Datasheet

92
ATmega32A [DATASHEET]
Atmel-8155D-AVR-ATmega32A-Datasheet_02/2014
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 17-3. 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 (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 (WGM1[3:0]) 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” on page 94.
17.6.1 Input Capture Pin 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 (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*