Manual
70
AT90S/LS8535
1041H–11/01
higher sampling rate. See “ADC Characteristics” on page 75 for more details. The ADC
module contains a prescaler, which divides the system clock to an acceptable ADC
clock frequency.
The ADPS2..0 bits in ADCSR are used to generate a proper ADC clock input frequency
from any CPU frequency above 100 kHz. The prescaler starts counting from the
moment the ADC is switched on by setting the ADEN bit in ADCSR. The prescaler
keeps running for as long as the ADEN bit is set and is continuously reset when ADEN
is low.
When initiating a conversion by setting the ADSC bit in ADCSR, the conversion starts at
the following rising edge of the ADC clock cycle.
A normal conversion takes 13 ADC clock cycles. In certain situations, the ADC needs
more clock cycles for initialization and to minimize offset errors. Extended conversions
take 25 ADC clock cycles and occur as the first conversion after the ADC is switched on
(ADEN in ADCSR is set).
The actual sample-and-hold takes place 1.5 ADC clock cycles after the start of a normal
conversion and 13.5 ADC clock cycles after the start of an extended conversion. When
a conversion is complete, the result is written to the ADC data registers and ADIF is set.
In Single Conversion Mode, ADSC is cleared simultaneously. The software may then
set ADSC again and a new conversion will be initiated on the first rising ADC clock
edge. In Free Running Mode, a new conversion will be started immediately after the
conversion completes, while ADSC remains high. Using Free Running Mode and an
ADC clock frequency of 200 kHz gives the lowest conversion time with a maximum res-
olution, 65 µs, equivalent to 15 kSPS. For a summary of conversion times, see Table
26.
Figure 47. ADC Timing Diagram, Extended Conversion (Single Conversion Mode)
Sign and MSB of result
LSB of result
ADC clock
ADSC
Sample & hold
ADIF
ADCH
ADCL
Cycle number
ADEN
1 212
13
14 15
16 17
18
19 20 21 22 23
24 25
1 2
Extended Conversion
Next
Conversion
3
MUX and REFS
update
MUX and REFS
update
Conversion
complete