This week I was looking around for possible solutions for a hardware problem. I have a Kindle and would like to run some custom software on it, the factory experience is nice but Koreader makes for a compelling alternative.
So now I am looking at the serial Kindle Jailbreak thread and one thing caught my attention.
In the thread there is warning that tingled my OHNO sense
You want to specifically order option #3. This is necessary because your kindle uses 1.8v signals and if you use 3.3v or 5.0v, you can blow up your chips.
As I do like my dear Kindle a little too much to blow it up I was very concerned. I don’t like to buy more stuff, especially if it is a one off and especially because I already have tools around to do the job.
I spent some time looking at the alternatives
- use my usb-serial adapter and fry the Kindle (not only the VCC pin is 5V, the signals from my PC will be 5V logical high!)
- buy another usb-serial adapter, specifically a 1.8V to 5V, looks expensive
- find a thing that makes the 5V a 1.8V signal
The third option is called a logic level translator (LLT) and is a very common component because there are many people that have the same problem.
So now I have to find a logic level translator capable of translating 5V to 1.8V, hook it up to my previous usb-serial adapter and I should be done.
Did I tell you I know nothing about electronics? I searched for a LLT board around and I settled on one that has a TXB0104. This board should help me but it as a caveat. I have to supply the voltage reference for both sides, from now on called VCCA and VCCB.
Providing 5V in VCCB is easily done as I can just plug in my usb serial adapter but VCCA needs a “donor” so now I have to find it on the kindle or provide it using some other source. I could provide it with a 5V to 1.8V converter or maybe just a resistor but I don’t know if this would be a problem as this IC needs some current to work. How much current can I expect to get from my usb-serial? How much can my laptop provide?
While scouring the internet for this LLT board I landed on the product page from sparkfun. I had already ordered the LLT as a breakout board without supply so I definitely felt a little silly but at a second glance this would not have been the solution I envisioned.
It’s true that their VCCB is 5V but VCCA is listed as 3.3V, I’m just guessing but it looks like the 1.8V operating voltage for the Kindle is not that common.
While clicking around I got to the document tab and found the schematics for the board, it confirmed what I already knew, the board would bridge from 5V to 3.3V, even if the IC would allow it to go to 1.8V.
What I didn’t expect was instead finding that there was some more information!
In the VCCA voltage regulation section of the schematics there where alternative values of one resistor to get different VCCA values!
This is super handy because if I ever get around to learning some more about electronics I can make my own board! Having a reference like this will be extremely interesting for speeding up my learning and not only my first attempt is more likely to succeed but I can actually shape the board to make it easier to hook it up to the Kindle pcb!
I am very grateful to sparkfun for providing me this material, if I ever get done with jailbreaking my kindle they have definitely played a major part in this.