ATMEGA325-16AJ ATMEL [ATMEL Corporation], ATMEGA325-16AJ Datasheet - Page 132

• Manufacturer Part Number: ATMEGA325-16AJ
• Description: 8-bit Microcontroller with In-System Programmable Flash
• Manufacturer: ATMEL [ATMEL Corporation]

132
ATmega325/3250/645/6450
Figure 59. Phase Correct PWM Mode, Timing Diagram
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 62
on page 136). 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:
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.
At the very start of period 2 in Figure 59 OCn 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.
•
TCNTn
OCnx
OCnx
Period
OCR2A changes its value from MAX, like in Figure 59. When the OCR2A value is
MAX the OCn pin value is the same as the result of a down-counting compare
match. To ensure symmetry around BOTTOM the OCn value at MAX must
correspond to the result of an up-counting Compare Match.
1
f
OCnxPCPWM
2
=
----------------- -
N 510
f
clk_I/O
⋅
3
OCnx Interrupt Flag Set
OCRnx Update
TOVn Interrupt Flag Set
(COMnx1:0 = 2)
(COMnx1:0 = 3)
2570A–AVR–09/04