Atmel ARM-Based Evaluation Kit for SAM4S16C, 32-Bit ARM® Cortex® Microcontroller ATSAM4S-WPIR-RD ATSAM4S-WPIR-RD Data Sheet

SAM4S Series [DATASHEET]

Atmel-11100G-ATARM-SAM4S-Datasheet_27-May-14

1014

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