ATmega128 datasheet(121/386 Pages) ATMEL

121 / 386 page

121

2467S–AVR–07/09

ATmega128

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 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 wave-

form generator for handling the special cases of the extreme values in some modes of operation

(See “Modes of Operation” on page 124.)

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 49 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 output

compare unit are gray shaded.

Figure 49. Output Compare Unit, Block Diagram

The OCRnx Register is double buffered when using any of the twelve Pulse Width Modulation

(PWM) modes. For the normal and Clear Timer on Compare (CTC) modes of operation, the dou-

ble buffering is disabled. The double buffering synchronizes the update of the OCRnx Compare

occurrence of odd-length, non-symmetrical PWM pulses, thereby making the output glitch-free.

OCFnx (Int.Req.)

=(16-bit Comparator )

OCRnx Buffer (16-bit Register)

OCRnxH Buf. (8-bit)

OCnx

TEMP (8-bit)

DATABUS (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

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