Seagate Ultra 320, Ultra 160 manual Clocking methods for data transfers, LVD measurement points

SE SCSI devices with data transfer rates up to and including Fast-20 shall use the measurement points defined in Section 9 of ANSI standard SPI-4, for the measurement of the timing values. The rise and fall times for the SE REQ/ACK signals shall be nominally the same as for the SE Data, DB(P_CRCA), and DB(P1) sig- nals.

SE Fast-20 timing measurement points shall apply even if a slower transfer rate is negotiated.

SE SCSI devices are not capable of Fast-40 and Fast-80 data transfer rates. LVD circuits are required for these faster transfer rates.

2.6.3LVD measurement points

When transferring data using ST LVD phases, LVD SCSI devices shall use the measurement points defined in Section 9 Figure 60 of ANSI standard SPI-5, for the measurement of timing values. When transferring data using DT DATA phases, LVD SCSI devices shall use the measurement points defined in Section 9 Figure 61 of SPI-5, for the measurement of timing values. When transferring data using DT DATA phases LVD SCSI devices using paced transfers shall use the measurement points defined in Section 9 Figure 62 of SPI-5 for the measurement of the timing values. The rise and fall times for the LVD REQ/ACK signals shall be nominally the same as for the LVD Data, P_CRCA, and DP-1 signals.

2.7Clocking methods for data transfers

This manual defines optional methods of latching data from the REQ and ACK signals depending on whether ST DATA phases or DT DATA phases are being used for information transfers as shown in Figure 5. Data shall only be latched on the asserting edge of the REQ or ACK signal except in DT DATA phases. When DT DATA phases are used, data shall be latched on both the asserting edge and the negating edge of the REQ or ACK signal.

Regardless of whether ST or DT transfers are enabled, the negotiated transfer period sets the maximum rate at which the data is clocked at in megatransfers per second. As a result, the time from rising edge to rising edge for REQ and ACK signals for the same transfer rate is twice as long for a DT transfer as it is for an ST transfer. An example of a negotiated transfer period of 25 ns with ST transfers is shown in figure 6. An example of a negotiated transfer period of 25 ns with DT transfers is shown in Figure 7. For ST and DT synchronous transfers the clocking signal (i.e., REQ or ACK) occurs when the DATA BUS is in a steady state.

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