Question

Write an ALP to get the data rom channel 0 of the ADC of Atmega 32 and display the result on port A and port B . This should continue for ever.

Answer 1

ADC (Analog to Digital converter) is the most widely used device in embedded systems which is designed especially for data acquisition. In AVR ATmega series normally 10-bit ADC is inbuilt in the controller.

Let us see how to use the ADC of AVR ATmega16 / ATmega32.

ATmega16/32 supports eight ADC channels, means we can connect eight analogue inputs at a time. ADC channel 0 to channel 7 are present on PORTA. i.e. Pin no.33 to 40.

The controller has 10 bit ADC, means we will get digital output 0 to 1023.

i.e. When the input is 0V, the digital output will be 0V & when input is 5V (and Vref=5V), we will get the highest digital output corresponding to 1023 steps, which is 5V.

So controller ADC has 1023 steps and

• Step size with Vref=5V : 5/1023 = 4.88 mV.
• Step size with Vref=2.56 : 2.56/1023 = 2.5 mV.

So Digital data output will be Dout = Vin / step size.

• It is 10-bit ADC
• Converted output binary data is held in two special function 8-bit register ADCL (result Low) and ADCH (result High).
• ADC gives 10-bit output, so (ADCH : ADCL) only 10-bits are useful out of 16-bits.
• We have options to use this 10-bits as upper bits or lower bits.
• We also have three options for Vref. 1. AVcc (analog Vcc), 2. Internal 2.56 v3. External Aref. Pin.
• Total conversion time depends on crystal frequency and ADPS0 : 2 (frequency devisor)
• If you decided to use AVcc or Vref pin as ADC voltage reference, you can make it more stable and increase the precision of ADC by connecting a capacitor between that pin and GND.