Datasheet

N represents the prescale divider (1, 8, 64, 256, or 1024).
The extreme values for the OCRnx Register represents special cases when generating a PWM waveform
output in the fast PWM mode. If the OCRnx is set equal to BOTTOM (0x0000) the output will be a narrow
spike for each TOP+1 timer clock cycle. Setting the OCRnx equal to TOP will result in a constant high or
low output (depending on the polarity of the output set by the COMnx1:0 bits.)
A frequency (with 50% duty cycle) waveform output in fast PWM mode can be achieved by setting OCnA
to toggle its logical level on each Compare Match (COMnA1:0 = 1). This applies only if OCRnA is used to
define the TOP value (WGMn3:0 = 15). The waveform generated will have a maximum frequency of f
OCnA
= f
clk_I/O
/2 when OCRnA is set to zero (0x0000). This feature is similar to the OCnA toggle in CTC mode,
except the double buffer feature of the Output Compare unit is enabled in the fast PWM mode.
20.9.4. Phase Correct PWM Mode
The phase correct Pulse Width Modulation or phase correct PWM mode (WGMn3:0 = 1, 2, 3, 10, or 11)
provides a high resolution phase correct PWM waveform generation option. The phase correct PWM
mode is, like the phase and frequency correct PWM mode, based on a dual-slope operation. The counter
counts repeatedly from BOTTOM (0x0000) to TOP and then from TOP to BOTTOM. In non-inverting
Compare Output mode, the Output Compare (OCnx) is cleared on the Compare Match between TCNTn
and OCRnx while upcounting, and set on the Compare Match while downcounting. In inverting Output
Compare mode, the operation is inverted. The dual-slope operation has lower maximum operation
frequency than single slope operation. However, due to the symmetric feature of the dual-slope PWM
modes, these modes are preferred for motor control applications.
The PWM resolution for the phase correct PWM mode 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:

PCPWM
=
log TOP+1
log 2
In phase correct PWM mode the counter is incremented until the counter value matches either one of the
fixed values 0x00FF, 0x01FF, or 0x03FF (WGMn3:0 = 1, 2, or 3), the value in ICRn (WGMn3:0 = 10), or
the value in OCRnA (WGMn3:0 = 11). The counter has then reached the TOP and changes the count
direction. The TCNTn value will be equal to TOP for one timer clock cycle. The timing diagram for the
phase correct PWM mode is shown in the figure below. The figure shows phase correct 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 dual-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]
Atmel-8160E-ATmega64A_Datasheet_Complete-09/2015
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