Datasheet

994
SAM4S Series [DATASHEET]
11100F–ATARM–29-Jan-14
2. During the Status OUT stage, the host emits a zero length packet to the device (Data OUT transaction with no data).
40.6.2 Handling Transactions with USB 2.0 Device Peripheral
40.6.2.1 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 40-5. Setup Transaction Followed by a Data OUT Transaction
40.6.2.2 Data IN Transaction
Data IN transactions are used in control, isochronous, bulk and interrupt transfers and conduct the transfer of data from
the device to the host. Data IN transactions in isochronous transfer must be done using endpoints with ping-pong
attributes.
Using Endpoints Without Ping-pong Attributes
To perform a Data IN transaction using a non ping-pong endpoint:
1. The application checks if it is possible to write in the FIFO by polling TXPKTRDY in the endpoint’s UDP_CSRx
(TXPKTRDY must be cleared).
2. The application writes the first packet of data to be sent in the endpoint’s FIFO, writing zero or more byte values in
the endpoint’s UDP_FDRx.
3. The application notifies the USB peripheral it has finished by setting the TXPKTRDY in the endpoint’s UDP_CSRx.
RX_Data_BKO
(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