KL27Z256VFT4 Using external reference on VREFH?

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KL27Z256VFT4 Using external reference on VREFH?

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codemonk
Contributor I

We are using the KL27Z256VFT4 in a project currently in development.  It seems that the VREFH cannot be used with an external reference.  When attempting to use a 2.5 volt reference connected to VREFH the adc conversion consistently reads 0xFF.  I have verified that the internal reference is off and that the source for the ADC reference is set to the VREFH/VREFL pair.  Is there something I am missing?

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Robin_Shen
NXP TechSupport
NXP TechSupport

Hi Seth,

This device contains dedicated VREFH and VREFL pins on 64-pin and 48-pin(KL27Z256VFT4) packages.

So I think it's ok to use external reference on VREFH.

23.1.4 ADC analog supply and reference connections.png

The output of On-chip 1.2V high precision voltage reference VREF_OUT shares with VREFH on 64-pin and 48-pin  packages. When VREF_OUT is enabled, this pin needs to connect a capacitor to ground.

So you need to check if it was disabled or not.(SIM_SCGC4[VREF]=0)

And set ADCx_SC2[REFSEL]=00 Default voltage reference pin pair, that is, external pins VREFH and VREFL

2.3.2 Analog reference options.png

If you are testing the adc example in SDK2.x, please notice the '#define BOARD_ADC_USE_ALT_VREF 1U' in board.h

BOARD_ADC_USE_ALT_VREF.png

Please connect 1v at ADC input pin, what value will you get?

Best Regards,

Robin

 

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