SBPH400-3 Просмотр технического описания (PDF) - STMicroelectronics
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SBPH400-3 Datasheet PDF : 43 Pages
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SBPH400-3
2.8.5 Bus request
The SBPH400 obeys the rules specified in the P1394a proposal for the disposition of requests
received from a link device, provided that the link device follows the P1394a rules for when it
may issue requests.
The link device sends the request for the bus for fair (FairReq) or priority (PriReq) access at
least one clock after the interface becomes idle or during a status transfer from the SBPH400.
A cycle master node uses a priority request (PriReq) to send the cycle start packet.
A receive state (CTL[0:1]=10) at any time during or after the link device sends a fair or priority
request transfer indicates to the link device that the request cannot yet been granted, due to
the arrival of an incoming packet. If arbitration acceleration is enabled, and the incoming
packet is null or has no more than 8 bits, then the SBPH400 retains the request, otherwise the
request is discarded as soon as the SBPH400 determines that the incoming packet has more
than 8 bits. The request always discarded if arbitration acceleration is not enabled. The link
device should reissue a discarded request on the next idle or status.
A link device uses the IsoReq request at any time to request the SBPH400 to send an
isochronous packet. The SBPH400 will wait for a an isochronous gap before arbitrating for the
bus. The SBPH400 will clear the request only when the bus has been won, or if it performs a
status transfer indicating a subaction gap (this indicates an error condition and should not
occur).
A link device must issue a ImmReq request to send an acknowledge packet during the
reception of a packet addressed to it or no later than the fourth SClk cycle after the interface
went idle. As soon as the packet reception ends the SBPH400 immediately takes control of
the bus and grants the bus to the link device. If the header CRC of the packet happens to be
bad the link device should release the bus immediately. Note that the link device should not
use this request to send another type of packet. This can be ensured by making the link device
wait for 160 ns after the end of the receive packet to allow the SBPH400 to grant it the bus for
the acknowledge. The bus is then released before starting another request.
After the link issues a request for the access to the bus (immediate, iso, fair, or priority) it
cannot issue another bus request until the SBPH400 indicates that the request is either “lost”
(incoming packet, other than an ACK packet when accelerations are enabled) or “won” (grant).
When a previous bus request is pending the SBPH400 ignores new bus requests.
All outstanding requests are cancelled on a bus reset.
2.8.6 Register Read/Write requests
For write requests, the SBPH400 takes the data field of the LREQ transfer and loads it into the
addressed register as soon as the transfer is complete.
For read requests, the SBPH400 returns the contents of the addressed register at the next
opportunity through a 16-bit status transfer. The link device may perform a read or write
register operation at any time.
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