Datasheet

ATmega164A/PA/324A/PA/644A/PA/1284/P
 2018 Microchip Technology Inc. Data Sheet Complete DS40002070A-page 152
For generating a waveform output in CTC mode, the OC2A output can be set to toggle its logical level on each
compare match by setting the Compare Output mode bits to toggle mode (COM2A1:0 = 1). The OC2A 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
OC2A
= f
clk_I/O
/2 when OCR2A is set to zero (0x00). The waveform frequency is
defined by the following equation:
The N variable represents the prescale factor (1, 8, 32, 64, 128, 256, or 1024).
As for the Normal mode of operation, the
TOV2 Flag is set in the same timer clock cycle that the counter counts
from MAX to 0x00.
17.7.3 Fast PWM mode
The fast Pulse Width Modulation or fast PWM mode (WGM22:0 = 3 or 7) provides a high frequency PWM
waveform generation 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 WGM22:0 =
3, and OCR2A when WGM22:0 = 7. In non-inverting Compare Output mode, the Output Compare (OC2x) is
cleared on the compare match between TCNT2 and OCR2x, 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 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
17-6 on page 152. The TCNT2 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 TCNT2 slopes represent compare matches between OCR2x and TCNT2.
Figure 17-6. Fast PWM mode, timing diagram
The Timer/Counter Overflow Flag (TOV2) 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
OCnx
OCnx
(COMnx1:0 = 2)
(COMnx1:0 = 3)
OCRnx Interrupt Flag Set
4 5 6 7