Datasheet

be applied via the ICPn pin or alternatively, for the Timer/Counter1 only, via the Analog Comparator unit.
The time-stamps can then be used to calculate frequency, 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 below. 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
indicates the Timer/Counter number.
Figure 20-3 Input Capture Unit Block Diagram
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*
Note: 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 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 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.
Atmel ATmega64A [DATASHEET]
Atmel-8160E-ATmega64A_Datasheet_Complete-09/2015
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