Datasheet

256
7766F–AVR–11/10
ATmega16/32U4
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 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 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