Datasheet

ATmega164A/PA/324A/PA/644A/PA/1284/P
 2018 Microchip Technology Inc. Data Sheet Complete DS40002070A-page 123
16.7.1 Input Capture Trigger Source
The main trigger source for the Input Capture unit is the Input Capture pin (ICPn). 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.
Both the Input Capture pin (ICPn) and the Analog Comparator output (ACO) inputs are sampled using the same
technique as for the Tn pin (Figure 16-1 on page 116). The edge detector is also identical. However, when the
noise canceler is enabled, additional logic is inserted before the edge detector, which increases the delay by
four system clock cycles. Note that the input of the noise canceler and edge detector is always enabled unless
the Timer/Counter is set in a Waveform Generation mode that uses ICRn to define TOP.
An Input Capture can be triggered by software by controlling the port of the ICPn pin.
16.7.2 Noise Canceler
The noise canceler improves noise immunity by using a simple digital filtering scheme. The noise canceler input
is monitored over four samples, and all four must be equal for changing the output that in turn is used by the
edge detector.
The noise canceler is enabled by setting the Input Capture Noise Canceler (ICNCn) bit in Timer/Counter Control
Register B (TCCRnB). When enabled the noise canceler introduces additional four system clock cycles of delay
from a change applied to the input, to the update of the ICRn Register. The noise canceler uses the system
clock and is therefore not affected by the prescaler.
16.7.3 Using the Input Capture unit
The main challenge when using the Input Capture unit is to assign enough processor capacity for handling the
incoming events. The time between two events is critical. If the processor has not read the captured value in the
ICRn Register before the next event occurs, the ICRn will be overwritten with a new value. In this case the result
of the capture will be incorrect.
When using the Input Capture interrupt, the ICRn Register should be read as early in the interrupt handler
routine as possible. Even though the Input Capture interrupt has relatively high priority, the maximum interrupt
response time is dependent on the maximum number of clock cycles it takes to handle any of the other interrupt
requests.
Using the Input Capture unit in any mode of operation when the TOP value (resolution) is actively changed
during operation, is not recommended.
Measurement of an external signal’s duty cycle requires that the trigger edge is changed after each capture.
Changing the edge sensing must be done as early as possible after the ICRn Register has been read. After a
change of the edge, the Input Capture Flag (ICFn) must be cleared by software (writing a logical one to the I/O
bit location). For measuring frequency only, the clearing of the ICFn Flag is not required (if an interrupt handler
is used).
16.8 Output Compare units
The 16-bit comparator continuously compares TCNTn with the Output Compare Register (OCRnx). If TCNT
equals OCRnx the comparator signals a match. A match will set the Output Compare Flag (OCFnx) at the next
timer clock cycle. If enabled (OCIEnx = 1), the Output Compare Flag generates an Output Compare interrupt.
The OCFnx Flag is automatically cleared when the interrupt is executed. Alternatively the OCFnx Flag can be
cleared by software by writing a logical one to its I/O bit location. The Waveform Generator uses the match
signal to generate an output according to operating mode set by the Waveform Generation mode (WGMn3:0)
bits and Compare Output mode (COMnx1:0) bits. The TOP and BOTTOM signals are used by the Waveform