ATtiny87-15XZ datasheet(96/282 Pages) ATMEL

96 / 282 page

7728D–AVR–07/09

ATtiny87/ATtiny167

Figure 10-7. 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 BOT-

TOM 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 10-3 on page

101). 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 Regis-

ter 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 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 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.

TOVn Interrupt Flag Set

OCnx Interrupt Flag Set

TCNTn

Period

OCnx

(COMnx1:0 = 2)

(COMnx1:0 = 3)

OCRnx Update

fOCnxPCPWM

clk_I/O

510

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