DOC7707 datasheet(187/307 Pages) ETC1

187 / 307 page

187

7707F–AVR–11/10

AT90USB82/162

19.3.3

Design guidelines

• Serial resistors on USB Data lines must have 22 Ohms value (+/- 5%).

• Traces from the input USB receptable (or from the cable connection in the case of a tethered

device) to the USB microcontroller pads should be as short as possibles, and follow

differential traces routing rules (same length, as near as possible, avoid vias accumulation).

• Voltage transcient / ESD suppressors may also be used to prevent USB pads to be damaged

by external disturbances.

•U

cap capacitor should be 1µF (+/- 10%) for correct operation.

• A 10µF capacitor is highly recommended on VBUS line

19.4

General Operating Modes

19.4.1

Introduction

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

– Power on reset

– External reset

– Watchdog reset

– Brown out reset

– debugWIRE reset

But another available and optionnal reset source is :

– USB End Of Reset

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

19.4.2

Power-on and reset

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

Figure 19-7. USB controller states after reset

When the USB controller is in reset state:

• USBE is not set

Dev ice

Reset

USBE=0

<anyother

state>

USBE=1

Clockstopped

FRZCLK=1

Macrooff

USBE=0

HWRESET

(exceptedfromEOR)

HW RESET from EOR

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