ATmega16U4 datasheet(103/408 Pages) ATMEL

103 / 408 page

103

7766D–AVR–01/09

ATmega16U4/ATmega32U4

Note:

1. A special case occurs when OCR0B equals TOP and COM0B1 is set. In this case, the Com-

pare Match is ignored, but the set or clear is done at TOP. See “Phase Correct PWM Mode” on

page 98 for more details.

• Bits 3, 2 – Res: Reserved Bits

These bits are reserved bits in the ATmega16U4/ATmega32U4 and will always read as zero.

• Bits 1:0 – WGM01:0: Waveform Generation Mode

Combined with the WGM02 bit found in the TCCR0B Register, these bits control the counting

sequence of the counter, the source for maximum (TOP) counter value, and what type of wave-

form generation to be used, see Table 13-7. Modes of operation supported by the Timer/Counter

unit are: Normal mode (counter), Clear Timer on Compare Match (CTC) mode, and two types of

Pulse Width Modulation (PWM) modes (see “Modes of Operation” on page 95).

Notes:

1. MAX

= 0xFF

2. BOTTOM = 0x00

13.8.2

Timer/Counter Control Register B – TCCR0B

• Bit 7 – FOC0A: Force Output Compare A

The FOC0A bit is only active when the WGM bits specify a non-PWM mode.

However, for ensuring compatibility with future devices, this bit must be set to zero when

TCCR0B is written when operating in PWM mode. When writing a logical one to the FOC0A bit,

an immediate Compare Match is forced on the Waveform Generation unit. The OC0A output is

changed according to its COM0A1:0 bits setting. Note that the FOC0A bit is implemented as a

strobe. Therefore it is the value present in the COM0A1:0 bits that determines the effect of the

forced compare.

Table 13-7.

Waveform Generation Mode Bit Description

Mode

WGM2

WGM1

WGM0

Timer/Counter

Mode of

Operation

TOP

Update of

OCRx at

TOV Flag

Set on

(1)(2)

Normal

0xFF

Immediate

MAX

PWM, Phase

Correct

0xFF

TOP

BOTTOM

CTC

OCRA

Immediate

MAX

Fast PWM

0xFF

TOP

MAX

Reserved

–

PWM, Phase

Correct

OCRA

TOP

BOTTOM

Reserved

–

Fast PWM

OCRA

TOP

Bit

543

FOC0A

FOC0B

–

WGM02

CS02

CS01

CS00

TCCR0B

Read/Write

R/W

Initial Value

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