AT90CAN128
Figure 68. Phase Correct PWM Mode, Timing Diagram
OCnx Interrupt Flag Set
OCRnx Update
TOVn Interrupt Flag Set
TCNTn
(COMnx1:0 = 2)
(COMnx1:0 = 3)
OCnx
OCnx
1
2
3
Period
The Timer/Counter Overflow Flag (TOV2) is set each time the counter reaches BOT-
TOM. 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 OC2A pin. Setting the COM2A1:0 bits to two will produce a non-inverted PWM. An
inverted PWM output can be generated by setting the COM2A1:0 to three (See Table 69
on page 153). The actual OC2A value will only be visible on the port pin if the data direc-
tion for the port pin is set as output. The PWM waveform is generated by clearing (or
setting) the OC2A Register at the compare match between OCR2A and TCNT2 when
the counter increments, and setting (or clearing) the OC2A Register at compare match
between OCR2A and TCNT2 when the counter decrements. The PWM frequency for
the output when using phase correct PWM can be calculated by the following equation:
fclk_I/O
fOCnxPCPWM = -----------------
N ⋅ 510
The N variable represents the prescale 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.
149
4250E–CAN–12/04
ATMEL [ ATMEL ]
