Datasheet

OCR0A. The OCR0A defines the top value for the counter, hence also its resolution. This mode allows
greater control of the compare match output frequency. It also simplifies the counting of external events.
The timing diagram for the CTC mode is shown below. The counter value (TCNT0) increases until a
compare match occurs between TCNT0 and OCR0A, and then counter (TCNT0) is cleared.
Figure 15-5. CTC Mode, Timing Diagram
TCNTn
OCn
(Toggle)
OCnx Interrupt Flag Set
1 4
Period
2 3
(COMnx[1:0] = 0x1)
An interrupt can be generated each time the counter value reaches the TOP value by setting the OCF0A
Flag. If the interrupt is enabled, the interrupt handler routine can be used for updating the TOP value.
Note: Changing TOP to a value close to BOTTOM while the counter is running must be done with care,
since the CTC mode does not provide double buffering. If the new value written to OCR0A is lower than
the current value of TCNT0, the counter will miss the compare match. The counter will then count to its
maximum value (0xFF for a 8-bit counter, 0xFFFF for a 16-bit counter) and wrap around starting at 0x00
before the compare match will occur.
For generating a waveform output in CTC mode, the OC0A output can be set to toggle its logical level on
each compare match by writing the two least significant Compare Output mode bits in the Timer/Counter
Control Register A Control to toggle mode (TCCR0A.COM0A[1:0]=0x1). The OC0A value will only 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
= f
clk_I/O
/2 when OCR0A is written to 0x00. The waveform frequency
is defined by the following equation:

OCnx
=

clk_I/O
2   
1 + OCRnx
N represents the prescaler factor (1, 8, 64, 256, or 1024).
As for the Normal mode of operation, the Timer/Counter Overflow Flag TOV0 is set in the same clock
cycle that the counter wraps from MAX to 0x00.
15.7.3. 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,
Atmel ATmega16M1/32M1/64M1 [DATASHEET]
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