ATtiny261A datasheet(99/292 Pages) ATMEL

99 / 292 page

8197A–AVR–10/09

ATtiny261A/461A/861A

DAC applications. High frequency allows physically small sized external components (coils,

capacitors), and therefore reduces total system cost.

The timing diagram for the fast PWM mode is shown in Figure 12-12. The counter is incre-

mented until the counter value matches the TOP value. The counter is then cleared at the

following timer clock cycle. The TCNT1 value is in the timing diagram shown as a histogram for

illustrating the single-slope operation. 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 12-12. 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 Waveforn

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

TCNTn

OCRnx Update and

TOVn Interrupt Flag Set

Period

OCWnx

(COMnx1:0 = 2)

(COMnx1:0 = 3)

OCRnx Interrupt Flag Set

fOCnxPWM

fclkT1

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