AT90PWM2 datasheet(94/365 Pages) ATMEL

94 / 365 page

4317K–AVR–03/2013

AT90PWM2/3/2B/3B

inverted PWM outputs. The small horizontal line marks on the TCNT0 slopes represent compare

matches between OCR0x and TCNT0.

Figure 14-6. Fast PWM Mode, Timing Diagram

The Timer/Counter Overflow Flag (TOV0) is set each time the counter reaches TOP. If the inter-

rupt is enabled, the interrupt handler routine can be used for updating the compare value.

In fast PWM mode, the compare unit allows generation of PWM waveforms on the OC0x pins.

Setting the COM0x1:0 bits to two will produce a non-inverted PWM and an inverted PWM output

can be generated by setting the COM0x1:0 to three: Setting the COM0A1:0 bits to one allows

the OC0A pin to toggle on Compare Matches if the WGM02 bit is set. This option is not available

for the OC0B pin (see Table 14-6 on page 99). The actual OC0x value will only be visible on the

port pin if the data direction for the port pin is set as output. The PWM waveform is generated by

setting (or clearing) the OC0x Register at the compare match between OCR0x and TCNT0, and

clearing (or setting) the OC0x Register at the timer clock cycle the counter is cleared (changes

from TOP to BOTTOM).

The PWM frequency for the output can be calculated by the following equation:

The N variable represents the prescale factor (1, 8, 64, 256, or 1024).

The extreme values for the OCR0A Register represents special cases when generating a PWM

waveform output in the fast PWM mode. If the OCR0A is set equal to BOTTOM, the output will

be a narrow spike for each MAX+1 timer clock cycle. Setting the OCR0A equal to MAX will result

in a constantly high or low output (depending on the polarity of the output set by the COM0A1:0

bits.)

A frequency (with 50% duty cycle) waveform output in fast PWM mode can be achieved by set-

ting OC0x to toggle its logical level on each compare match (COM0x1:0 = 1). The waveform

generated will have a maximum frequency of f

OC0 = fclk_I/O/2 when OCR0A is set to zero. This

feature is similar to the OC0A toggle in CTC mode, except the double buffer feature of the Out-

put Compare unit is enabled in the fast PWM mode.

TCNTn

OCRnx Update and

TOVn Interrupt Flag Set

Period

OCn

(COMnx1:0 = 2)

(COMnx1:0 = 3)

OCRnx Interrupt Flag Set

OCnxPWM

fclk_I/O

N 256



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AT90PWM2 Datasheet(PDF) 94 Page - ATMEL Corporation

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