LM3S9997_15 Datasheet, PDF (933/1337 Pages) Texas Instruments – Stellaris LM3S9997 Microcontroller

Stellaris^® LM3S9997 Microcontroller
Single-Packet Buffering
If the size of the transmit endpoint's FIFO is less than twice the maximum packet size for this endpoint
(as set in the USB Transmit Dynamic FIFO Sizing (USBTXFIFOSZ) register), only one packet
can be buffered in the FIFO and single-packet buffering is required. When each packet is completely
loaded into the transmit FIFO, the TXRDY bit in the USB Transmit Control and Status Endpoint
n Low (USBTXCSRLn) register must be set. If the AUTOSET bit in the USB Transmit Control and
Status Endpoint n High (USBTXCSRHn) register is set, the TXRDY bit is automatically set when
a maximum-sized packet is loaded into the FIFO. For packet sizes less than the maximum, the
TXRDY bit must be set manually. When the TXRDY bit is set, either manually or automatically, the
packet is ready to be sent. When the packet has been successfully sent, both TXRDY and FIFONE
are cleared, and the appropriate transmit endpoint interrupt signaled. At this point, the next packet
can be loaded into the FIFO.
Double-Packet Buffering
If the size of the transmit endpoint's FIFO is at least twice the maximum packet size for this endpoint,
two packets can be buffered in the FIFO and double-packet buffering is allowed. As each packet is
loaded into the transmit FIFO, the TXRDY bit in the USBTXCSRLn register must be set. If the
AUTOSET bit in the USBTXCSRHn register is set, the TXRDY bit is automatically set when a
maximum-sized packet is loaded into the FIFO. For packet sizes less than the maximum, TXRDY
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.
19.3.1.3
OUT Transactions as a Device
When in Device mode, 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
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