ATtiny45 datasheet(68/196 Pages) ATMEL

68 / 196 page

7598H–AVR–07/09

ATtiny25/45/85

This high frequency makes the fast PWM mode well suited for power regulation, rectification,

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

capacitors), and therefore reduces total system cost.

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 12-6. The TCNT0 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 TCNT0 slopes represent Com-

pare Matches between OCR0x and TCNT0.

Figure 12-6. 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 allowes

the AC0A pin to toggle on Compare Matches if the WGM02 bit is set. This option is not available

for the OC0B pin (See Table 12-2 on page 72). 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.

TCNTn

OCRnx Update and

TOVn Interrupt Flag Set

Period

OCn

(COMnx1:0 = 2)

(COMnx1:0 = 3)

OCRnx Interrupt Flag Set

fOCnxPWM

clk_I/O

256

⋅

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ATtiny45 Datenblatt(PDF) 68 Page - ATMEL Corporation