ATmega16U4 datasheet(152/408 Pages) ATMEL

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152

7766D–AVR–01/09

ATmega16U4/ATmega32U4

The Output Compare Pin configurations in Normal Mode are described in Table 15-2.

15.8.2

Fast PWM Mode

The fast Pulse Width Modulation or fast PWM mode (PWM4x = 1 and WGM40 = 0) provides a

high frequency PWM waveform generation option. The fast PWM differs from the other PWM

option by its single-slope operation. The counter counts from BOTTOM to TOP (defined as

OCR4C) then restarts from BOTTOM. In non-inverting Compare Output mode the Waveform

Output (OCW4x) is cleared on the Compare Match between TCNT4 and OCR4x and set at

BOTTOM. In inverting Compare Output mode, the Waveform Output is set on Compare Match

and cleared at BOTTOM. In complementary Compare Output mode the Waveform Output is

cleared on the Compare Match and set at BOTTOM.

Due to the single-slope operation, the operating frequency of the fast PWM mode can be twice

as high as the Phase and Frequency Correct PWM mode that use dual-slope operation. This

high frequency makes the fast PWM mode well suited for power regulation, rectification, and

DAC applications. High frequency allows physically small sized external components (coils,

capacitors), and therefore reduces total system cost.

The timing diagram for the fast PWM mode is shown in Figure 15-13. The counter is incre-

mented until the counter value matches the TOP value. The counter is then cleared at the

following timer clock cycle. The TCNT4 value is in the timing diagram shown as a histogram for

illustrating the single-slope operation. The diagram includes the Waveform Output in non-

inverted and inverted Compare Output modes. The small horizontal line marks on the TCNT4

slopes represent Compare Matches between OCR4x and TCNT4.

Figure 15-13. Fast PWM Mode, Timing Diagram

Table 15-2.

Output Compare Pin Configurations in Normal Mode

COM4x1

COM4x0

OC4x Pin

Disconnected

OC4x

Disconnected

OC4x

Disconnected

OC4x

TCNTn

OCRnx Update and

TOVn Interrupt Flag Set

Period

OCWnx

(COMnx1:0 = 2)

(COMnx1:0 = 3)

OCRnx Interrupt Flag Set

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