SLLS418G
–
JUNE 2000
–
REVISED JANUARY 2003
35
POST OFFICE BOX 655303
DALLAS, TEXAS 75265
PRINCIPLES OF OPERATION
LLC service request (continued)
For fair or priority access, the LLC sends the bus request (FairReq or PriReq) at least one clock after the
PHY-LLC interface becomes idle. If the CTL terminals are asserted to the receive state (10b) by the PHY, then
any pending fair or priority request is lost (cleared). Additionally, the PHY ignores any fair or priority requests
if the receive state is asserted while the LLC is sending the request. The LLC may then reissue the request one
clock after the next interface idle.
The cycle master node uses a priority bus request (PriReq) to send a cycle start message. After receiving or
transmitting a cycle start message, the LLC can issue an isochronous bus request (IsoReq). The PHY clears
an isochronous request only when the serial bus has been won.
To send an acknowledge packet, the LLC must issue an immediate bus request (ImmReq) during the reception
of the packet addressed to it. This is required in order to minimize the idle gap between the end of the received
packet and the start of the transmitted acknowledge packet. As soon as the receive packet ends, the PHY
immediately grants control of the bus to the LLC. The LLC sends an acknowledgment to the sender unless the
header CRC of the received packet is corrupted. In this case, the LLC does not transmit an acknowledge, but
instead cancels the transmit operation and releases the interface immediately; the LLC must not use this grant
to send another type of packet. After the interface is released the LLC may proceed with another request.
The LLC may make only one bus request at a time. Once the LLC issues any request for bus access (ImmReq,
IsoReq, FairReq, or PriReq), it cannot issue another bus request until the PHY indicates that the bus request
was lost (bus arbitration lost and another packet received), or won (bus arbitration won and the LLC granted
control). The PHY ignores new bus requests while a previous bus request is pending. All bus requests are
cleared upon a bus reset.
For write register requests, the PHY loads the specified data into the addressed register as soon as the request
transfer is complete. For read register requests, the PHY returns the contents of the addressed register to the
LLC at the next opportunity through a status transfer. If a received packet interrupts the status transfer, then the
PHY continues to attempt the transfer of the requested register until it is successful. A write or read register
request may be made at any time, including while a bus request is pending. Once a read register request is
made, the PHY ignores further read register requests until the register contents are successfully transferred to
the LLC. A bus reset does not clear a pending read register request.
The TSB41AB3 includes several arbitration acceleration enhancements, which allow the PHY to improve bus
performance and throughput by reducing the number and length of inter-packet gaps. These enhancements
include autonomous (fly-by) isochronous packet concatenation, autonomous fair and priority packet
concatenation onto acknowledge packets, and accelerated fair and priority request arbitration following
acknowledge packets. The enhancements are enabled when the EAA bit in PHY register 5 is set.
The arbitration acceleration enhancements may interfere with the ability of the cycle master node to transmit
the cycle start message under certain circumstances. The acceleration control request is therefore provided
to allow the LLC to temporarily enable or disable the arbitration acceleration enhancements of the TSB41AB3
during the asynchronous period. The LLC typically disables the enhancements when its internal cycle counter
rolls over indicating that a cycle start message is imminent, and then reenables the enhancements when it
receives a cycle start message. The acceleration control request may be made at any time, however, and is
immediately serviced by the PHY. Additionally, a bus reset or isochronous bus request causes the
enhancements to be reenabled, if the EAA bit is set.