Texas Instruments TMS320DM357 - Manual (Page 41)

3.1.4 Isochronous Transactions
3.1.4.1 Isochronous IN Transactions
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An Isochronous IN transaction is used to transfer periodic data from the function controller to the host.
The following optional features are available for use with a Tx endpoint used in Peripheral mode for
Isochronous IN transactions:
•
Double packet buffering: When enabled, up to two packets can be stored in the FIFO awaiting
transmission to the host. Double packet buffering is enabled by setting the DPB bit of TXFIFOSZ
register (bit 4).
Note: Double packet buffering is generally advisable for Isochronous transactions in order to avoid
Underrun errors as described in later section.
•
DMA: If DMA is enabled for the endpoint, a DMA request will be generated whenever the endpoint is
able to accept another packet in its FIFO. This feature allows the DMA controller to load packets into
the FIFO without processor intervention.
However, this feature is not particularly useful with Isochronous endpoints because the packets
transferred are often not maximum packet size and the PERI_TXCSR register needs to be accessed
following every packet to check for Underrun errors.
When DMA is enabled and DMAMODE bit of PERI_TXCSR is set, endpoint interrupt will not be
generated for completion of packet transfer. Endpoint interrupt will be generated only in the error
conditions.
3.1.4.1.1 Setup
In configuring a Tx endpoint for Isochronous IN transactions, the TXMAXP register must be written with
the maximum packet size (in bytes) for the endpoint. This value should be the same as the
wMaxPacketSize field of the Standard Endpoint Descriptor for the endpoint. In addition, the relevant
interrupt enable bit in the INTRTXE register should be set (if an interrupt is required for this endpoint) and
the PERI_TXCSR register should be set as shown in
Table 4 .
Table 4. PERI_TXCSR Register Bit Configuration for Isochronous IN Transactions
Bit Position Bit Field Name Configuration
Bit 14 ISO Set to 1 to enable Isochronous transfer protocol
Bit 13 MODE Set to 1 to ensure the FIFO is enabled (only necessary if the FIFO is shared with an Rx
endpoint).
Bit 12 DMAEN Set to 1 if DMA Requests have to be enabled
Bit 11 FRCDATATOG Ignored in Isochronous mode
Bit 10 DMAMODE Set it to 1, when DMA is enabled and EP interrupt is not needed for each packet transmission
3.1.4.1.2 Operation
An Isochronous endpoint does not support data retries, so if data underrun is to be avoided, the data to be
sent to the host must be loaded into the FIFO before the IN token is received. The host will send one IN
token per frame (or microframe in High-speed mode), however the timing within the frame (or microframe)
can vary. If an IN token is received near the end of one frame and then at the start of the next frame,
there will be little time to reload the FIFO. For this reason, double buffering of the endpoint is usually
necessary.
SPRUGH3 – November 2008 Universal Serial Bus (USB) Controller 41
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