Datasheet

270
ATmega16U4/32U4 [DATASHEET]
Atmel-7766J-USB-ATmega16U4/32U4-Datasheet_04/2016
22. USB Device Operating Modes
22.1 Introduction
The USB device controller supports full speed and low speed data transfers. In addition to the default control
endpoint, it provides six other endpoints, which can be configured in control, bulk, interrupt or isochronous
modes:
 Endpoint 0:programmable size FIFO up to 64 bytes, default control endpoint
 Endpoints 1 programmable size FIFO up to 256 bytes in ping-pong mode
 Endpoints 2 to 6: programmable size FIFO up to 64 bytes in ping-pong mode
The controller starts in the “idle” mode. In this mode, the pad consumption is reduced to the minimum.
22.2 Power-on and Reset
The next diagram explains the USB device controller main states on power-on:
Figure 22-1. USB Device Controller States after Reset
The reset state of the Device controller is:
 the macro clock is stopped in order to minimize the power consumption (FRZCLK set)
 the USB device controller internal state is reset (all the registers are reset to their default value. Note that
DETACH is set.)
 the endpoint banks are reset
 the D+ or D- pull up are not activated (mode Detach)
The D+ or D- pull-up will be activated as soon as the DETACH bit is cleared and VBUS is present.
The macro is in the ‘Idle’ state after reset
with a minimum power consumption and does not need to have the
PLL activated to enter this state.
The USB device controller can at any time be reset by clearing USBE (disable USB interface).
22.3 Endpoint Reset
An endpoint can be reset at any time by setting in the UERST register the bit corresponding to the endpoint
(EPRSTx). This resets:
 the internal state machine on that endpoint
 the Rx and Tx banks are cleared and their internal pointers are restored
 the UEINTX, UESTA0X and UESTA1X are restored to their reset value
The data toggle field remains unchanged.
The other registers remain unchanged.
The endpoint configuration remains active and the endpoint is still enabled.
Reset
Idle
HW
RESET
USBE=0
<any
other
state>
USBE=0
USBE=1