ATmega32U2 datasheet(189/314 Pages) ATMEL

189 / 314 page

189

7799B–AVR–07/09

ATmega8U2/16U2/32U2

20.3.3

Design guidelines

The following design guidelines should be met:

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

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

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

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

• Voltage transient / 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.

In addition it is highly recommended to connect a 10µF capacitor to the VBUS line

20.4

General Operating Modes

20.4.1

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

– USB End Of Reset

In the case of USB End Of Reset (EOR), the USB controller is reset, but not disabled. Therefore

the device remains attached.

20.4.2

Power-on and reset

Figure 20-7 on page 189 illustrates the USB controller main states on power-on:

Figure 20-7. USB controller states after reset

Reset

Device

Any other

state

USBE = 0

USBE = 1

HW RESET

(except from EOR)

USBE = 0

HW RESET

(from EOR)

Clock stopped

FRZCLK = 1

(macro off)

ATmega32U2 Datasheet(PDF) 189 Page - ATMEL Corporation

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