9 years, 5 months ago.

Can AnalogIn configure _any_ ADC pin on the KL25Z?

I have a FRDM-KL25Z on which I have wired the coin cell voltage (TP8) over to J1-3 (PTC0/ADC0_SE14) hoping to monitor the coin cell voltage with the ADC using AnalogIn. When I configure an AnalogIn object for PTC0, it compiles and seems to run just fine (i.e. no error messages to the USB serial port or blinking lights of death), but the output is always 1.0, no matter if the coin cell is present or not. I have verified the voltage the header pin comes and goes as expected when the battery is installed and removed, and I assume it's present at the KL25Z pin 55 also (small pins not easy to probe).

Are there some ADC input pins that AnalogIn will not configure, or am I doing something else wrong? Here is my code:

#include  "mbed.h"

Serial pc(USBTX, USBRX);  // Declare the serial port 

int main( void )
{
    AnalogIn Vbatt( PTC0 );
    
    while ( 1 )
    {
        pc.printf( "Vbatt = %f\n", 3.3 * Vbatt.read() );
        wait( 1 );
    }
}

It runs and behaves as expected, except Vbatt is always 3.3, even when I remove the battery. Perhaps there is a pullup on that input that needs to be turned off?

Thanks for your help.