ATmega88 datasheet(144/340 Pages) ATMEL

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7530H–AVR–02/09

ATmega48/88/168 Automotive

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

WGM2:0 = 3, and OCR2A when MGM2:0 = 7 (See Table 15-3 on page 147). 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

PWM frequency for the output when using phase correct PWM can be calculated by the follow-

ing 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 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 BOT-

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

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

15.7

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 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

ATmega88 Datasheet(PDF) 144 Page - ATMEL Corporation

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