Datasheet
Figure 21-3. 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*
When a change of the logic level (an event) occurs on the Input Capture Pin (ICP1), 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 (TCNT1) is written to the
Input Capture Register (ICR1). The Input Capture Flag (ICF1) is set at the same system clock as the
TCNT1 value is copied into ICR1 Register. If enabled (TICIE1 = 1), the Input Capture Flag generates an
Input Capture interrupt. The ICF1 Flag is automatically cleared when the interrupt is executed.
Alternatively the ICF1 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 (ICR1) is done by first reading the Low byte
(ICR1L) and then the High byte (ICR1H). When the Low byte is read the High byte is copied into the High
byte temporary register (TEMP). When the CPU reads the ICR1H I/O location it will access the TEMP
Register.
The ICR1 Register can only be written when using a Waveform Generation mode that utilizes the ICR1
Register for defining the counter’s TOP value. In these cases the Waveform Generation mode
(WGM13:0) bits must be set before the TOP value can be written to the ICR1 Register. When writing the
ICR1 Register the High byte must be written to the ICR1H I/O location before the Low byte is written to
ICR1L.
For more information on how to access the 16-bit registers refer to Accessing 16-bit Registers.
21.6.1 Input Capture Pin Source
The main trigger source for the Input Capture unit is the Input Capture Pin (ICP1). Timer/Counter 1 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.
AVR 8-Bit Microcontroller
16-bit Timer/Counter1
© 2017 Microchip Technology Inc.
Datasheet Complete
40001974A-page 121