Datasheet

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small horizontal line marks on the TCNT0 slopes represent compare matches between OCR0 and

TCNT0.

Figure 21-6 Fast PWM Mode, Timing Diagram

TCNTn

OCRn Upda te

and

TOVn Interrupt Fla g Se t

Pe riod

2 3

OCn

(COMn1:0 = 2)

(COMn1:0 = 3)

OCRn Inte rrupt Flag Se t

4 5 6 7

The Timer/Counter Overflow Flag (TOV0) is set each time the counter reaches MAX. If the interrupt 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 OC0 pin. Setting the

COM01:0 bits to 2 will produce a non-inverted PWM and an inverted PWM output can be generated by

setting the COM01:0 to 3. The actual OC0 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 OC0 Register at

the Compare Match between OCR0 and TCNT0, and clearing (or setting) the OC0 Register at the timer

clock cycle the counter is cleared (changes from MAX to BOTTOM).

The PWM frequency for the output can be calculated by the following equation:



OCnPWM



clk_I/O

  256

The N variable represents the prescaler factor (1, 8, 32, 64, 128, 256, or 1024).

The extreme values for the OCR0 Register represent special cases when generating a PWM waveform

output in the fast PWM mode. If the OCR0 is set equal to BOTTOM, the output will be a narrow spike for

each MAX+1 timer clock cycle. Setting the OCR0 equal to MAX will result in a constantly high or low

output (depending on the polarity of the output set by the COM01:0 bits.)

A frequency (with 50% duty cycle) waveform output in fast PWM mode can be achieved by setting OC0 to

toggle its logical level on each Compare Match (COM01:0 = 1). The waveform generated will have a

maximum frequency of f

oc0

= f

clk_I/O

/2 when OCR0 is set to zero. This feature is similar to the OC0 toggle

in CTC mode, except the double buffer feature of the Output Compare unit is enabled in the fast PWM

mode.