104
ATmega16M1/32M1/64M1 [DATASHEET]
8209E–AVR–11/2012
15.6
Input Capture unit
The Timer/Counter incorporates an Input Capture unit that can capture external events and give them a time-
stamp indicating time of occurrence. The external signal indicating an event, or multiple events, can be applied via
the ICPn pin or alternatively, via the analog-comparator unit. The time-stamps can then be used to calculate fre-
quency, duty-cycle, and other features of the signal applied. Alternatively the time-stamps can be used for creating
a log of the events.
The Input Capture unit is illustrated by the block diagram shown in
Figure 15-3. 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 15-3. Input Capture unit block diagram.
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 (ICIEn = 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
writing 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 Generating mode that utilizes the ICRn Register for
defining the counter’s TOP value. In these cases the Waveform Generating 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.
The ICF1 output can be used to retrigger the timer counter. It has the same effect than the TOP signal.
ICFn (Int.req.)
WRITE
ICRn (16-bit register)
ICRnH (8-bit)
Noise
canceler
ICPnB
Edge
detector
TEMP (8-bit)
DATA BUS (8-bit)
ICRnL (8-bit)
TCNTn (16-bit counter)
TCNTnH (8-bit)
TCNTnL (8-bit)
ICPSEL1
ICNC
ICES
ICPnA
AC1ICE
Analog Comparator 1 interrupt