ATmega324P datasheet(119/377 Pages) ATMEL

119 / 377 page

119

7674F–AVR–09/09

ATmega164P/324P/644P

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).

14.7

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 “Modes of Operation” on page 122.)

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 14-4 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 (A/B/C). The elements of the block diagram that are not directly a part of the Out-

put Compare unit are gray shaded.

Figure 14-4. Output Compare Unit, Block Diagram

OCFnx (Int.Req.)

= (16-bit Comparator )

OCRnx Buffer (16-bit Register)

OCRnxH Buf. (8-bit)

OCnx

TEMP (8-bit)

DATA BUS (8-bit)

OCRnxL Buf. (8-bit)

TCNTn (16-bit Counter)

TCNTnH (8-bit)

TCNTnL (8-bit)

COMnx1:0

WGMn3:0

OCRnx (16-bit Register)

OCRnxH (8-bit)

OCRnxL (8-bit)

Waveform Generator

TOP

BOTTOM

ATmega324P Datasheet(PDF) 119 Page - ATMEL Corporation

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