114
ATmega16M1/32M1/64M1 [DATASHEET]
8209E–AVR–11/2012
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 15-9. The figure shows phase and fre-
quency 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 15-9. 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 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 15-9 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 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 gen-
erated by setting 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
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)