Can VDD be measured? (without routing it to an GPIO)

RE: Can VDD be measured? (without routing it to an GPIO)

muhkuhns — Tue, 21 Apr 2015 21:37:36 GMT

Yep, that's exactly what I was imagining it to look from Nikolaj's answer and my addition :) Thanks!

RE: Can VDD be measured? (without routing it to an GPIO)

Erik Sanchez — Tue, 21 Apr 2015 21:31:03 GMT

I use the following configuration to measure VDD and compare to internal reference. No external resistor network or reference necessary:

 NRF_ADC->CONFIG     = (ADC_CONFIG_RES_8bit                        << ADC_CONFIG_RES_Pos)     |
                          (ADC_CONFIG_INPSEL_SupplyOneThirdPrescaling << ADC_CONFIG_INPSEL_Pos)  |
                          (ADC_CONFIG_REFSEL_VBG                      << ADC_CONFIG_REFSEL_Pos)  |
                          (ADC_CONFIG_PSEL_Disabled                   << ADC_CONFIG_PSEL_Pos)    |
                          (ADC_CONFIG_EXTREFSEL_None                  << ADC_CONFIG_EXTREFSEL_Pos);

RE: Can VDD be measured? (without routing it to an GPIO)

Nikolaj — Tue, 21 Apr 2015 20:47:57 GMT

Sorry, my bad! Of course you can do that

RE: Can VDD be measured? (without routing it to an GPIO)

muhkuhns — Tue, 21 Apr 2015 20:46:00 GMT

Oops. Converted it to answer by missclicking.. I am confused by "(you can only choose 1/3 or 2/3 of the VDD however" So I thought I could measure 1/3 of my supply voltage to the internal reference?

RE: Can VDD be measured? (without routing it to an GPIO)

Nikolaj — Tue, 21 Apr 2015 20:43:29 GMT

Since Internal Band Gap ref is 1.2 V and minimum VDD for most chips are 1.8 V you'll always get a reading of 0xFF (if you use 8 bit). Remember when sampling, the reference is the maximum readable voltage level.

RE: Can VDD be measured? (without routing it to an GPIO)

muhkuhns — Tue, 21 Apr 2015 20:34:25 GMT

Thanks! But can't I use the Internal Band Gap reference? (As I wanted to use a 3rd Party Module which has no external reference)

RE: Can VDD be measured? (without routing it to an GPIO)

Nikolaj — Tue, 21 Apr 2015 20:19:49 GMT

Yes it is possible.

When you setup your ADC you have the option to choose VDD on the INPSEL (you can only choose 1/3 or 2/3 of the VDD however..

check out the nrf51_bitfields.h

I would config it something like:

	//Configure ADC
	NRF_ADC->CONFIG = (ADC_CONFIG_EXTREFSEL_AnalogReference0<< ADC_CONFIG_EXTREFSEL_Pos)	// AnalogRef 0 as external reference
													| (ADC_CONFIG_PSEL_Disabled<< ADC_CONFIG_PSEL_Pos)	// Disable input pin 
													| (ADC_CONFIG_REFSEL_SupplyOneThirdPrescaling << ADC_CONFIG_REFSEL_Pos)	// Use VDD/3 as reference
													| (ADC_CONFIG_INPSEL_SupplyOneThirdPrescaling<< ADC_CONFIG_INPSEL_Pos)	// Supply voltage with 1/3 prescaling used as input for the conversion
													| (ADC_CONFIG_RES_8bit << ADC_CONFIG_RES_Pos);	// 8 bit

Notice that I'm using an external reference to measure the VDD, because you have to compare it to something.