192
ATmega8A [DATASHEET]
8159E–AVR–02/2013
Bit 7 – ADEN: 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 con-
version is in progress, will terminate this conversion.
Bit 6 – ADSC: 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 – ADFR: ADC Free Running Select
When this bit is set (one) the ADC operates in Free Running mode. In this mode, the ADC samples and updates
the Data Registers continuously. Clearing this bit (zero) will terminate Free Running mode.
Bit 4 – ADIF: 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
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 – ADPS2:0: ADC Prescaler Select Bits
These bits determine the division factor between the XTAL frequency and the input clock to the ADC.
Table 23-4.
ADC Prescaler Selections
ADPS2
ADPS1
ADPS0
Division Factor
000
2
001
2
010
4
011
8
100
16
101
32
110
64
111
128