltage/current might be on the low side with two 10k resistors.
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Again, when the Kindle is connected to USB it works fine.
Blogpost: http://pepijndevos.nl/2012/10/26/spa...to-serial.html |
We have seen this behavior before. Yifanlu reported it, and I have seen it on my K4 and K5 with serial ports. It appears that the supposed GND pin on the serial port is not effective. We noticed that serial I/O works reliably when the USB cable is also attached, to provide an alternate ground path back to the host PC.
It also works if you get your ground from a known good internal ground, such as a metallic shield cover, instead of from the serial I/O pad or connector. This information has been posted to this forum multiple times in the past.
Of course, using a 1.8v reference voltage as
qlob suggested is also a good idea.
EDIT: I use part of the circuit posted here (the top part with R1 and D1) on my kindles:
https://www.mobileread.com/forums/sho....php?p=1903472
I also had to remove an LED on my 3.3v USB serial adapter that was supplying too much current for the kindle to sink to a logic zero. I replace the LED with the 22K resistor shown in the circuit at the post linked above. That allows me to use a 3.3v adapter just fine on my K4 and K5. I actually soldered the resistor and diode inside one of my kindles, so that kindle now supports 1.8v to 5v TTL serial.