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Adafruit Ultimate GPS pulling enable pin high

Posted: Fri Jan 27, 2017 1:13 pm
by TheRayTracer
Hi,

I am using Adafruit's Ultimate GPS unit. All is going great, and with Vcc, Gnd, Tx, and Rx pins setup the Rasp Pi and GPS unit are talking to each other. Moving on from an early prototype stage, I would like to make use of other pins. These would include the EN and FIX pins. Focusing on the EN pin, I would like to enable and disable the GPS unit from a GPIO pin of the Rasp Pi. At the following page (https://learn.adafruit.com/adafruit-ult ... ps/pinouts) it states:
EN is the Enable pin, it is pulled high with a 10K resistor. When this pin is pulled to ground, it will turn off the GPS module.
This confuses me, and I don't want to leave the EN pin floating. Can I simply connect the EN pin to a GPIO pin to toggle the GPS unit on and off without adding my own resistor? If an external resistor is required, what would a circuit diagram look like between the GPS unit and the Rasp Pi?

Thanks in advance!!!

Re: Adafruit Ultimate GPS pulling enable pin high

Posted: Fri Jan 27, 2017 5:05 pm
by pcmanbob
Hi.

Having looked at the schematic and the layout diagrams in the pdf file, it would appear that R2 is in fact connected to the EN pin holding it high, so no you don't need an external resistor.

If you want to prove this just use a multi meter set to volts and connect it between EN and GND while the board is powered.

Re: Adafruit Ultimate GPS pulling enable pin high

Posted: Fri Jan 27, 2017 9:08 pm
by TheRayTracer
Thanks for looking up the schematic. I too now see the presence of the R2 resister.
pcmanbob wrote:If you want to prove this just use a multi meter set to volts and connect it between EN and GND while the board is powered.
Shouldn't I set the multi meter to measure Ohms and connect it between EN and GND to measure 10k (of the R2)?

Re: Adafruit Ultimate GPS pulling enable pin high

Posted: Sat Jan 28, 2017 10:22 am
by pcmanbob
TheRayTracer wrote:Thanks for looking up the schematic. I too now see the presence of the R2 resister.
pcmanbob wrote:If you want to prove this just use a multi meter set to volts and connect it between EN and GND while the board is powered.
Shouldn't I set the multi meter to measure Ohms and connect it between EN and GND to measure 10k (of the R2)?
No the resistor pulls the pin high so it must be connected to the 3.3v rail by putting the meter on volts and measuring as I suggested you should see a voltage of around 3.3v, as it says in the instructions pulling this pin low will turn the device off, if the resistor were connected to GND the device would never switch on.

Re: Adafruit Ultimate GPS pulling enable pin high

Posted: Sat Jan 28, 2017 10:29 am
by Burngate
TheRayTracer wrote:
pcmanbob wrote:If you want to prove this just use a multi meter set to volts and connect it between EN and GND while the board is powered.
Shouldn't I set the multi meter to measure Ohms and connect it between EN and GND to measure 10k (of the R2)?
No.

The pin is connected to the "works" of the device, and to one end of the 10k resistor, the other end of which is connected to the power rail.
If nothing else is connected to it, the 10k provides a path for current to flow into your meter (and back out of your meter to the ground)
If you connect a GPIO to it and drive it low, or just stick a screwdrider between it and the ground, the 10k isn't strong enough to hold it high.

That is the long version of Adafruit's statement "the Enable pin ... is pulled high with a 10K resistor."

If you turn your meter to Ohms, it will provide some voltage between its prods and measure the current that flows.
Having the device powered when you do that would probably confuse both the meter and the device.

Re: Adafruit Ultimate GPS pulling enable pin high

Posted: Sat Jan 28, 2017 10:08 pm
by TheRayTracer
Thanks guys. I think I've got it in combination with the following reference: https://learn.sparkfun.com/tutorials/pull-up-resistors