Datasheet
ATmega164A/PA/324A/PA/644A/PA/1284/P
2018 Microchip Technology Inc. Data Sheet Complete DS40002070A-page 132
The extreme values for the OCRnx Register represent special cases when generating a PWM waveform output
in the phase correct PWM mode. If the OCRnx is set equal to BOTTOM the output will be continuously low and
if set equal to TOP the output will be continuously high for non-inverted PWM mode. For inverted PWM the
output will have the opposite logic values. If OCR1A is used to define the TOP value (WGM13:0 = 11) and
COM1A1:0 = 1, the OC1A output will toggle with a 50% duty cycle.
16.10.5 Phase and Frequency Correct PWM mode
The phase and frequency correct Pulse Width Modulation, or phase and frequency correct PWM mode
(WGMn3:0 = 8 or 9) provides a high resolution phase and frequency correct PWM waveform generation option.
The phase and frequency correct PWM mode is, like the phase 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 Compare
Output mode, the operation is inverted. The dual-slope operation gives a lower maximum operation frequency
compared to the single-slope operation. However, due to the symmetric feature of the dual-slope PWM modes,
these modes are preferred for motor control applications.
The main difference between the phase correct, and the phase and frequency correct PWM mode is the time
the OCRnx Register is updated by the OCRnx Buffer Register, (see Figure 16-8 and Figure 16-9).
The PWM resolution for the phase and frequency correct PWM mode can be 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 using the following equation:
In phase and frequency correct PWM mode the counter is incremented until the counter value matches either
the value in ICRn (WGMn3:0 = 8), or the value in OCRnA (WGMn3:0 = 9). 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 and frequency correct PWM mode is shown on Figure 16-9. The figure
shows phase and frequency 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.
R
PFCPWM
TOP 1+log
2log
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