20.7.3 Fast PWM Mode
The Fast Pulse Width Modulation or Fast PWM modes (WGM0[2:0]=0x3 or WGM0[2:0]=0x7) provide a high frequency PWM waveform generation option. The Fast PWM modes differ from the other PWM options by their single-slope operation. The counter counts from BOTTOM to TOP, then restarts from BOTTOM. TOP is defined as 0xFF when WGM0[2:0]=0x3. TOP is defined as OCR0A when WGM0[2:0]=0x7.
In non-inverting Compare Output mode, the Output Compare register (OC0x) is cleared on the compare match between TCNT0 and OCR0x, and set at BOTTOM. In inverting Compare Output mode, the output is set on compare match and cleared at BOTTOM. Due to the single-slope operation, the operating frequency of the Fast PWM mode can be twice as high as the phase correct PWM modes, which use dual-slope operation. 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 below. The TCNT0 value is in the timing diagram shown as a histogram for illustrating the single-slope operation. The diagram includes non-inverted and inverted PWM outputs. The small horizontal lines on the TCNT0 slopes mark compare matches between OCR0x and TCNT0.
The Timer/Counter Overflow Flag (TOV0) is set each time the counter reaches TOP. 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 OC0x pins. Writing the TCCR0A.COM0x[1:0] bits to 0x2 will produce a non-inverted PWM; TCCR0A.COM0x[1:0]=0x3 will produce an inverted PWM output. Writing the TCCR0A.COM0A[1:0] bits to 0x1 allows the OC0A pin to toggle on Compare Matches if the TCCRnB.WGMn2 bit is set. This option is not available for the OC0B pin. 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:
N represents the prescale divider (1, 8, 64, 256, or 1024).
The extreme values for the OCR0A register represents special cases for PWM waveform output in the Fast PWM mode: If OCR0A is written equal to BOTTOM, the output will be a narrow spike for each MAX+1 timer clock cycle. Writing OCR0A=MAX will result in a constantly high or low output (depending on the polarity of the output set by the COM0A[1:0] bits.)
A frequency waveform output with 50% duty cycle can be achieved in Fast PWM mode by selecting OC0x to toggle its logical level on each compare match (COM0x[1:0]=0x1). The waveform generated will have a maximum frequency of fOC0 = fclk_I/O/2 when OCR0A=0x00. This feature is similar to the OC0A toggle in CTC mode, except double buffering of the Output Compare unit is enabled in the Fast PWM mode.