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.
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.
When sending data while not on USB, I observe the following transmission:
Code:
pressed: g(1100111)
received: G(1000111)/g(1100111)
pressed: o(1101111)
received: o(1101111)/O(1001111)
pressed: s(1110011)
received: o(1110011)/c(1100011)
pressed: t(1110100)
received: t(1110100)/p(1110000)
pressed: p(1110000)
received: p(1110000)/`(1100000)
A couple of other characters degrade to `, while a newline seems to degrade to a tab or carriage return and backspace has no visible effect.
Again, when the Kindle is connected to USB it works fine.
Blogpost: http://pepijndevos.nl/2012/10/26/spa...to-serial.html |
Sounds like a floating ground.
Check (again) where you have connected the adapter board ground to the Kindle ground plane (the black lead connection point to your voltage divider board).
Also, check that connection point where you are picking up the 3.3v at, be sure that is the output pin of the FR232's 3.3 volt regulator (pin 17).
I don't see (at only a quick glance) the current requirement of VCCIO, but it should be low. Low enough so that the 20K divider should provide enough current.
BUT the data sheet says that 1.8v is the minimum input for that sense lead.
So, as qlob says, change your divider -
Both the FT232 and the Kindle's SoC are designed to work with 1.8v logic levels.