ATmega32 ADC for Light and Temperature Sensors

This tutorial shows how to implement the Analogue to Digital Converter (ADC) function on ATMega32 using C code. It consists of code examples, and the meaning of some nomenclature such as sampling rate, and resolution. However before we get to the code, let us start from the beginning. The ATMega32 has a built-in 10-bit ADC. The input to this ADC has a multiplexer to provide eight single-ended channels, where each channel can have a dedicated sensor such as an LDR or a Thermistor to provide an input voltage. These channels share the GPIO pins 33 to 40 on Port A. The output from the multiplexer feeds the non-inverting input of an operational amplifier with programmable gain. The gain is available in the following steps, 0 dB (1×), 20 dB (10×), and 46 dB (200×).

There is also an option to have three differential inputs by way of ADC5, ADC6, and ADC7, which feeds a second multiplexer connecting to the inverting input side of the op-amp.

From a hardware consideration, the AVCC pin provides voltage to the ADC circuitry and Port A also. This pin requires connection to the Vcc voltage supply, even when the ADC is not used. However when using the ADC, you should also include a filter capacitor as the ADC requires an extremely clean power supply.

The ADC also has a dedicated clock circuit, which allows the programmer to shut-off all the other clocks to reduce noise and therefore increase the precision of the ADC.

ADMUX Register

7	6	5	4	3	2	1
REFS1	REFS0	ADLAR	MUX4	MUX3	MUX2	MUX1	MUX0

REFS1	REFS0	Methods for selecting Voltage Reference
		AREF, Internal Vref turned OFF
	1	AVCC with external capacitor at AREF pin
1		Reserved
1	1	Internal 2.56 V Voltage Reference with external capacitor at AREF pin

The ADC requires a reference voltage to determine the conversion range. The analogue signal for sampling must be between ground and Vref for capture.

For most applications, Vcc is sufficient by setting bit REFS0 to binary 1. Vcc is prone to noise so as a result an external decoupling capacitor helps filter this noise.

ADMUX |= _BV (REFS0);

Analog Channel and Gain Selection Bits

In the ADMUX register, the first four “MUX bits” organise as follows. The first seven numbers select single-ended input configuration. From eight onwards you can select a combination of differential inputs with different gain factors. The gain factor is available only for differential inputs.