ATmega16 datasheet(91/315 Pages) ATMEL

91 / 315 page

ATmega16(L)

2466E–AVR–10/02

For more information on how to access the 16-bit registers refer to “Accessing 16-bit

Registers” on page 86.

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 set-

ting 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 cap-

ture. The input capture flag must therefore be cleared after the change.

Both the Input Capture pin (ICP1) and the Analog Comparator output (ACO) inputs are

sampled using the same technique as for the T1 pin (Figure 38 on page 81). 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 ICR1 to

define TOP.

An input capture can be triggered by software by controlling the port of the ICP1 pin.

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 (ICNC1) bit

in Timer/Counter Control Register B (TCCR1B). When enabled the noise canceler intro-

duces additional four system clock cycles of delay from a change applied to the input, to

the update of the ICR1 Register. The noise canceler uses the system clock and is there-

fore not affected by the prescaler.

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 ICR1 Register before the next event

occurs, the ICR1 will be overwritten with a new value. In this case the result of the cap-

ture will be incorrect.

When using the input capture interrupt, the ICR1 Register should be read as early in the

interrupt handler routine as possible. Even though the input capture interrupt has rela-

tively 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 ICR1 Register has been read. After a change of the edge, the input capture flag

(ICF1) must be cleared by software (writing a logical one to the I/O bit location). For

measuring frequency only, the clearing of the ICF1 flag is not required (if an interrupt

handler is used).

Output Compare Units

The 16-bit comparator continuously compares TCNT1 with the Output Compare Regis-

ter (OCR1x). If TCNT equals OCR1x the comparator signals a match. A match will set

the Output Compare Flag (OCF1x) at the next timer clock cycle. If enabled (OCIE1x =

1), the Output Compare flag generates an output compare interrupt. The OCF1x flag is

automatically cleared when the interrupt is executed. Alternatively the OCF1x flag can

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