Functional Description
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The Standard Device Requests received by a USB peripheral device can be divided into three categories:
Zero Data Requests (in which all the information is included in the command; no additional data is
required), Write Requests (in which the command will be followed by additional data), and Read Requests
(in which the device is required to send data back to the host).
This section looks at the sequence of actions that the software must perform to process these different
types of device request.
NOTE: The Setup packet associated with any standard device request should include an 8-byte
command. Any setup packet containing a command field other than 8 bytes will be automatically rejected
by the controller.
16.3.8.1.1.1 Zero Data Requests: Peripheral Mode
Zero data requests have all their information included in the 8-byte command and require no additional
data to be transferred. Examples of Zero Data standard device requests are:
• SET_FEATURE
• CLEAR_FEATURE
• SET_ADDRESS
• SET_CONFIGURATION
• SET_INTERFACE
The sequence of events will begin, as with all requests, when the software receives an endpoint 0
interrupt. The RXPKTRDY bit of PERI_CSR0 (bit 0) will also have been set. The 8-byte command should
then be read from the endpoint 0 FIFO, decoded and the appropriate action taken.
For example, if the command is SET_ADDRESS, the 7-bit address value contained in the command
should be written to the FADDR register at the completion of the command. The PERI_CSR0 register
should be written by setting the SERV_RXPKTRDY bit (bit 6) (indicating that the command has been read
from the FIFO) and also setting the DATAEND bit (bit 3) (indicating that no further data is expected for this
request). The interval between setting SERV_RXPKTRDY bit and DATAEND bit should be very small to
avoid getting a SETUPEND error condition. It is highly recommended to set both bits at the same time.
When the host moves to the status stage of the request, a second endpoint 0 interrupt will be generated to
indicate that the request has completed. No further action is required from the software. The second
interrupt is just a confirmation that the request completed successfully. For SET_ADDRESS command,
the address should be written to FADDR register at the completion of the command, i.e. when the status
stage interrupt is received.
If the command is an unrecognized command, or for some other reason cannot be executed, then when it
has been decoded, the PERI_CSR0 register should be written to set the SERV_RXPKTRDY bit (bit 6) and
to set the SENDSTALL bit (bit 5). When the host moves to the status stage of the request, the controller
will send a STALL packet telling the host that the request was not executed. A second endpoint 0 interrupt
will be generated and the SENTSTALL bit (bit 2 of PERI_CSR0) will be set.
If the host sends more data after the DATAEND bit has been set, then the controller will send a STALL
packet automatically. An endpoint 0 interrupt will be generated and the SENTSTALL bit (bit 2 of
PERI_CSR0) will be set.
NOTE: DMA is not supported for endpoint 0; endpoint 0 is always serviced via CPU.
16.3.8.1.1.2 Write Requests: Peripheral Mode
Write requests involve an additional packet (or packets) of data being sent from the host after the 8-byte
command. An example of a Write standard device request is: SET_DESCRIPTOR.
The sequence of events will begin, as with all requests, when the software receives an endpoint 0
interrupt. The RXPKTRDY bit of PERI_CSR0 will also have been set. The 8-byte command should then
be read from the Endpoint 0 FIFO and decoded.
As with a zero data request, the PERI_CSR0 register should then be written to set the SERV_RXPKTRDY
bit (bit 6) (indicating that the command has been read from the FIFO) but in this case the DATAEND bit
(bit 3) should not be set (indicating that more data is expected).
1702
Universal Serial Bus (USB) SPRUH73H–October 2011–Revised April 2013
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