Datasheet

ATmega48A/PA/88A/PA/168A/PA/328/P
 2018 Microchip Technology Inc. Data Sheet Complete DS40002061A-page 137
Figure 16-9. Phase and Frequency Correct PWM Mode, Timing Diagram
The Timer/Counter Overflow Flag (TOV1) is set at the same timer clock cycle as the OCR1x Registers are
updated with the double buffer value (at BOTTOM). When either OCR1A or ICR1 is used for defining the TOP
value, the OC1A or ICF1 Flag set when TCNT1 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 TCNT1 and the OCR1x.
As Figure 16-9 on page 137 shows the output generated is, in contrast to the phase correct mode, symmetrical
in all periods. Since the OCR1x 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 ICR1 Register for defining TOP works well when using fixed TOP values. By using ICR1, the OCR1A
Register is free to be used for generating a PWM output on OC1A. However, if the base PWM frequency is
actively changed by changing the TOP value, using the OCR1A 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
OC1x pins. Setting the COM1x1:0 bits to two will produce a non-inverted PWM and an inverted PWM output can
be generated by setting the COM1x1:0 to three (See Table 16-3 on page 141). The actual OC1x value will only
be visible on the port pin if the data direction for the port pin is set as output (DDR_OC1x). The PWM waveform
is generated by setting (or clearing) the OC1x Register at the compare match between OCR1x and TCNT1
when the counter increments, and clearing (or setting) the OC1x Register at compare match between OCR1x
and TCNT1 when the counter decrements. The PWM frequency for the output when using phase and frequency
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 OCR1x Register represents special cases when generating a PWM waveform
output in the phase correct PWM mode. If the OCR1x 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
OCRnx/TOP Updateand
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
OCnx
(COMnx1:0 = 2)
(COMnx1:0 = 3)
f
OCnxPFCPWM
f
clk_I/O
2 NTOP
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