How to: Decoding mini and micro-USB cables' identity resistor with a Garmin GPS

This method is valid for my Nuvi 1350. It should work with all Garmin models as long as internal R1 is the same.

It's no easy task sticking probes into mini-USB male cord end, so I hope you find this useful for reverse-engineering all kinds of USB-cables, both mini, and micro(with the use of mini-to-micro conversion dongle.

Garmin uses assortment of resistors to identify the cable type.

Places your Garmin GPS into diagnostic mode.

Use a web calculator like this for quickest result.

http://www.raltron.com/cust/tools/voltage_divider.asp

Plug in R1 = 10,000
R2 = (cable resistance you're solving for)
V in = 2.25
V out =(key in the "voltage ID" value form diag screen)

On a regulator calculator you would do:
(10,000 x displayed volts)/(2.25-displayed volts) to solve for cable.

I wasn't sure if Nuvi used constant current method like a DMM or voltage divider. Turns out its the latter.
Here is how it works.
+2.25v--(R)--(pin-x)(cable resistor)--Gnd

On my unit, I determined that open circuit voltage is 2.25v, and short circuit current from pin-x to ground is 225µA. I found that the current varies with resistance as well. So the resistance is 2.25/2.25E-06, therefore 10kohm.

---
Determining the internal divider value:

If not all Garmin models use R1=10kOhm... Do let me know if this is the case. You will need a precision resistor of a known value between 10 and 50k ohms. It doesn't really matter what value as long as its about between those values and you know the exact value.

Plug in the value shown for Voltage ID with nothing connected for Vin. Plug in the known resistor's value for R1. Attach the resistor between the frame ground and 2nd pin from left with the wide end facing down and looking into the connector. This is probably the hardest part. Press it up against a window and have someone read the number from the other side. Once you get this value, simply key this in for Vin.