113
ATmega16M1/32M1/64M1 [DATASHEET]
8209E–AVR–11/2012
masked to zero when any of the OCRnx Registers are written. As the third period shown in
Figure 15-8 illustrates,
changing the TOP actively while the Timer/Counter is running in the phase correct mode can result in an unsym-
metrical output. The reason for this can be found in the time of update of the OCRnx Register. Since the OCRnx
update occurs at TOP, the PWM period starts and ends at TOP. This implies that the length of the falling slope is
determined by the previous TOP value, while the length of the rising slope is determined by the new TOP value.
When these two values differ the two slopes of the period will differ in length. The difference in length gives the
unsymmetrical result on the output.
It is recommended to use the phase and frequency correct mode instead of the phase correct mode when chang-
ing the TOP value while the Timer/Counter is running. When using a static TOP value there are practically no
differences between the two modes of operation.
In phase correct PWM mode, the compare units allow generating of PWM waveforms on the OCnx pins. Setting
the COMnx1:0 bits to two will produce a non-inverted PWM and an inverted PWM output can be generated by set-
ting the COMnx1:0 to three (see
Table 15-4 on page 118). 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 when the counter increments,
and clearing (or setting) the OCnx Register at compare match between OCRnx and TCNTn when the counter dec-
rements. The PWM frequency for the output when using phase correct PWM can be calculated by the following
equation:
The N variable represents the prescaler divider (1, 8, 64, 256, or 1024).
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.
15.9.5
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 generating 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 Com-
pare 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 sin-
gle-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
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
f
OCnxPCPWM
fclk_I/O
2 NTOP
----------------------------
=
R
PFCPWM
TOP
1
+
log
2
log
-----------------------------------
=