Datasheet

94
7728G–AVR–06/10
ATtiny87/ATtiny167
For generating a waveform output in CTC mode, the OC0A output can be set to toggle its log-
ical level on each compare match by setting the Compare Output mode bits to toggle mode
(COM0A1:0 = 1). The OC0A value will not be visible on the port pin unless the data direction
for the pin is set to output. The waveform generated will have a maximum frequency of f
OC
0
A
=
f
clk_I/O
/2 when OCR0A is set to zero (0x00). The waveform frequency is defined by the follow-
ing equation:
The N variable represents the prescale factor (1, 8, 32, 64, 128, 256, or 1024).
As for the Normal mode of operation, the TOV0 flag is set in the same timer clock cycle that
the counter counts from MAX to 0x00.
10.7.3 Fast PWM Mode
The fast Pulse Width Modulation or fast PWM mode (WGM01:0 = 3) provides a high frequency
PWM waveform generation option. The fast PWM differs from the other PWM option by its sin-
gle-slope operation. The counter counts from BOTTOM to MAX then restarts from BOTTOM.
In non-inverting Compare Output mode, the Output Compare (OC0A) is cleared on the com-
pare match between TCNT0 and OCR0A, 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 mode that uses 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 MAX value.
The counter is then cleared at the following timer clock cycle. The timing diagram for the fast
PWM mode is shown in Figure 10-6. 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 line marks on the TCNT0 slopes represent com-
pare matches between OCR0A and TCNT0.
Figure 10-6. Fast PWM Mode, Timing Diagram
f
OCnx
f
clk_I/O
2 N 1 OCRnx+()⋅⋅
-------------------------------------------------------=
TCNTn
OCRnx Update and
TOVn Interrupt Flag Set
1
Period
2 3
OCnx
OCnx
(COMnx1:0 = 2)
(COMnx1:0 = 3)
OCRnx Interrupt Flag Set
4 5 6 7