Datasheet

591
SAM9261 [DATASHEET]
Atmel-6062O-ATARM-SAM9261-Datasheet_21-Jun-16
36.6.2 Handling Transactions with USB V2.0 Device Peripheral
36.6.2.1 Setup Transaction
Setup is a special type of host-to-device transaction used during control transfers. Control transfers must be
performed using endpoints with no ping-pong attributes. A setup transaction needs to be handled as soon as
possible by the firmware. It is used to transmit requests from the host to the device. These requests are then
handled by the USB device and may require more arguments. The arguments are sent to the device by a Data
OUT transaction which follows the setup transaction. These requests may also return data. The data is carried out
to the host by the next Data IN transaction which follows the setup transaction. A status transaction ends the
control transfer.
When a setup transfer is received by the USB endpoint:
 The USB device automatically acknowledges the setup packet?
 RXSETUP is set in the UDP_CSRx
 An endpoint interrupt is generated while the RXSETUP is not cleared. This interrupt is carried out to the
microcontroller if interrupts are enabled for this endpoint.
Thus, firmware must detect the RXSETUP polling the UDP_CSRx or catching an interrupt, read the setup packet
in the FIFO, then clear the RXSETUP. RXSETUP cannot be cleared before the setup packet has been read in the
FIFO. Otherwise, the USB device would accept the next Data OUT transfer and overwrite the setup packet in the
FIFO.
Figure 36-5. Setup Transaction Followed by a Data OUT Transaction
RX_DATA_BK0
(UDP_CSRx)
ACK
PID
Data OUT
Data OUT
PID
NAK
PID
ACK
PID
Data Setup
Setup
PID
USB
Bus Packets
RXSETUP Flag
Set by USB Device Cleared by Firmware
Set by USB
Device Peripheral
FIFO (DPR)
Content
Data Setup
Data
XX XX
OUT
Interrupt Pending
Setup Received Setup Handled by Firmware Data Out Received
Data OUT
Data OUT
PID