Atmel ARM-Based Evaluation Kit AT91SAM9N12-EK AT91SAM9N12-EK Data Sheet

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SAM9N12/SAM9CN11/SAM9CN12 [DATASHEET]

11063K–ATARM–05-Nov-13

A stall handshake can be used in one of two distinct occasions. (For more information on the stall handshake, refer to
Chapter 8 of the Universal Serial Bus Specification, Rev 2.0.)



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.

Data IN 

Stall PID

PID

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