Specific ADC input not working on Microchip PIC PIC24FJ128GA202, other inputs work fine

Question

I am trying to get an analog input working on my microcontroller. I am using the PIC 24FJ128GA204. The problem is that a specific input is not working, while other do work.

I want pin 26, AN9/C3INA/RP15/T3CK/T2CK/CTED6/CN11/RB15, to be the input. Other analog inputs seem to be working fine, but when I configure the input pin to be AN9 the results seem to be floating, even if I ground the input. The samples I get: 3987, 3987, 3988, 3989, 3989, 3987, etc. Below is the code I am using:

void initAD(){
//Pin used for ADC is pin 26 = RB15 = AN9

TRISBbits.TRISB15 = 1; //RB15 is input
ANSBbits.ANSB15 = 1; //analog mode

AD1CON1bits.MODE12 = 1; //set 12 bit mode operation
AD1CON1bits.SSRC = 0b0111; //auto-convert
AD1CON1bits.ASAM = 1; //SAMP bit is auto-set

AD1CON2 = 0; //AD1CON2 all 0
AD1CON2bits.PVCFG = 0b00; //00 = Vss, 01 = External VREF+
AD1CON2bits.NVCFG0 = 0; //AVSS

AD1CON3bits.SAMC = 0b11111; //Auto sample time = 31 TAD
AD1CON3bits.ADCS = 0b00000101; //AD conversion clock 5*Tcy = TAD

AD1CHSbits.CH0NA = 0; //negative is VREF-/AVSS
AD1CHSbits.CH0SA = 0b01001; //channel A AN9

AD1CHSbits.CH0NB = 0; //negative is VREF-/AVSS
AD1CHSbits.CH0SB = 0b01001; //channel B AN9

AD1CSSL = 0;            //No scanned inputs
AD1CON1bits.ADON = 1;   //enable AD module
}

and:

uint16_t ADread(){
while(!AD1CON1bits.DONE);

return(ADC1BUF0);
}

Thanks for reading. If you have any suggestions, that would be great.

Answer 1

I had the same problem..

PIC24FJ128GA204 Family Silicon Errata and Data Sheet Clarification

AN9 has been removed from all 28-Pin diagrams and tables. This analog channel is unimplemented in 28-pin devices.

But datasheet is not repaired..

Answer 2

The code looks correct to me. Check the rest of your code to see if you accidentally assigned some digital peripheral output to this channel. Better yet, instead of posting code fragments write a short sketch demonstrating this particular issue only, i.e., conf.bits plus main with AD init and poll and nothing else. It would also be helpful to know how you test.

Answer 3

Do you use DMA?

You are reading ADCBUF0 all the time.

I think some dsPICs have dedicated ADCBUFx for every analog pin, others just use ADCBUF0 for every conversion.

You also need to configure other stuff...

Tell adc to scan channels:

AD1CON2bits.CSCNA=1; //SCAN INPUTS FOR CH0+

Tell adc to convert for instance CH0:

AD1CON2bits.CHPS=0; //CONVERT CH0

Select which channels to scan:

• AD1CSSH
• AD1CSSL

The analog channels and if you have two ADCs on board both configuration must be identical for registers:

• AD1PCFGH
• AD1PCFGL
• AD2PCFGH
• AD2PCFGL

The dsPIC ADC can be a pain to configure.

Answer 4

The analog Input AN9 does not exist on this device