Figure 16-5. CTC Mode, Timing Diagram
OCnx Interrupt Flag Set
TCNTn
OCnx
(Toggle)
(COMnx1:0 = 1)
1
2
3
4
Period
An interrupt can be generated each time the counter value reaches the TOP value by using the
OCF2A Flag. If the interrupt is enabled, the interrupt handler routine can be used for updating
the TOP value. However, changing the TOP to a value close to BOTTOM when the counter is
running with none or a low prescaler value must be done with care since the CTC mode does
not have the double buffering feature. If the new value written to OCR2A is lower than the cur-
rent value of TCNT2, the counter will miss the compare match. The counter will then have to
count to its maximum value (0xFF) and wrap around starting at 0x00 before the compare match
can occur.
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 fOC2A
clk_I/O/2 when OCR2A is set to zero (0x00). The waveform frequency is defined by the following
equation:
=
f
f
clk_I/O
f
= -------------------------------------------------
OCnx
2 ⋅ N ⋅ (1 + OCRnx)
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.
16.6.3
Fast PWM Mode
The fast Pulse Width Modulation or fast PWM mode (WGM21:0 = 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 (OC2A) is cleared on the compare
match between TCNT2 and OCR2A, 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
144
ATmega169P
8018A–AVR–03/06
ATMEL [ ATMEL ]
