Avago Technologies LSI53C1020 - Manual (Page 45)

Ultra320 SCSI Functional Description 2-19
Version 2.4 Copyright © 2001–2004 by LSI Logic Corporation. All rights reserved.
device sourcing the data drives the REQ/ACK signal as a free-running
clock. The transition of the REQ/ACK signal, either the assertion or the
negation, clocks data across the bus. For successful completion of a
paced transfer, the number of ACK transitions must equal the number of
REQ transitions and both the REQ and ACK lines must be negated.
The P1 line indicates valid data in 4-byte quantities by using its phase.
The transmitting device indicates the start of valid data state by holding
the state of the P1 line for the first two data transfer periods. Beginning
on the third data transfer period, the transmitting device continues the
valid data state by toggling the state of the P1 line every two data transfer
periods for as long as the data is valid. The transmitting device must
toggle the P1 line coincident with the REQ/ACK assertion. The method
provides a minimum data valid period of two transfer periods.
To pause the data transfer, the transmitting device reverses the phase of
P1 by withholding the next transition of P1 at the start of the first two
invalid data transfer periods. Beginning with the third invalid data transfer
period, the transmitting device toggles the P1 line every two invalid data
transfer periods until it sends valid data. The transmitting device returns
to the valid data state by reversing the phase of the P1 line. The invalid
data state must experience at least one P1 transition before returning to
the valid data state. This method provides a minimum data invalid period
of four transfer periods.
Figure 2.2 provides a waveform diagram of paced data transfers and
illustrates the use of the P1 line.
Figure 2.2 Paced Transfer Example
The LSI53C1020 uses the PPR negotiation that the SPI-4 draft standard
describes to establish a paced transfer agreement for each
initiator-target pair.
Data Not Valid Data Valid Data Valid Data Not Valid
REQ
ACK
P1
DATA