86
ATmega16M1/32M1/64M1 [DATASHEET]
8209E–AVR–11/2012
For generating a waveform output in CTC mode, the OC0A output can be set to toggle its logical 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
OC0 = fclk_I/O/2 when OCR0A is set to zero (0x00). The waveform frequency is defined by
the following equation:
The N variable represents the prescale factor (1, 8, 64, 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.
14.7.3
Fast PWM mode
The fast Pulse Width Modulation or fast PWM mode (WGM02:0 = 3 or 7) provides a high frequency PWM wave-
form generating option. The fast PWM differs from the other PWM option by its single-slope operation. The counter
counts from BOTTOM to TOP then restarts from BOTTOM. TOP is defined as 0xFF when WGM2:0 = 3, and
OCR0A when WGM2:0 = 7. In non-inverting Compare Output mode, the Output Compare (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 mode that 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 sys-
tem 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 14-6. 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
compare matches between OCR0x and TCNT0.
Figure 14-6. Fast PWM mode, timing diagram.
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.
f
OCnx
f
clk_I/O
2 N
1
OCRnx
+
--------------------------------------------------
=
TCNTn
OCRnx update and
TOVn interrupt flag set
1
Period
2
3
OCn
(COMnx1:0 = 2)
(COMnx1:0 = 3)
OCRnx interrupt flag set
4
5
6
7