AT90PWM2 datasheet(120/365 Pages) ATMEL

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4317K–AVR–03/2013

AT90PWM2/3/2B/3B

Figure 15-8. Phase Correct PWM Mode, Timing Diagram

The Timer/Counter Overflow Flag (TOVn) is set each time the counter reaches BOTTOM. When

either OCRnA or ICRn is used for defining the TOP value, the OCnA or ICFn Flag is set accord-

ingly at the same timer clock cycle as the OCRnx Registers are updated with the double buffer

value (at TOP). The Interrupt Flags can be used to generate an interrupt each time the counter

reaches the TOP or BOTTOM value.

When changing the TOP value the program must ensure that the new TOP value is higher or

equal to the value of all of the Compare Registers. If the TOP value is lower than any of the

Compare Registers, a compare match will never occur between the TCNTn and the OCRnx.

Note that when using fixed TOP values, the unused bits are masked to zero when any of the

OCRnx Registers are written. As the third period shown in Figure 15-8 illustrates, changing the

TOP actively while the Timer/Counter is running in the phase correct mode can result in an

unsymmetrical output. The reason for this can be found in the time of update of the OCRnx Reg-

ister. Since the OCRnx update occurs at TOP, the PWM period starts and ends at TOP. This

implies that the length of the falling slope is determined by the previous TOP value, while the

length of the rising slope is determined by the new TOP value. When these two values differ the

two slopes of the period will differ in length. The difference in length gives the unsymmetrical

result on the output.

It is recommended to use the phase and frequency correct mode instead of the phase correct

mode when changing the TOP value while the Timer/Counter is running. When using a static

TOP value there are practically no differences between the two modes of operation.

In phase correct PWM mode, the compare units allow generation of PWM waveforms on the

OCnx pins. Setting the COMnx1:0 bits to two will produce a non-inverted PWM and an inverted

PWM output can be generated by setting the COMnx1:0 to three (See Table on page 125). The

actual OCnx value will only be visible on the port pin if the data direction for the port pin is set as

output (DDR_OCnx). The PWM waveform is generated by setting (or clearing) the OCnx Regis-

ter at the compare match between OCRnx and TCNTn when the counter increments, and

clearing (or setting) the OCnx Register at compare match between OCRnx and TCNTn when

OCRnx/TOP Update and

OCnA Interrupt Flag Set

or ICFn Interrupt Flag Set

(Interrupt on TOP)

TOVn Interrupt Flag Set

(Interrupt on Bottom)

TCNTn

Period

OCnx

(COMnx1:0 = 2)

(COMnx1:0 = 3)

AT90PWM2 Datasheet(PDF) 120 Page - ATMEL Corporation

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