AVR Freaks

STOP

Don't use your buffer circuit, or you could damage the uC.

You have powered the op-amp with +12/-5 V. Pots are noisy devices, the wipers bounce a bit. If there is a bad spot on the pot or much noise there is transiently no wiper connection, and your input is floating. It can then swing the output from rail to rail, i.e. +12 V out to -5 V out.

If one powers the op-amp from +5/Gnd then a full swing is within spec for the ADC input. If you use a bi-polar power supply for the op-amp, and/or have V+ > 5V, then you need to be sure that the output can not exceed 5V. You haven't done this.

The ADC inputs on AVRs don't accept a negative input voltage at all, and the positive input should not be greater than the micro's V+, (+5 V usually).

Although it is common to use an op-amp to pre-process an analog signal for an ADC this isn't necessary for a simple POT input.

So: Remove your op-amp from the circuit. Reconnect just the wiper to the ADC input pin.

Add a small cap, perhaps 0.1 uF, from the ADC input to ground. This filters out noise on the POT, makes the input a form of LPF, and lowers in impedance of your source providing the signal to the ADC.

Pots come in linear, log, and other configurations. Once you can correctly read the ADC input voltage you can correct for any non-linearity in software, if needed, but that discussion can be held later.

JC

indianajones11 wrote:

Your adc_read() returns a ZERO, EVERY TIME !

YOu don't select a new channel correctly . Example, previous channel = 7 and now you want channel = 1...1 | with 7 is still 7 . See the adc tutorials about that .

You shouldn't mess with ISR version until regular code's working .

I'm not sure what you mean by this. Connect AVcc to +5V and you may add an extra 100nF to the ground.

To make small steps, you could set a program in the ATmega16 that doesn't initialize anything (just a program loop). All I/O-pins are inputs and you could measure the voltage of the potmeter.
If that is already wrong, your hardware is wrong. If it's okay, make the next small step. Initialize the ADC.
And so on.

If you show us the schematics and initialization code, we could be more specific.

void ini(void){

	//ports
	DDRA = 0x00;
	DDRC = 0x1F;
	DDRB |= 0xF0;
	DDRD |= 0x0E;
	
	//debug 
	i2c_init();
	
	//UART:
	unsigned char ubrr_val;
	ubrr_val = ((F_CPU/(MIDI_BAUD*16.0)) - 1);
	UBRRH = (unsigned char) (ubrr_val>>8);
	UBRRL = (unsigned char)  ubrr_val;
	UCSRB = (1<<RXEN) | (1<<RXCIE) | (1<<TXCIE);
    UCSRC = (1<<URSEL) | (1<<UCSZ1) | (1<<UCSZ0); 	// 8 bit data, asynchonous, 1 stop bit, no parity
	
	//timer
	TCCR0 = (1<<CS02) | (1<<CS00); //prescaler 1024 = 15625cps
	TIFR = (1<<TOV0); //clear pending interrupts
	TIMSK  = (1<<TOIE0); //Enable timer 0 overflow
	//ADC
	ADC_init();
	CLEARBIT(PINA,PA0);

	//Machine ready
	CLEARBIT(DAC_PORT, DAC_DA); //low
	CLEARBIT(DAC_PORT, DAC_CLK); //low
	SETBIT(DAC_PORT, DAC_CS); //low
	CLEARBIT(LED_PORT, LED_SCK); //low
	CLEARBIT(LED_PORT, LED_SDA); //low
	decodeSwitch(OFF);
	ledWrite(0); 
	sei(); //global interrupt enable

void ADC_init(void)	
{
	ADMUX=(1<<REFS0);	// AVcc with external capacitor at AREF
	ADCSRA=(1<<ADEN)|(1<<ADPS2)|(1<<ADPS1)|(1<<ADPS0)|(1<<ADIE);	
}
	
}