Datasheet

259
ATmega16U4/32U4 [DATASHEET]
Atmel-7766J-USB-ATmega16U4/32U4-Datasheet_04/2016
Figure 21-6. Typical Self Powered Application with 3.0V to 3.6 I/O
21.4 Crystal-less Operation
To reduce external components count and BOM cost, the USB module can be configured to operate in low-
speed mode with internal RC oscillator as input source clock for the PLL. The internal RC oscillator is factory
calibrated to satisfy the USB low speed frequency accuracy within the 0°C and +40°C temperature range.
For USB full-speed operation only external crystal oscillator or external source clock can be used.
21.5 Design Guidelines
 Serial resistors on USB Data lines must have 22 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 possible, and follow differential traces routing rules
(same length, as near as possible, avoid via 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
 A 10µF capacitor is highly recommended on VBUS line
1µF
External 3.0V - 3.6V
Power Supply
UDP
UDM
VBUS
UVSS
UID
UCAP
D-
D+
VBUS
UID
UGND
UVCC AVCC
VCC
XTAL1 XTAL2 GND GND
Rs=22
Rs=22