Datasheet

257
7766F–AVR–11/10
ATmega16/32U4
21.6 General Operating Modes
21.6.1 Introduction
The USB controller is disabled and reset after an hardware reset generated by:
– Power on reset
– External reset
– Watchdog reset
– Brown out reset
– JTAG reset
But another available and optional CPU reset source is:
– USB End Of Reset
In this case, the USB controller is reset, but not disabled (so that the device remains attached).
21.6.2 Power-on and reset
The next diagram explains the USB controller main states on power-on:
Figure 21-7. USB controller states after reset
USB Controller state after an hardware reset is ‘Reset’. In this state:
• USBE is not set
• the USB controller clock is stopped in order to minimize the power consumption
(FRZCLK=1),
• the USB controller is disabled,
• the USB pad is in the suspend mode,
• the Device USB controller internal state is reset.
After setting USBE, the USB Controller enters the Device state. The controller is ‘Idle’.
The USB Controller can at any time be stopped by clearing USBE. In fact, clearing USBE acts
as an hardware reset.
21.6.3 Interrupts
Two interrupts vectors are assigned to USB interface.
Device
Reset
USBE=0
<any other
state>
USBE=1
Clock stopped
FRZCLK=1
M acr o off
USBE=0
USBE=0
HW
RESET