ATmega324P datasheet(149/377 Pages) ATMEL

149 / 377 page

149

7674F–AVR–09/09

ATmega164P/324P/644P

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 fol-

lowing 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 15-7 on page 148 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.

• OCR2A changes its value from MAX, like in Figure 15-7 on page 148. 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.

• The timer starts counting from a value higher than the one in OCR2A, and for that reason

misses the Compare Match and hence the OCn change that would have happened on the way

up.

15.8

Timer/Counter Timing Diagrams

The following figures show the Timer/Counter in synchronous mode, and the timer clock (clk

T2)

is therefore shown as a clock enable signal. In asynchronous mode, clk

I/O should be replaced by

the Timer/Counter Oscillator clock. The figures include information on when Interrupt Flags are

set. Figure 15-8 on page 149 contains timing data for basic Timer/Counter operation. The figure

shows the count sequence close to the MAX value in all modes other than phase correct PWM

mode.

Figure 15-8. Timer/Counter Timing Diagram, no Prescaling

fOCnxPCPWM

clk_I/O

510

⋅

---------------------

clk

(clk

I/O/1)

TOVn

clk

I/O

TCNTn

MAX - 1

MAX

BOTTOM

BOTTOM + 1

ATmega324P Datasheet(PDF) 149 Page - ATMEL Corporation

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