ATmega16U4 datasheet(151/408 Pages) ATMEL

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7766D–AVR–01/09

ATmega16U4/ATmega32U4

Waveform Output is cleared on the Compare Match. In inverting Compare Output Mode the

Waveform Output is set on Compare Match.

The timing diagram for the Normal mode is shown in Figure 15-12. The counter value (TCNT4)

that is shown as a histogram in the timing diagram is incremented until the counter value

matches the TOP value. The counter is then cleared at the following clock cycle The diagram

includes the Waveform Output (OCW4x) in toggle Compare Mode. The small horizontal line

marks on the TCNT4 slopes represent Compare Matches between OCR4x and TCNT4.

Figure 15-12. Normal Mode, Timing Diagram

The Timer/Counter Overflow Flag (TOV4) is set in the same clock cycle as the TCNT4 becomes

zero. The TOV4 Flag in this case behaves like a 11th bit, except that it is only set, not cleared.

However, combined with the timer overflow interrupt, that automatically clears the TOV4 Flag,

the timer resolution can be increased by software. There are no special cases to consider in the

Normal mode, a new counter value can be written anytime.

The Output Compare Unit can be used to generate interrupts at some given time. Using the Out-

put Compare to generate waveforms in Normal mode is not recommended, since this will

occupy too much of the CPU time. For generating a waveform, the OCW4x output can be set to

toggle its logical level on each Compare Match by setting the Compare Output mode bits to tog-

gle mode (COM4x1:0 = 1). The OC4x value will not be visible on the port pin unless the data

direction for the pin is set to output. The waveform generated will have a maximum frequency of

OC4x = fclkT4/4 when OCR4C is set to zero. The waveform frequency is defined by the following

equation:

Resolution shows how many bit is required to express the value in the OCR4C register. It is cal-

culated by following equation:

Resolution

PWM = log2(OCR4C + 1).

TCNTn

OCWnx

(COMnx=1)

OCnx Interrupt Flag Set

Period

TOVn Interrupt Flag Set

OC4x

clkT4

OCR4C

()

⋅

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