ATmega328P datasheet(151/448 Pages) ATMEL

151 / 448 page

151

8161D–AVR–10/09

ATmega48PA/88PA/168PA/328P

In fast PWM mode, the counter is incremented until the counter value matches the TOP value.

The counter is then cleared at the following timer clock cycle. The timing diagram for the fast

PWM mode is shown in Figure 17-6. The TCNT2 value is in the timing diagram shown as a his-

togram for illustrating the single-slope operation. The diagram includes non-inverted and

inverted PWM outputs. The small horizontal line marks on the TCNT2 slopes represent compare

matches between OCR2x and TCNT2.

Figure 17-6. Fast PWM Mode, Timing Diagram

The Timer/Counter Overflow Flag (TOV2) 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 OC2x pin.

Setting the COM2x1:0 bits to two will produce a non-inverted PWM and an inverted PWM output

can be generated by setting the COM2x1:0 to three. TOP is defined as 0xFF when WGM2:0 = 3,

and OCR2A when MGM2:0 = 7. (See Table 17-3 on page 158). The actual OC2x value will only

be visible on the port pin if the data direction for the port pin is set as output. The PWM wave-

form is generated by setting (or clearing) the OC2x Register at the compare match between

OCR2x and TCNT2, and clearing (or setting) the OC2x 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, 32, 64, 128, 256, or 1024).

The extreme values for the OCR2A Register represent special cases when generating a PWM

waveform output in the fast PWM mode. If the OCR2A is set equal to BOTTOM, the output will

be a narrow spike for each MAX+1 timer clock cycle. Setting the OCR2A equal to MAX will result

in a constantly high or low output (depending on the polarity of the output set by the COM2A1:0

bits.)

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

ting OC2x to toggle its logical level on each compare match (COM2x1:0 = 1). The waveform

TCNTn

OCRnx Update and

TOVn Interrupt Flag Set

Period

OCnx

(COMnx1:0 = 2)

(COMnx1:0 = 3)

OCRnx Interrupt Flag Set

OCnxPWM

clk_I/O

N 256

⋅

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