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154

7679H–CAN–08/08

AT90CAN32/64/128

Figure 14-8. Phase Correct PWM Mode, Timing Diagram

The Timer/Counter Overflow Flag (TOV2) 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

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 14-4 on page 159).

The actual OC2A 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 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 calcu-

lated 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 14-8 on page 154 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 14-8 on page 154. When the OCR2A

value is MAX the OCn pin value is the same as the result of a down-counting compare

TOVn Interrupt Flag Set

OCnx Interrupt Flag Set

1 2 3

TCNTn

Period

OCnx

(COMnx1:0 = 2)

(COMnx1:0 = 3)

OCRnx Update

OCnxPCPWM

clk_I/O

N 510⋅

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Atmel AVR AT90CAN32 User Manual

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