Datasheet

ATmega164A/PA/324A/PA/644A/PA/1284/P
 2018 Microchip Technology Inc. Data Sheet Complete DS40002070A-page 122
The Timer/Counter Overflow Flag (TOVn) is set according to the mode of operation selected by the WGMn3:0
bits. TOVn can be used for generating a CPU interrupt.
16.7 Input Capture Unit
The Timer/Counter incorporates an Input Capture unit that can capture external events and give them a time-
stamp indicating time of occurrence. The external signal indicating an event, or multiple events, can be applied
via the ICPn pin 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.
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 indicates the Timer/Counter number.
Figure 16-3. Input Capture unit block diagram
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 (ICIEn = 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.
For more information on how to access the 16-bit registers refer to ”Accessing 16-bit Registers” on page 117.
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*