SBPH400-3 datasheet(20/43 Pages) STMICROELECTRONICS

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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

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