93
7734Q–AVR–02/12
AT90PWM81/161
Figure 11-4. Input capture unit block diagram.
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 (ICIE1 = 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 Genera-
tion mode (WGM13) 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.
11.5.1
Input Capture Trigger Source
The main trigger source for the Input Capture unit is the Input Capture pin (ICP1).
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 (AC1ICE) bit in the Analog Comparator Extended Control Register
(AC1ECON). Be aware that changing trigger source can trigger a capture. The Input Capture
Flag must therefore be cleared after the change.
ICFn (Int.Req.)
WRITE
ICRn (16-bit Register)
ICRnH (8-bit)
Noise
Canceler
Edge
Detector
TEMP (8-bit)
DATA BUS
(8-bit)
ICRnL (8-bit)
TCNTn (16-bit Counter)
TCNTnH (8-bit)
TCNTnL
(8-bit)
ICNC
ICES
ICPnA