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ATmega64A [DATASHEET]
8160D–AVR–02/2013
Figure 16-9. Phase and Frequency Correct PWM Mode, Timing Diagram
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 set when TCNTn has reached TOP. The interrupt flags can then be used to generate an inter-
rupt 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 16-9 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 gen-
erated by setting the COMnx1:0 to three (See
Table 16-4 on page 127). 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 cal-
culated 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 set to high for non-inverted PWM mode. For inverted PWM the output will have the
OCRnx / TOP Update and
TOVn Interrupt Flag Set
(Interrupt on Bottom)
OCnA Interrupt Flag Set
or ICFn Interrupt Flag Set
(Interrupt on TOP)
1
2
3
4
TCNTn
Period
OCnx
(COMnx1:0 = 2)
(COMnx1:0 = 3)
fOCnxPFCPWM
f
clk_I/O
2
NTOP
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=