Electrical – MCP3221 ADC always returns zero in Proteus

adci2cmplabpicproteus

I am interfacing PIC18F2320 with MCP3221 ADC over I2C. When I try to read the converted value from ADC, only zeros are returned. I'm using MPLAB libraries. ADC is in A5 address mode. The AIN voltage is 2.503 V, what should be around 2048 after conversion.

Schematic looks like this (sorry for the mess, there was other stuff around and I'm debugging it so it isn't clean, just ignore MAX232, as removing it doesn't affect the result):

Captured lines look like this (upper is clock; it's easy to observe the three groups of 9b each):

The code I'm using is following:

// Set the ports to input, as the manual instructs
TRISCbits.TRISC3 = 1;
TRISCbits.TRISC4 = 1;

// Open I2C in master mode, slew off, as I'm aiming for 100kHz
OpenI2C(MASTER, SLEW_OFF);
SSPADD = 10; // A bit under 100kHz, because the MCU is on 4.1925 MHz

StartI2C();
WriteI2C(0x9B);      // Address of the ADC is 0x9A, 0x01 is read mode
voltage = ReadI2C(); // voltage is a global int; read upper byte
AckI2C();
voltage <<= 8;       // Make room for the lower byte
voltage |= ReadI2C();// Read lower byte
NotAckI2C();
StopI2C();

I have tried adding a delay after the address byte, but the result stays the same.

I have tried addressing with 0x9A only, in which case the data line is always high during reading.

Any other ideas?

Best Answer

To answer my own question almost a year later:

The teaching assistant who gave me this task told me that this behavior is a known "bug" in Proteus and not my fault. Quite anticlimactic.

Related Solutions

PIC18f45k20 ADC – always returns zero

I don't like "fix my code" too :) I will not analyse your code, I'm not PIC expert, I just started with Microchip microcontrollers - they are awesome! :)

Edited:

So this is how I do it on ~~PIC18F4550~~ PIC18F25k50 and it works for sure. ~~It should work on PIC18F4520 too~~.

End of edited.

You have to change port and probably voltage reference configuration.

volatile unsigned int millivolts = 0; // variable for result

void config_adc(void)
{
    // voltage reference selection

    // VREFCONbits.FVRS
    // 0b00 off
    // 0b01 1024mV
    // 0b10 2048mV
    // 0b11 4096mV
    VREFCON0bits.FVRS = 0b01;
    VREFCON0bits.FVREN = 1; // internal fixed voltage reference turned on

    ANSELAbits.ANSA0 = 1; // analog input A0 on
    TRISAbits.TRISA0 = 1; // pin A0 as input

    ADCON0bits.CHS = 0;   // analog channel select

    ADCON1bits.PVCFG = 0b10; // 0b10 for positive voltage reference = FVR BUF2 ("FVRS" configured earlier)
    ADCON1bits.NVCFG = 0b00; // 0b00 for negative voltage reference = AVss

    ADCON2bits.ADCS = 0b110; // 0b011 use FRC clock (internal clock dedicated for ADC)
    ADCON2bits.ACQT = 0b000; // manual acquisition mode
    ADCON2bits.ADFM = 1; // result bits adjusted to right

    ADCON0bits.ADON = 1; // ADC converter on
}

void main()
{
    config_adc();
    while(1)
    {

        ADCON0bits.GODONE = 1; // start conversion

        while (ADCON0bits.GODONE) // wait until conversion is done
        {
            asm("nop"); // im not sure if this is necessary
        }

    millivolts = (ADRESH * 256 + ADRESL); // im writing result into volatile variable to use it in interrupt routine

    }
}

Im using XC8 too. Code is not complete, this is just ADC part.

Electronic – trouble getting two PICs to communicate via I2C

[SOLVED]

Well, after trying to track down the problem for some time, I have found my main mistakes:

Due to my lack of knowledge with I2C, I have used a master Read function to read the data; in PIC16F886 I2C is handled by hardware - just read from SSPBUF and you got your data. Plus, that Read function was actually trying to pull the clock down (as the master is supposed to do), from the slave side (thus voltage was pulled down to almost 0 => no clock).
Resistor values were apparently wrong - voltage on SDA was lower than specified on the I2C specs.

Will try to publish a tutorial on this issue with code for XC8 ASAP. Cheers

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