8-bit AVR Microcontrollers

ADC Control and Status Register A


Bit 7 – ADEN: ADC Enable

ADC Enable

Writing this bit to one enables the ADC. By writing it to zero, the ADC is turned off. Turning the ADC off while a conversion is in progress, will terminate this conversion.

Bit 6 – ADSC: ADC Start Conversion

ADC Start Conversion

In Single Conversion mode, write this bit to one to start each conversion. In Free Running mode, write this bit to one to start the first conversion. The first conversion after ADSC has been written after the ADC has been enabled, or if ADSC is written at the same time as the ADC is enabled, will take 25 ADC clock cycles instead of the normal 13. This first conversion performs initialization of the ADC.

ADSC will read as one as long as a conversion is in progress. When the conversion is complete, it returns to zero. Writing zero to this bit has no effect.

Bit 5 – ADATE: ADC Auto Trigger Enable

ADC Auto Trigger Enable

When this bit is written to one, auto triggering of the ADC is enabled. The ADC will start a conversion on a positive edge of the selected trigger signal. The trigger source is selected by setting the ADC trigger select bits, ADTS in ADCSRB.

Bit 4 – ADIF: ADC Interrupt Flag

ADC Interrupt Flag

This bit is set when an ADC conversion completes and the data registers are updated. The ADC conversion complete interrupt is executed if the ADIE bit and the I-bit in SREG are set. ADIF is cleared by hardware when executing the corresponding interrupt handling vector. Alternatively, ADIF is cleared by writing a logical one to the flag. Beware that if doing a Read-Modify-Write on ADCSRA, a pending interrupt can be disabled. This also applies if the SBI and CBI instructions are used.

Bit 3 – ADIE: ADC Interrupt Enable

ADC Interrupt Enable

When this bit is written to one and the I-bit in SREG is set, the ADC conversion complete interrupt is activated.

Bits 2:0 – ADPS [2:0]: ADC Prescaler Select

ADC Prescaler Select

These bits determine the division factor between the system clock frequency and the input clock to the ADC.

Table 1. Input Channel Selection
ADPS[2:0] Division Factor
000 2
001 2
010 4
011 8
100 16
101 32
110 64
111 128