ATmega16U4 datasheet(258/408 Pages) ATMEL

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258

7766D–AVR–01/09

ATmega16U4/ATmega32U4

21.4

General Operating Modes

21.4.1

Introduction

The USB controller is disabled and reset after an hardware reset generated by:

– Power on reset

– External reset

– Watchdog reset

– Brown out reset

– JTAG reset

But another available and optional CPU reset source is:

– USB End Of Reset

In this case, the USB controller is reset, but not disabled (so that the device remains attached).

21.4.2

Power-on and reset

The next diagram explains the USB controller main states on power-on:

Figure 21-7. USB controller states after reset

USB Controller state after an hardware reset is ‘Reset’. In this state:

• USBE is not set

• the USB controller clock is stopped in order to minimize the power consumption

(FRZCLK=1),

• the USB controller is disabled,

• the USB pad is in the suspend mode,

• the Device USB controller internal state is reset.

After setting USBE, the USB Controller enters the Device state. The controller is ‘Idle’.

The USB Controller can at any time be stopped by clearing USBE. In fact, clearing USBE acts

as an hardware reset.

21.4.3

Interrupts

Two interrupts vectors are assigned to USB interface.

Dev ice

Reset

USBE=0

<any other

state>

USBE=1

Clock stopped

FRZCLK=1

M acro off

USBE=0

RESET

ATmega16U4 Datasheet(PDF) 258 Page - ATMEL Corporation

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