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First:
Major Credit to FTEN for being first with his very original idea and micro Usb version of this hardware mod. If it weren't for him, I would not have dared to do this.
See his original thread:
Self Powered USB Host on the HP Touchpad! and at SDX

SDX Version of this thread
XDA Version of this thread

DISCLAIMER: I'm not a dev, nor do I play one on t.v. Use the information and links at your own risk. Read and understand what you're doing FIRST is the best bet. I take no credit for the original idea, just ran a bit farther out on the branch before it broke.

Afterwards be sure to check out:
[Tips and Tricks] OTG (USB Host Support) - Requires kernel support
and
My 2 Port 2 Amp OTG Charger & Mod post further down.

This is a difficult modification that requires you to completely disassemble Touchpad (correct tools a plus), cut and dremel out excess material from the back cover (risk of permanent damage) and display frame. You also need to have some soldering equipment plus skill with very small circuit boards/traces (if you want more than a full size power port) and loads of patience (this took several hours over 4-6 separate days).
Still interested? Have a look:

Parts:
- USB-A Female Connector from a USB Extension cable (and it's spare wire)
(or optional microUSB breakout as FTEN used in his original version)
- Small, thin, SPDT switch (single pole double throw) from a Dollar Store toy, ie "DollarTree"
- VPack PRT-08290 (5v Regulator) and it's PDF Reference file
- micro USB (M) connector from a cable or OTG Adapter (if you can't get the Enable switch landed you'll need this cut up into a plug with the ID pin shorted)
( or maybe AppleCable's Powered OTG Cable )

Tools:
- Spudger Tool set (or similar for cracking open the TP)
- Variable Speed Dremel with "Rotary File" bit
- Decent Soldering Iron (ie: Weller WES51) with small conical tip, and small electrical solder (LeadFree compatible recommended)
- Hobby knife Set (ie X-Acto with various blades and handles)
- Hot Glue Gun and sticks

Basic Disassembly/Reassembly: iFixit's HP Touchpad Battery Removal Guide
(You should be confident with the above. Be prepared to break a few of the 7 clips; superglue helps).

The Finished Product:
 

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Part 0 - Parts, Planning, & Schematics:

My Original sketch of the wiring schematic for the mod


The 3.7v to 5vdc Step-Up Regulator board


My wife's Simon Toy supplied the OTG Enable Switch (she wasn't thrilled)



Here's another option for the switch from my local "DollarTree" store



Here's the Full Size USB-A Extension Cable before and after modifiying


Lesson Learned: Go ahead and strip the Potting Material off the USB-A (F) connector to expose the 4 solder points and remove the wires (or at least the 2 DATA lines). I think you will have better success if you "land" the Data wires and OTG Enable Switch wire to the TP's USB Controller Board first and then connect the leads to the USB-A Connector and Enable Switch since those solder points are more durable.
** Make sure that the cable wiring is "Copper Colored" not silver, etc. The copper wire will solder more easily without damage to the USB Controller Board later on **
 

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Part 1 Pictures (Regulator/Connector Placement and Wiring)

Soldering the Battery leads to the Regulator input (take time to get the length & routing correct)


* See Previous note about soldering Data and OTG Switch wire to USB Controller Board First *


Here the 5vdc to the USB connector is soldered to the Regulator Output & a DC Rtn from the regulator is soldered to the switch's "common".




 

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Part 3 Pictures (USB Controller Board info)







Part 3 Pictures (USB Controller Board info)

Note: You can skip this step if you just want a full size USB-A power port and don't care about it having working "data" communication and OTG Enable Switch (I screwed up the enable switch solder pad).

Please read the notes in the pictures for this section, they will help avoid my mistakes.







 

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Additional Reference Info

Here'a an OTG Cable reference schematic that I made to help people make their own OTG cables and OTG shorting plugs. It works as a good reference to figure out how a purchased cable is wired up (some are different).


And here's a picture of the micro-USB breakout board (4pins only) from Sparkfun.com that Ften used in his original mod mentioned at the top of the OP. If you prefer, you can squeeze this into the sim slot without dremeling the back cover to a "paper thinness" and hook up your data and pwr to it, but you will have to make or purchase a male microUSB to USB-A adapter (an OTG one would work since there isn't an ID pin hooked up). I avoided this option because I don't want someone hooking it up to a USB wall charger by accident and possibly killing the regulator
 

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Here's an update...

My CrazyOnDigital 2 port charger doesn't have the data pins shorted (D+ and D-) like the stock HTC unit. This causes any "Android" phone to charge at the lower 500ma Max "Charging USB" rate vs "AC Charging" (explains my slow charging when travelling), but a bonus is that it allows me to pass data b/w the 2 ports like a simple hub.

- If the D+ and D- pins are shorted when an OTG USB cable is plugged in, the phone/device will go into "AC Charging" mode, but not allow OTG devices to work, but if the phone is still connected and the data pins opened back up, you retain "AC Charging" mode and have USB OTG functions.
(I'm thinking that a simple on/off switch installed across the data pins of my charger will give me a dual mode charger).

More info at: [Tips and Tricks] OTG (USB Host Support) - Requires kernel support

AC CHARGING / USB CHARGING (Data Pass Through) MOD 12-1-12
- I got tired of having a 2 port 2 Amp charger that only allowed 500ma "USB Charging" with Android & OTG. See pix:







 

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I will likely post this question separately to see if I can get more exposure, but for those looking through this thread:

Q: Does anyone know for certain if it's possible on the touchpad to echo the OTG "mode" command (ie host vs none) ?

On the evo3d there is a sysfs command that can be "echoed" to enable OTG with any standard cable and I was able to get the Kernel Tuner dev to add a toggle in his app for it.

If I can find someone who knows & can verify the sysfs command for our CM touchpad's I can have him add it into the app easily.
Note: I haven't found it yet by digging around with EsFileExplorer and activating OTG with a cable.
Mpgrimm2 said:
Didn't know OTG could be activated (on the Evo3D) using a standard cable with a sysfs command like:
Code:
echo host > /sys/kernel/debug/msm_otg/mode
Options are "host" or "none" (maybe this command can get added to "kernel Tuner" app! ). Wonder if it's the same for the Touchpad.

Fyi, don't know how different it would be on each device (ie GS3 vs. Evo3d & HP Touchpad), but it looks like for the Touchpad on CM9 it's
Code:
 /sys/kernel/debug/otg/mode
 

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Got a new toy for the TP... Apple Ipad case with softgel BT keyboard (it fits!). Who needs a netbook/Transformer tablet when you can tweak a TP. Yay!
IMAG0233.jpg

$23 at Amazon... http://amzn.com/B004YADU4A

12-3-2013 edit:

BT keyboard worked well with CM9 and Jc's 11-21-2013 CM10.1 Snapshot Build, however, the microUSB charge/power port connector had been getting loose and finally broke free this morning. May even consider getting another one. Shame it wasn't more durable.
 

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Just posting to try and get my thread to show up in my subscribed/participated list since it won't show as a started thread in my profile. Darn it Rootz!

Additional purchase links, info and pictures for off the shelf OTG items i use with my TP can be found in HTC Evo3D OTG Tips and Tricks (Same chipset as the TP)
Additional diagrams and details for making your own are also available in the rest of the thread. If there is enough interest, I can create/duplicate the thread for the TP.

Sent from my "Up All Night Sleep All Day" Nexus 5!
 
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