105
Atmel ATmega16/32/64/M1/C1 [DATASHEET]
7647K–AVR–12/13
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 OCR1A is used to define the TOP value (WGM13:0 = 15).
The waveform generated will have a maximum frequency of fOCnA = fclk_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.
13.8.4 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:
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 on
Figure 13-8. 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.
Figure 13-8. Phase Correct PWM Mode, Timing Diagram
R
PCPWM
TOP
1
+
()
log
2
()
log
----------------------------------
=
12
3
4
TCNTn
(COMnx1:0 = 2)
(COMnx1:0 = 3)
OCnx
Period
TOVn Interrupt Flag Set
(Interrupt on Bottom)
OCRnx/ TOP Update and
OCnA Interrupt Flag Set
or ICFn Interrupt Flag Set
(Interrupt on TOP)