RM0444 Rev 5 1277/1390
RM0444 Universal serial bus full-speed host/device interface (USB)
1307
37.5.5 Isochronous transfers in Device mode
The USB standard supports full speed peripherals requiring a fixed and accurate data
production/consume frequency, defining this kind of traffic as ‘isochronous’. Typical
examples of this data are: audio samples, compressed video streams, and in general any
sort of sampled data having strict requirements for the accuracy of delivered frequency.
When an endpoint is defined to be ‘isochronous’ during the enumeration phase, the host
allocates in the frame the required bandwidth and delivers exactly one IN or OUT packet
each frame, depending on endpoint direction. To limit the bandwidth requirements, no re-
transmission of failed transactions is possible for isochronous traffic; this leads to the fact
that an isochronous transaction does not have a handshake phase and no ACK packet is
expected or sent after the data packet. For the same reason, isochronous transfers do not
support data toggle sequencing and always use DATA0 PID to start any data packet.
Table 219. Bulk double-buffering memory buffers usage (Host mode)
Endpoint
type
DTOG SW_BUF
Packet buffer used by
USB peripheral
Packet buffer used by
Application Software
Transmit
(OUT)
01
USB_CHEP_TXRXBD_0
(ADDRn_TX_0 / COUNTn_TX_0)
Buffer description table locations.
USB_CHEP_RXTXBD_0
(ADDRn_TX_1 / COUNTn_TX_1)
Buffer description table locations
10
USB_CHEP_RXTXBD_0
(ADDRn_TX_0 / COUNTn_TX_0)
Buffer description table locations
USB_CHEP_TXRXBD_0
(ADDRn_TX_0 / COUNTn_TX_0)
Buffer description table locations.
00 None
(1)
1. Endpoint in NAK Status.
USB_CHEP_TXRXBD_0
(ADDRn_TX_0 / COUNTn_TX_0)
Buffer description table locations.
11 None
(1)
USB_CHEP_RXTXBD_0
(ADDRn_TX_0 / COUNTn_TX_0)
Buffer description table locations.
Receive
(IN)
01
USB_CHEP_RXTXBD_0
(ADDRn_RX_0 /
COUNTn_RX_0)
Buffer description table locations.
USB_CHEP_TXRXBD_0
(ADDRn_RX_0 /
COUNTn_RX_0)
Buffer description table locations.
10
USB_CHEP_TXRXBD_0
(ADDRn_RX_0 /
COUNTn_RX_0)
Buffer description table locations
USB_CHEP_RXTXBD_0
(ADDRn_RX_0 /
COUNTn_RX_0)
Buffer description table locations.
00 None
(1)
USB_CHEP_RXTXBD_0
(ADDRn_RX_0 /
COUNTn_RX_0)
Buffer description table locations.
11 None
(1)
USB_CHEP_TXRXBD_0
(ADDRn_RX_0 /
COUNTn_RX_0)
Buffer description table locations.
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