Datasheet

ATmega164A/PA/324A/PA/644A/PA/1284/P
 2018 Microchip Technology Inc. Data Sheet Complete DS40002070A-page 131
Figure 16-8. Phase Correct PWM mode, timing diagram
The Timer/Counter Overflow Flag (TOVn) is set each time the counter reaches BOTTOM. When either OCRnA
or ICRn is used for defining the TOP value, the OCnA or ICFn Flag is set accordingly at the same timer clock
cycle as the OCRnx Registers are updated with the double buffer value (at TOP). The Interrupt Flags can 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. Note that when using fixed TOP values, the unused bits
are masked to zero when any of the OCRnx Registers are written. As the third period shown in Figure 16-8
illustrates, changing the TOP actively while the Timer/Counter is running in the phase correct mode can result in
an unsymmetrical 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
changing 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 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 137). 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 correct PWM can be calculated
by the following equation:
The N variable represents the prescaler divider (1, 8, 64, 256, or 1024).
OCRnx/TOP Update and
OCnA Interrupt Flag Set
or ICFn Interrupt Flag Set
(Interrupt on TOP)
1 2 3 4
TOVn Interrupt Flag Set
(Interrupt on Bottom)
TCNTn
Period
OCnx
OCnx
(COMnx1:0 = 2)
(COMnx1:0 = 3)
f
OCnxPCPWM
f
clk_I/O
2 NTOP
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