Datasheet

1000
SAM4S Series [DATASHEET]
11100F–ATARM–29-Jan-14
 A functional stall is used when the halt feature associated with the endpoint is set. (Refer to Chapter 9 of the
Universal Serial Bus Specification, Rev 2.0, for more information on the halt feature.)
 To abort the current request, a protocol stall is used, but uniquely with control transfer.
The following procedure generates a stall packet:
1. The microcontroller sets the FORCESTALL flag in the UDP_CSRx endpoint’s register.
2. The host receives the stall packet.
3. The microcontroller is notified that the device has sent the stall by polling the STALLSENT to be set. An endpoint
interrupt is pending while STALLSENT is set. The microcontroller must clear STALLSENT to clear the interrupt.
When a setup transaction is received after a stall handshake, STALLSENT must be cleared in order to prevent interrupts
due to STALLSENT being set.
Figure 40-12. Stall Handshake (Data IN Transfer)
Figure 40-13. Stall Handshake (Data OUT Transfer)
Data IN Stall PIDPID
USB Bus
Packets
Cleared by Firmware
Set by Firmware
FORCESTALL
STALLSENT
Set by
USB Device
Cleared by Firmware
Interrupt Pending
Data OUT PID
Stall PID
Data OUT
USB Bus
Packets
Cleared by Firmware
Set by Firmware
FORCESTALL
STALLSENT
Set by USB Device
Interrupt Pending