Functional Description
1676
SLAU723A – October 2017 – Revised October 2018
Copyright © 2017–2018, Texas Instruments Incorporated
Universal Serial Bus (USB) Controller
NOTE:
Double-packet buffering is disabled if a corresponding EPn bit of an endpoint 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.
27.3.1.3 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 seven 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.
27.3.1.3.1 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.
27.3.1.3.2 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 controller first clears
the FULL bit, then sets RXRDY again to indicate that there is another packet waiting in the FIFO to be
unloaded.
NOTE:
Double-packet buffering is disabled if the corresponding EPn bit of an endpoint is set in the
USB Receive Double Packet Buffer Disable (USBRXDPKTBUFDIS) register. This bit is set
by default, so it must be cleared to enable double-packet buffering.