Datasheet

use the fast PWM mode using OCRnA for defining TOP (WGMn3:0 = 15) since the OCRnA then will be
double buffered.
For generating a waveform output in CTC mode, the OCnA output can be set to toggle its logical level on
each Compare Match by setting the Compare Output mode bits to toggle mode (COMnA1:0 = 1). The
OCnA value will not be visible on the port pin unless the data direction for the pin is set to output
(DDR_OCnA = 1). The waveform generated will have a maximum frequency of f
OCnA
= f
clk_I/O
/2 when
OCRnA is set to zero (0x0000). The waveform frequency is defined by the following equation:

OCnA
=

clk_I/O
2   
1 + OCRnA
N represents the prescaler factor (1, 8, 64, 256, or 1024).
As for the Normal mode of operation, the Timer Counter TOVn Flag is set in the same timer clock cycle
that the counter counts from MAX to 0x0000.
20.9.3. Fast PWM Mode
The fast Pulse Width Modulation or fast PWM mode (WGMn3:0 = 5, 6, 7, 14, or 15) provides a high
frequency PWM waveform generation option. The fast PWM differs from the other PWM options by its
single-slope operation. The counter counts from BOTTOM to TOP then restarts from BOTTOM. In non-
inverting Compare Output mode, the Output Compare (OCnx) is cleared on the Compare Match between
TCNTn and OCRnx, and set at BOTTOM. In inverting Compare Output mode output is set on Compare
Match and cleared at BOTTOM. Due to the singleslope operation, the operating frequency of the fast
PWM mode can be twice as high as the phase correct and phase and frequency correct PWM modes 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), hence reduces total system cost.
The PWM resolution for fast PWM can be fixed to 8-, 9-, or 10-bit, or defined by either ICRn or OCRnA.
The minimum resolution allowed is 2-bit (ICRn or OCRnA set to 0x0003), and the maximum resolution is
16-bit (ICRn or OCRnA set to MAX). The PWM resolution in bits can be calculated by using the following
equation:

FPWM
=
log TOP+1
log 2
In fast PWM mode the counter is incremented until the counter value matches either one of the fixed
values 0x00FF, 0x01FF, or 0x03FF (WGMn3:0 = 5, 6, or 7), the value in ICRn (WGMn3:0 = 14), or the
value in OCRnA (WGMn3:0 = 15). The counter is then cleared at the following timer clock cycle. The
timing diagram for the fast PWM mode is shown in the figure below. The figure shows fast PWM mode
when OCRnA or ICRn is used to define TOP. The TCNTn 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 TCNTn slopes represent compare matches
between OCRnx and TCNTn. The OCnx Interrupt Flag will be set when a Compare Match occurs.
Atmel ATmega64A [DATASHEET]
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