LM3S5C36 Datasheet, PDF (802/1068 Pages) Texas Instruments – Stellaris® LM3S5C36 Microcontroller

Universal Serial Bus (USB) Controller
must be set manually. When the TXRDY bit is set, either manually or automatically, the packet is
ready to be sent. After the first packet is loaded, TXRDY is immediately cleared and an interrupt is
generated. A second packet can now be loaded into the transmit FIFO and TXRDY set again (either
manually or automatically if the packet is the maximum size). At this point, both packets are ready
to be sent. After each packet has been successfully sent, TXRDY is automatically cleared and the
appropriate transmit endpoint interrupt signaled to indicate that another packet can now be loaded
into the transmit FIFO. The state of the FIFONE bit in the USBTXCSRLn register at this point
indicates how many packets may be loaded. If the FIFONE bit is set, then another packet is in the
FIFO and only one more packet can be loaded. If the FIFONE bit is clear, then no packets are in
the FIFO and two more packets can be loaded.
Note: Double-packet buffering is disabled if an endpoint’s corresponding EPn bit is set in the USB
Transmit Double Packet Buffer Disable (USBTXDPKTBUFDIS) register. This bit is set
by default, so it must be cleared to enable double-packet buffering.
17.3.1.3
OUT Transactions
OUT transactions are handled through the USB controller receive FIFOs. The sizes of the receive
FIFOs for the 15 configurable OUT endpoints are determined by the USB Receive FIFO Start
Address (USBRXFIFOADD) register. The maximum amount of data received by an endpoint in
any packet is determined by the value written to the USB Maximum Receive Data Endpoint n
(USBRXMAXPn) register for that endpoint. When double-packet buffering is enabled, two data
packets can be buffered in the FIFO. When double-packet buffering is disabled, only one packet
can be buffered even if the packet is less than half the FIFO size.
Note: In all cases, the maximum packet size must not exceed the FIFO size.
Single-Packet Buffering
If the size of the receive endpoint FIFO is less than twice the maximum packet size for an endpoint,
only one data packet can be buffered in the FIFO and single-packet buffering is required. When a
packet is received and placed in the receive FIFO, the RXRDY and FULL bits in the USB Receive
Control and Status Endpoint n Low (USBRXCSRLn) register are set and the appropriate receive
endpoint is signaled, indicating that a packet can now be unloaded from the FIFO. After the packet
has been unloaded, the RXRDY bit must be cleared in order to allow further packets to be received.
This action also generates the acknowledge signaling to the Host controller. If the AUTOCL bit in the
USB Receive Control and Status Endpoint n High (USBRXCSRHn) register is set and a
maximum-sized packet is unloaded from the FIFO, the RXRDY and FULL bits are cleared
automatically. For packet sizes less than the maximum, RXRDY must be cleared manually.
Double-Packet Buffering
If the size of the receive endpoint FIFO is at least twice the maximum packet size for the endpoint,
two data packets can be buffered and double-packet buffering can be used. When the first packet
is received and loaded into the receive FIFO, the RXRDY bit in the USBRXCSRLn register is set
and the appropriate receive endpoint interrupt is signaled to indicate that a packet can now be
unloaded from the FIFO.
Note: The FULL bit in USBRXCSRLn is not set when the first packet is received. It is only set if
a second packet is received and loaded into the receive FIFO.
After each packet has been unloaded, the RXRDY bit must be cleared to allow further packets to be
received. If the AUTOCL bit in the USBRXCSRHn register is set and a maximum-sized packet is
unloaded from the FIFO, the RXRDY bit is cleared automatically. For packet sizes less than the
maximum, RXRDY must be cleared manually. If the FULL bit is set when RXRDY is cleared, the USB
802
January 23, 2012
Texas Instruments-Production Data