ATtiny43U datasheet(87/182 Pages) ATMEL

87 / 182 page

8048B–AVR–03/09

in a constantly high or low output (depending on the polarity of the output set by the COMnA1:0

bits.)

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

ting OCnx to toggle its logical level on each Compare Match (COMnx1:0 = 1). The waveform

generated will have a maximum frequency of f

OCnx = fclk_I/O/2 when OCRnA is set to zero. This

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

put Compare unit is enabled in the fast PWM mode.

12.7.4

Phase Correct PWM Mode

The phase correct PWM mode (WGMn2:0 = 1 or 5) provides a high resolution phase correct

PWM waveform generation option. The phase correct PWM mode is based on a dual-slope

operation. The counter counts repeatedly from BOTTOM to TOP and then from TOP to BOT-

TOM. TOP is defined as 0xFF when WGMn2:0 = 1, and OCRnA when WGMn2:0 = 5. In non-

inverting Compare Output mode, the Output Compare (OCnx) is cleared on the Compare Match

between TCNTn and OCRnx while upcounting, and set on the Compare Match while down-

counting. In inverting Output Compare mode, the operation is inverted. The dual-slope operation

has lower maximum operation frequency than single slope operation. However, due to the sym-

metric feature of the dual-slope PWM modes, these modes are preferred for motor control

applications.

In phase correct PWM mode the counter is incremented until the counter value matches TOP.

When the counter reaches TOP, it changes the count direction. The TCNTn value will be equal

to TOP for one timer clock cycle. The timing diagram for the phase correct PWM mode is shown

on Figure 12-7 on page 87. The TCNTn value is in the timing diagram shown as a histogram for

illustrating the dual-slope operation. The diagram includes non-inverted and inverted PWM out-

puts. The small horizontal line marks on the TCNTn slopes represent Compare Matches

between OCRnx and TCNTn.

Figure 12-7. Phase Correct PWM Mode, Timing Diagram

TOVn Interrupt Flag Set

OCnx Interrupt Flag Set

TCNTn

Period

OCn

(COMnx1:0 = 2)

(COMnx1:0 = 3)

OCRnx Update

ATtiny43U датащи(PDF) 87 Page - ATMEL Corporation