Datasheet

107
ATtiny828 [DATASHEET]
8371A–AVR–08/12
or alternatively, 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.
Figure 39. Input Capture Unit Block Diagram
The Input Capture unit is illustrated by the block diagram shown in Figure 39. 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.
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 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 120.
12.5.1 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 (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* ICNCn ICES1
ACO*