Atmega128rfa1 – Rainbow Electronics ATmega128RFA1 User Manual
Page 315
315
8266A-MCU Wireless-12/09
ATmega128RFA1
includes non-inverted and inverted PWM outputs. The small horizontal line marks on
the TCNT2 slopes represent compare matches between OCR2x and TCNT2.
Figure 21-5. Phase Correct PWM Mode, Timing Diagram
TOVn Interrupt Flag Set
OCnx Interrupt Flag Set
1
2
3
TCNTn
Period
OCnx
OCnx
(COMnx1:0 = 2)
(COMnx1:0 = 3)
OCRnx Update
The Timer/Counter Overflow Flag (TOV2) is set each time the counter reaches
BOTTOM. The Interrupt Flag can be used to generate an interrupt each time the
counter reaches the BOTTOM value.
In phase correct PWM mode, the compare unit allows generation of PWM waveforms
on the OC2x pin. Setting the COM2x1:0 bits to two will produce a non-inverted PWM.
An inverted PWM output can be generated by setting the COM2x1:0 to three. TOP is
defined as 0xFF when WGM22:0 = 3, and OCR2A when WGM22:0 = 7 (see section
"Register Description" on page 323
for register TCCR2A). The actual OC2x value will
only be visible on the port pin if the data direction for the port pin is set as output. The
PWM waveform is generated by clearing (or setting) the OC2x Register at the compare
match between OCR2x and TCNT2 when the counter increments, and setting (or
clearing) the OC2x Register at compare match between OCR2x and TCNT2 when the
counter decrements. The PWM frequency for the output when using phase correct
PWM can be calculated by the following equation:
510
/
_
⋅
=
N
f
f
O
I
clk
OCnxPCPWM
The N variable represents the pre-scale factor (1, 8, 32, 64, 128, 256, or 1024).
The extreme values for the OCR2A Register represent special cases when generating
a PWM waveform output in the phase correct PWM mode. If the OCR2A is set equal to
BOTTOM, the output will be continuously low and if set equal to MAX the output will be
continuously high for non-inverted PWM mode. For inverted PWM the output will have
the opposite logic values.
At the very start of period 2 in
OCnx has a transition from high to low
even though there is no Compare Match. The point of this transition is to guarantee
symmetry around BOTTOM. There are two cases that give a transition without
Compare Match.