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Old 12-27-2011, 03:40 PM   #1
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Post Kindle serial adapter level shifter circuit

UPDATE: A Kindle Touch can now be debricked in cases like mine by using only the USB Drive, even if it is only visible to the host PC for a few seconds during the reboot cycle like mine was. It has been reported to work if your Kindle is stuck at the startup screen with no progress bar, and it also works if you kindle is stuck at the dreaded "Repair Needed" screen (like mine was). No serial port connection is required, so no need to open up the Kindle back cover.

EDIT: This schematic is used for Kindle Touch serial recovery (debricking) as described in this thread:
http://www.mobileread.com/forums/sho...d.php?t=162292


I would recommend just getting a USB to 1.8v TTL serial adapter, but for those who requested a schematic when I described this circuit in various posts and multiple times on the IRC channel, here is the level shifter circuit that I used with my 3.3v USB serial adapter to recover my bricked Kindle Touch:


You can harvest resistors and diodes from discarded circuit boards. To use salvaged surface mount components, I recommend soldering small wires to them first. You can harvest small wire from motors or speakers. You can also peel small wires from old computer ribbon cables. I peeled a strip of 3-wires from an old IDE cable, which soldered nicely to my kindle after stripping and solder-tinning both ends of the wires. EDIT: You can substitute a common red LED in place of the two diodes in the kindle RxD level shifter circuit. A red LED has 2.2v across it while conducting current, which although a bit higher than specified input voltage, should work fine with the kindle RxD. Do not use other colors or a super bright red LED, because they have different operating voltage drops. Although I did use a red LED in a level shifter circuit for a cellphone USB adapter cable when I "debricked" my OLPC (One Laptop Per Child a/k/a XO computer), I did not test an LED in this circuit on my kindle touch.

EDIT: You can also harvest silicon diodes from the electronic ballast circuit in a Compact Flourescent Lamp (CFL, the "twisty" kind). Even discarded CFLs may have useful parts inside.

Enjoy, but please be careful to not damage anything with it. If you use this, you agree that you will use it at your own risk. And please be careful to not poke your eye out with that hot soldering iron!

Last edited by geekmaster; 11-11-2012 at 12:43 PM.
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Old 12-27-2011, 08:14 PM   #2
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TTL Level Shifter "Theory of Operation"

TTL voltage level definitons:
For both 5v and 3.3v TTL devices, a logic ONE is defined as a voltage level between 2v and 5v. All 3.3v devices must tolerate up to 5v on their inputs. All 5v and 3.3v TTL devices must output at least 2v for a logic ONE under normal operating loads, and must accept any voltage from 2v to 5v as a logic ONE. All 5v and 3.3v TTL must accept any input voltage below 0.8v as a logic zero, but *should* output 0.5v for logic ZERO (to allow for some voltage drop due to long narrow conductive traces on a circuit board). 1.8v TTL must accept any input voltage above 0.65v as a logic ONE, and any input voltage below 0.35v as a logic ZERO. Voltage levels between the logic ONE low voltage and logic ZERO high voltage specifications are undefined and cause TTL circuits to operate in the linear range where they may draw excessive current, and must be avoided except in special (hack) circuits specifically designed to use their analog properties. Read more about TTL voltage levels and level shifting here:
http://www.analog.com/static/importe...als/MT-098.pdf
and here:
http://www.pericom.com/pdf/applications/AN066.pdf


Kindle TxD level shifter circuit description:
When the kindle outputs a TxD logic ZERO, the level shifter uses diode D1 to let the kindle pull the adapter RxD pin down to 0.7v (the voltage drop across one silicon diode while conducting current through it). When the kindle outputs a TxD logic ONE, diode D1 blocks current flow, and resistor R1 pulls the adapter RxD pin up to its logic ONE level. Anything from 2v to 5v should work fine for the adapter RxD circuit supply voltage. For some USB serial adapters, you may not need resistor R1 or the supply voltage (do to an internal pull-up resistor), but it will not hurt to leave them in the circuit.

