Rainbow Electronics ATmega32L - Manual (Page 91)

91
ATmega32(L)
2503C–AVR–10/02
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 ICP1 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 42. 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 42. 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 (WGM13:0) 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 writ-
ten to the ICR1H I/O location before the low byte is written to ICR1L.
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*