If the result is left adjusted and no more than 8-bit precision is required, it is sufficient to read
ADCH. Otherwise, ADCL must be read first, then ADCH, to ensure that the content of the Data
Registers belongs to the same conversion. Once ADCL is read, ADC access to Data Registers
is blocked. This means that if ADCL has been read, and a conversion completes before ADCH is
read, neither register is updated and the result from the conversion is lost. When ADCH is read,
ADC access to the ADCH and ADCL Registers is re-enabled.
The ADC has its own interrupt which can be triggered when a conversion completes. When ADC
access to the Data Registers is prohibited between reading of ADCH and ADCL, the interrupt
will trigger even if the result is lost.
Starting a
Conversion
A single conversion is started by writing a logical one to the ADC Start Conversion bit, ADSC.
This bit stays high as long as the conversion is in progress and will be cleared by hardware
when the conversion is completed. If a different data channel is selected while a conversion is in
progress, the ADC will finish the current conversion before performing the channel change.
In Free Running mode, the ADC is constantly sampling and updating the ADC Data Register.
Free Running mode is selected by writing the ADFR bit in ADCSRA to one. The first conversion
must be started by writing a logical one to the ADSC bit in ADCSRA. In this mode the ADC will
perform successive conversions independently of whether the ADC Interrupt Flag, ADIF is
cleared or not.
Prescaling and
Figure 91. ADC Prescaler
Conversion Timing
ADEN
START
Reset
7-BIT ADC PRESCALER
CK
ADPS0
ADPS1
ADPS2
ADC CLOCK SOURCE
By default, the successive approximation circuitry requires an input clock frequency between 50
kHz and 200 kHz to get maximum resolution. If a lower resolution than 10 bits is needed, the
input clock frequency to the ADC can be higher than 200 kHz to get a higher sample rate.
The ADC module contains a prescaler, which generates an acceptable ADC clock frequency
from any CPU frequency above 100 kHz. The prescaling is set by the ADPS bits in ADCSRA.
The prescaler starts counting from the moment the ADC is switched on by setting the ADEN bit
in ADCSRA. The prescaler keeps running for as long as the ADEN bit is set, and is continuously
reset when ADEN is low.
When initiating a single ended conversion by setting the ADSC bit in ADCSRA, the conversion
starts at the following rising edge of the ADC clock cycle. A normal conversion takes 13 ADC
clock cycles. The first conversion after the ADC is switched on (ADEN in ADCSRA is set) takes
25 ADC clock cycles in order to initialize the analog circuitry.
198
ATmega8(L)
2486T–AVR–05/08
ATMEL [ ATMEL ]
