ATmega16M1 datasheet(116/365 Pages) ATMEL

116 / 365 page

116

7647F–AVR–04/09

ATmega16/32/64/M1/C1

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

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).

13.5.4

Using the Input Capture Unit as TCNT1 Retrigger Input

TCNT1 counts from BOTTOM to TOP. The TOP value can be a fixed value, ICR1, or OCR1A.

When enabled the Retrigger Input forces to reach the TOP value. It means that ICF1 output is

ored with the TOP signal.

13.6

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 Com-

pare 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 writ-

ing 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 Generator for handling the special cases of the extreme values in

some modes of operation (See “16-bit Timer/Counter1 with PWM” on page 107.)

A special feature of Output Compare unit A allows it to define the Timer/Counter TOP value (i.e.,

counter resolution). In addition to the counter resolution, the TOP value defines the period time

for waveforms generated by the Waveform Generator.

Figure 13-4 shows a block diagram of the Output Compare unit. The small “n” in the register and

bit names indicates the device number (n = n for Timer/Counter n), and the “x” indicates Output

Compare unit (x). The elements of the block diagram that are not directly a part of the Output

Compare unit are gray shaded.

ATmega16M1 Datasheet(PDF) 116 Page - ATMEL Corporation