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 13-9. 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.
Figure 13-9. Phase and Frequency Correct PWM Mode, Timing Diagram
OCnA Interrupt Flag Set
or ICFn Interrupt Flag Set
(Interrupt on TOP)
OCRnx/ TOP Update and
TOVn Interrupt Flag Set
(Interrupt on Bottom)
TCNTn
(COMnx1:0 = 2)
OCnx
OCnx
(COMnx1:0 = 3)
1
2
3
4
Period
The Timer/Counter overflow flag (TOVn) is set at the same timer clock cycle as the OCRnx registers are updated with the
double buffer value (at BOTTOM). When either OCRnA or ICRn is used for defining the TOP value, the OCnA or ICFn flag is
set when TCNTn has reached TOP. The interrupt flags can then be used to generate an interrupt each time the counter
reaches the TOP or BOTTOM value.
When changing the TOP value the program must ensure that the new TOP value is higher or equal to the value of all of the
compare registers. If the TOP value is lower than any of the compare registers, a compare match will never occur between
the TCNTn and the OCRnx.
As Figure 13-9 shows the output generated is, in contrast to the phase correct mode, symmetrical in all periods. Since the
OCRnx registers are updated at BOTTOM, the length of the rising and the falling slopes will always be equal. This gives
symmetrical output pulses and is therefore frequency correct.
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 phase and frequency correct PWM mode, the compare units allow generation 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 setting
the COMnx1:0 to three (See Table on page 111). 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 decrements. The PWM frequency for the output
when using phase and frequency correct PWM can be calculated by the following equation:
f
clk_I/O
---------------------------
f
=
OCnxPFCPWM
2 N TOP
The N variable represents the prescaler divider (1, 8, 64, 256, or 1024).
ATmega16/32/64/M1/C1 [DATASHEET]
107
7647O–AVR–01/15
MICROCHIP [ MICROCHIP ]
