LM4F232H5QCFIG Datasheet, PDF (1127/1472 Pages) Texas Instruments – Stellaris® LM4F232H5QC Microcontroller

Stellaris^® LM4F232H5QC Microcontroller
18.3.2.2
IN Transactions as a Host
IN transactions are handled in a similar manner to the way in which OUT transactions are handled
when the USB controller is in Device mode except that the transaction first must be initiated by
setting the REQPKT bit in the USBCSRL0 register, indicating to the transaction scheduler that there
is an active transaction on this endpoint. The transaction scheduler then sends an IN token to the
target Device. When the packet is received and placed in the receive FIFO, the RXRDY bit in the
USBCSRL0 register is set, and the appropriate receive endpoint interrupt is signaled to indicate
that a packet can now be unloaded from the FIFO.
When the packet has been unloaded, RXRDY must be cleared. The AUTOCL bit in the USBRXCSRHn
register can be used to have RXRDY automatically cleared when a maximum-sized packet has been
unloaded from the FIFO. The AUTORQ bit in USBRXCSRHn causes the REQPKT bit to be automatically
set when the RXRDY bit is cleared. The AUTOCL and AUTORQ bits can be used with µDMA accesses
to perform complete bulk transfers without main processor intervention. When the RXRDY bit is
cleared, the controller sends an acknowledge to the Device. When there is a known number of
packets to be transferred, the USB Request Packet Count in Block Transfer Endpoint n
(USBRQPKTCOUNTn) register associated with the endpoint should be configured to the number
of packets to be transferred. The USB controller decrements the value in the USBRQPKTCOUNTn
register following each request. When the USBRQPKTCOUNTn value decrements to 0, the AUTORQ
bit is cleared to prevent any further transactions being attempted. For cases where the size of the
transfer is unknown, USBRQPKTCOUNTn should be cleared. AUTORQ then remains set until cleared
by the reception of a short packet (that is, less than the MAXLOAD value in the USBRXMAXPn
register) such as may occur at the end of a bulk transfer.
If the Device responds to a bulk or interrupt IN token with a NAK, the USB Host controller keeps
retrying the transaction until any NAK Limit that has been set has been reached. If the target Device
responds with a STALL, however, the USB Host controller does not retry the transaction but sets
the STALLED bit in the USBCSRL0 register. If the target Device does not respond to the IN token
within the required time, or the packet contained a CRC or bit-stuff error, the USB Host controller
retries the transaction. If after three attempts the target Device has still not responded, the USB
Host controller clears the REQPKT bit and sets the ERROR bit in the USBCSRL0 register.
18.3.2.3
OUT Transactions as a Host
OUT transactions are handled in a similar manner to the way in which IN transactions are handled
when the USB controller is in Device mode. The TXRDY bit in the USBTXCSRLn register must be
set as each packet is loaded into the transmit FIFO. Again, setting the AUTOSET bit in the
USBTXCSRHn register automatically sets TXRDY when a maximum-sized packet has been loaded
into the FIFO. Furthermore, AUTOSET can be used with the µDMA controller to perform complete
bulk transfers without software intervention.
If the target Device responds to the OUT token with a NAK, the USB Host controller keeps retrying
the transaction until the NAK Limit that has been set has been reached. However, if the target Device
responds with a STALL, the USB controller does not retry the transaction but interrupts the main
processor by setting the STALLED bit in the USBTXCSRLn register. If the target Device does not
respond to the OUT token within the required time, or the packet contained a CRC or bit-stuff error,
the USB Host controller retries the transaction. If after three attempts the target Device has still not
responded, the USB controller flushes the FIFO and sets the ERROR bit in the USBTXCSRLn register.
18.3.2.4
Transaction Scheduling
Scheduling of transactions is handled automatically by the USB Host controller. The Host controller
allows configuration of the endpoint communication scheduling based on the type of endpoint
transaction. Interrupt transactions can be scheduled to occur in the range of every frame to every
November 08, 2011
Texas Instruments-Advance Information
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