113
ATmega64A [DATASHEET]
8160D–AVR–02/2013
The Input Capture unit is illustrated by the block diagram shown in
Figure 16-3. The elements of the block diagram
that are not directly a part of the Input Capture unit are gray shaded. The small “n” in register and bit names indi-
cates the Timer/Counter number.
Figure 16-3. Input Capture Unit Block Diagr
am Note:
1. The Analog Comparator Output (ACO) can only trigger the Timer/Counter1 ICP – not Timer/Counter3.
When a change of the logic level (an event) occurs on the Input Capture pin (ICPn), alternatively on the Analog
Comparator output (ACO), and this change confirms to the setting of the edge detector, a capture will be triggered.
When a capture is triggered, the 16-bit value of the counter (TCNTn) is written to the Input Capture Register
(ICRn). The Input Capture Flag (ICFn) is set at the same system clock as the TCNTn value is copied into ICRn
Register. If enabled (TICIEn = 1), the Input Capture Flag generates an Input Capture interrupt. The ICFn flag is
automatically cleared when the interrupt is executed. Alternatively the ICFn flag can be cleared by software by writ-
ing a logical one to its I/O bit location.
Reading the 16-bit value in the Input Capture Register (ICRn) is done by first reading the low byte (ICRnL) and
then the high byte (ICRnH). When the low byte is read the high byte is copied into the high byte temporary register
(TEMP). When the CPU reads the ICRnH I/O location it will access the TEMP Register.
The ICRn Register can only be written when using a Waveform Generation mode that utilizes the ICRn Register for
defining the counter’s TOP value. In these cases the Waveform Generation mode (WGMn3:0) bits must be set
before the TOP value can be written to the ICRn Register. When writing the ICRn Register the high byte must be
written to the ICRnH I/O location before the low byte is written to ICRnL.
16.6.1
Input Capture Source
The main trigger source for the Input Capture unit is the Input Capture pin (ICPn). Timer/Counter1 can alternatively
use the Analog Comparator output as trigger source for the Input Capture unit. The Analog Comparator is selected
as trigger source by setting the Analog Comparator Input Capture (ACIC) bit in the Analog Comparator Control and
Status Register (ACSR). Be aware that changing trigger source can trigger a capture. The Input Capture Flag must
therefore be cleared after the change.
ICFn (Int.Req.)
Analog
Comparator
WRITE
ICRn (16-bit Register)
ICRnH (8-bit)
Noise
Canceler
ICPn
Edge
Detector
TEMP (8-bit)
DATA BUS (8-bit)
ICRnL (8-bit)
TCNTn (16-bit Counter)
TCNTnH (8-bit)
TCNTnL (8-bit)
ACIC*
ICNC
ICES
ACO*