Datasheet

is written the value written will be put into the OCRnA Buffer Register. The OCRnA Compare Register will
then be updated with the value in the Buffer Register at the next timer clock cycle the TCNTn matches
TOP. The update is done at the same timer clock cycle as the TCNTn is cleared and the TOVn Flag is
set.
Using the ICRn Register for defining TOP works well when using fixed TOP values. By using ICRn, the
OCRnA Register is free to be used for generating a PWM output on OCnA. However, if the base PWM
frequency is actively changed (by changing the TOP value), using the OCRnA as TOP is clearly a better
choice due to its double buffer feature.
In fast PWM mode, the compare units allow generation of PWM waveforms on the OCnx pins. Setting the
COMnx1:0 bits to 2 will produce a non-inverted PWM and an inverted PWM output can be generated by
setting the COMnx1:0 to 3. Refer to table Table 19-3. The actual OCnx value will only be visible on the
port pin if the data direction for the port pin is set as output (DDR_OCnx). The PWM waveform is
generated by setting (or clearing) the OCnx Register at the Compare Match between OCRnx and TCNTn,
and clearing (or setting) the OCnx Register at the timer clock cycle the counter is cleared (changes from
TOP to BOTTOM).
The PWM frequency for the output can be calculated by the following equation:

OCnxPWM
=

clk_I/O
 
1 + TOP
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.
19.9.4. 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
Atmel ATmega32A [DATASHEET]
Atmel-8155I-ATmega32A_Datasheet_Complete-08/2016
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