ATmega325P datasheet(98/364 Pages) ATMEL

98 / 364 page

8023F–AVR–07/09

ATmega325P/3250P

Figure 14-7. Phase Correct PWM Mode, Timing Diagram

The Timer/Counter Overflow Flag (TOV0) 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

OC0A pin. Setting the COM0A1:0 bits to two will produce a non-inverted PWM. An inverted

PWM output can be generated by setting the COM0A1:0 to three (See Table 14-5 on page 103).

The actual OC0A 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 OC0A Register at the

compare match between OCR0A and TCNT0 when the counter increments, and setting (or

clearing) the OC0A Register at compare match between OCR0A and TCNT0 when the counter

decrements. The PWM frequency for the output when using phase correct PWM can be calcu-

lated by the following equation:

The N variable represents the prescale factor (1, 8, 64, 256, or 1024).

The extreme values for the OCR0A Register represent special cases when generating a PWM

waveform output in the phase correct PWM mode. If the OCR0A 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 14-7 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 BOT-

TOM. There are two cases that give a transition without Compare Match.

• OCR0A changes its value from MAX, like in Figure 14-7. When the OCR0A value is MAX the

OCn pin value is the same as the result of a down-counting Compare Match. To ensure

TOVn Interrupt Flag Set

OCnx Interrupt Flag Set

TCNTn

Period

OCn

(COMnx1:0 = 2)

(COMnx1:0 = 3)

OCRnx Update

fOCnxPCPWM

clk_I/O

N 510

⋅

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