how waveforms are generated on the Output Compare outputs OCnx. For more details
about advanced counting sequences and waveform generation, see “Modes of Opera-
tion” on page 147.
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.
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, for the
Timer/Counter1 only, via the Analog Comparator unit. The time-stamps can then be
used to calculate frequency, duty-cycle, and other features of the signal applied. Alter-
natively 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 51. The ele-
ments 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 51. Input Capture Unit Block Diagram
DATA BUS (8-bit)
TEMP (8-bit)
ICRnH (8-bit)
ICRnL (8-bit)
TCNTnH (8-bit)
TCNTnL (8-bit)
ICRn (16-bit Register)
TCNTn (16-bit Counter)
WRITE
ACO*
ACIC*
ICNC
ICES
Analog
Comparator
Noise
Canceler
Edge
Detector
ICFn (Int.Req.)
ICPn
Note:
The Analog Comparator Output (ACO) can only trigger the Timer/Counter1 ICP – not
Timer/Counter3, 4 or 5.
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 (TICIEn = 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.
142
ATmega640/1280/1281/2560/2561
2549A–AVR–03/05
ATMEL [ ATMEL ]
