ATtiny261-ESSZ datasheet(104/227 Pages) ATMEL

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104

7753C–AVR–07/09

ATtiny261/ATtiny461/ATtiny861

The diagram includes the Waveform Output in non-inverted and inverted Compare Output

modes. The small horizontal line marks on the TCNT1 slopes represent Compare Matches

between OCR1x and TCNT1.

Figure 16-11. Fast PWM Mode, Timing Diagram

The Timer/Counter Overflow Flag (TOV1) 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 OC1x pins.

Setting the COM1x1:0 bits to two will produce a non-inverted PWM and setting the COM1x1:0 to

three will produce an inverted PWM output. Setting the COM1x1:0 bits to one will enable com-

plementary Compare Output mode and produce both the non-inverted (OC1x) and inverted

output (OC1x). The actual 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 Waveform

Output (OCW1x) at the Compare Match between OCR1x and TCNT1, and clearing (or setting)

the Waveform Output 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 number of steps in single-slope operation. The value of N equals

either to the TOP value.

The extreme values for the OCR1C Register represents special cases when generating a PWM

waveform output in the fast PWM mode. If the OCR1C is set equal to BOTTOM, the output will

be a narrow spike for each MAX+1 timer clock cycle. Setting the OCR1C equal to MAX will result

in a constantly high or low output (depending on the polarity of the output set by the COM1x1:0

bits.)

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

ting the Waveform Output (OCW1x) to toggle its logical level on each Compare Match

(COM1x1:0 = 1). The waveform generated will have a maximum frequency of f

OC1 = fclkT1/4 when

OCR1C is set to three.

TCNTn

OCRnx Update and

TOVn Interrupt Flag Set

Period

OCWnx

(COMnx1:0 = 2)

(COMnx1:0 = 3)

OCRnx Interrupt Flag Set

fOCnxPWM

clkT1

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