USB Serial adapter modifications (if needed):
If the serial adapter contains an LED that lights to show data on its RxD line, that diode might supply too much current for the kindle to pull it down to a logic one. You can measure this with an oscilloscope as the lowest voltage level on the pulsing data line while serial traffic is flowing across it. If the PC connected to the adapter USB port does not see data, you might have to remove that RxD LED (like I did on mine) to get it to work.

Kindle RxD level shifter circuit description:
The kindle RxD level shifter uses diodes D2 and D3 to limit the adapter TxD output to 1.4v (the voltage drop across two series-connected silicon diodes while conducting current through them). Resistor R2 limits the current flowing though the diodes so to a safe value that will not overload the adapter TxD output. When the adapter TxD is at a logic ZERO level, there will be no current flowing through diodes D2 and D3, and the kindle will see a logic ZERO.

Possible improvements:
You can improve (and complicate) this circuit design by adding transistors so the kindle TxD can sink more current (no need to remove an adapter RxD LED). You can use a level shifter IC designed for this purpose. EDIT: Although in theory, 1.8v TTL used in the kindle is not guaranteed to treat voltage levels between 0.35v and 0.8v as a logic ZERO, in practice, real devices such as the kindle are fairly tolerant of logic voltages a little ways into the "no man's land" between logic ZERO and logic ONE, and the kindle does just fine seeing the logic ZERO output from the higher-voltage USB TTL serial adapter. A proper TTL level shifter IC will also shift the logic ZERO voltage levels, unlike this "Super Simple" version. Or, you can instead buy a USB 1.8v TTL serial adapter that needs no extra circuitry such as is provided in this thread (recommended).

Any questions?

Last edited by geekmaster; 12-30-2011 at 06:22 AM.
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Old 12-31-2011, 04:19 AM   #3
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Kindle Touch serial connector pinout, as reported by hondamarlboro here:with photos here:are "GND, RxD, TxD in order from the left to the right".

When I connected to mine, I discovered that "left" is when the kindle is rotated so the connector is at the top.

Last edited by geekmaster; 12-31-2011 at 04:34 AM.
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Old 12-31-2011, 04:23 AM   #4
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(Bump)
News Flash: please read update at the top of the first post to this thread. Kindle touch may now be debricked in many cases with no serial port connection and no usbNetwork needed. Only USB Drive mode is required, even if it is only visible for a few seconds during reboot (like mine was).

Last edited by geekmaster; 12-31-2011 at 04:30 AM.
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Old 11-11-2012, 11:20 AM   #5
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I have the circuit equipped as the following: Is it right equipped ?
Diode
1N4148 - 200mA do-35 Switching Diode Semtech
VRM = 100V, VR = 70V, IF(AV) = 200mA, Ptot=500mW

Resistor R1/4W-J22K 22K±5% 1/4W Though Hole Resistor

And USB TO COM PL2303 V1

Last edited by ppoo; 11-11-2012 at 11:33 AM.
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Old 11-11-2012, 12:46 PM   #6
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Quote:
Originally Posted by ppoo View Post
I have the circuit equipped as the following: Is it right equipped ?
Diode
1N4148 - 200mA do-35 Switching Diode Semtech
VRM = 100V, VR = 70V, IF(AV) = 200mA, Ptot=500mW

Resistor R1/4W-J22K 22K±5% 1/4W Though Hole Resistor

And USB TO COM PL2303 V1
Those parts should work. If not, you may have an LED that you need to remove, as mentioned above.

One of my USB serial adapters has the resistor and diode soldered onto it. One of my K4s has them soldered inside it. The "bottom half" of the circuit was not needed for any of my kindles after using a better ground point (not the GND pin at the serial connector.

Last edited by geekmaster; 11-11-2012 at 12:51 PM.
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