ATmega16U4 datasheet(144/408 Pages) ATMEL

144 / 408 page

144

7766D–AVR–01/09

ATmega16U4/ATmega32U4

15.4

Output Compare Unit

The comparator continuously compares TCNT4 with the Output Compare Registers (OCR4A,

OCR4B, OCR4C and OCR4D). Whenever TCNT4 equals to the Output Compare Register, the

comparator signals a match. A match will set the Output Compare Flag (OCF4A, OCF4B or

OCF4D) at the next timer clock cycle. If the corresponding interrupt is enabled, the Output Com-

pare Flag generates an Output Compare interrupt. The Output Compare Flag is automatically

cleared when the interrupt is executed. Alternatively, the flag can be cleared by software by writ-

ing a logical one to its I/O bit location. The Waveform Generator uses the match signal to

generate an output according to operating mode set by the PWM4x, WGM40 and Compare Out-

put mode (COM4x1:0) bits. The top and bottom signals are used by the Waveform Generator for

handling the special cases of the extreme values in some modes of operation (See “Modes of

Operation” on page 150.). Figure 15-4 shows a block diagram of the Output Compare unit.

Figure 15-4. Output Compare Unit, Block Diagram

The OCR4x Registers are double buffered when using any of the Pulse Width Modulation

(PWM) modes. For the normal mode of operation, the double buffering is disabled. The double

buffering synchronizes the update of the OCR4x Compare Registers to either top or bottom of

the counting sequence. The synchronization prevents the occurrence of odd-length, non-sym-

metrical PWM pulses, thereby making the output glitch-free. See Figure 15-5 for an example.

During the time between the write and the update operation, a read from OCR4A, OCR4B,

OCR4C or OCR4D will read the contents of the temporary location. This means that the most

recently written value always will read out of OCR4A, OCR4B, OCR4C or OCR4D.

OCFnx (Int.Req.)

= (10-bit Comparator )

8-BIT DATA BUS

TCNTn

WGMn0

Waveform Generator

COMnX1:0

PWMnx

TCnH

OCWnx

10-BIT TCNTn

10-BIT OCRnx

OCRnx

FOCn

TOP

BOTTOM

ATmega16U4 Datasheet(PDF) 144 Page - ATMEL Corporation