89
ATmega8A [DATASHEET]
8159E–AVR–02/2013
The N variable represents the prescaler divider (1, 8, 64, 256, or 1024).
The extreme values for the OCR1x Register represent 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 continuously high for non-inverted PWM mode. For inverted PWM the output will
have the opposite logic values.
If OCR1A is used to define the TOP value (WMG13:0 = 11) and COM1A1:0 = 1, the OC1A output will toggle with a
50% duty cycle.
17.9.5
Phase and Frequency Correct PWM Mode
The phase and frequency correct Pulse Width Modulation, or phase and frequency correct PWM mode (WGM13:0
= 8 or 9) provides a high resolution phase and frequency correct PWM waveform generation option. The phase
and frequency correct PWM mode is, like the phase correct PWM mode, based on a dual-slope operation. The
counter counts repeatedly from BOTTOM (0x0000) to TOP and then from TOP to BOTTOM. In non-inverting Com-
pare Output mode, the Output Compare (OC1x) is cleared on the Compare Match between TCNT1 and OCR1x
while upcounting, and set on the Compare Match while downcounting. In inverting Compare Output mode, the
operation is inverted. The dual-slope operation gives a lower maximum operation frequency compared to the sin-
gle-slope operation. However, due to the symmetric feature of the dual-slope PWM modes, these modes are
preferred for motor control applications.
The main difference between the phase correct, and the phase and frequency correct PWM mode is the time the
The PWM resolution for the phase and frequency correct PWM mode can be defined by either ICR1 or OCR1A.
The minimum resolution allowed is 2-bit (ICR1 or OCR1A set to 0x0003), and the maximum resolution is 16-bit
(ICR1 or OCR1A set to MAX). The PWM resolution in bits can be calculated using the following equation:
In phase and frequency correct PWM mode the counter is incremented until the counter value matches either the
value in ICR1 (WGM13:0 = 8), or the value in OCR1A (WGM13:0 = 9). The counter has then reached the TOP and
changes the count direction. The TCNT1 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 17-9. The figure shows phase and
frequency correct PWM mode when OCR1A or ICR1 is used to define TOP. The TCNT1 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 TCNT1 slopes represent compare matches between
OCR1x and TCNT1. The OC1x Interrupt Flag will be set when a Compare Match occurs.
RPFCPWM
TOP 1
+
log
2
log
-----------------------------------
=