SCSI Functional Description 2-35

Figure 2.6 Determining the Synchronous TransferRate
SCLK Clock
Quadrupler QCLK
SCF
Divider
CCF
Divider
Synchronous
Divider
Asynchronous
SCSILogic
Divideby 4
SCF2 SCF1 SCF0 SCF
Divisor
0011
0101.5
0112
1003
0003
1014
1106
1118
TP2 T P1 TP0 XFERP
Divisor
0004
0015
0106
0117
1008
1019
11010
11111
CCF2 CCF1 CCF0 Divisor QCLK (MHz)
0 0 1 1 50.1–66.00
0 1 0 1.5 16.67–25.00
0 1 1 2 25.1–37.50
1 0 0 3 37.51–50.00
0 0 0 3 50.01–66.00
1 0 1 4 75.01–80.00
1 1 0 6 120
1 1 1 8 160
Example1 (using 40 MHz clock)
QCLK(Quadr upled SCSI Clock) = 160 MHz
SCF= 1 (/ 1), XFERP = 4 (/8), CCF = 7 (/8)
Synchronoussend rate = (QCLK/SCF)/XFER P =
(160/1)/81= 20 Mbytes/s
Synchronousreceive rate = (QCLK/SCF) /4 =
(160/1)/42= 40 Mbytes/s
Thispoint
mustnot
exceed
160MHz
Receive
Clock
SendClock
(toSCSI Bus)
Thispoint must
notexceed 20 MHz.
SCLK= 40 MHz Example2 (using 20 MHz clock)
QCLK(Quadr upled SCSI Clock) = 80 MHz
SCF= 1 (/1), XFERP = 0 (/4), CCF = 5 (/4)
Synchronoussend rate = (QCLK/SCF)/XF ERP =
(80/1)/4 = 20 Mbytes/s
Synchronousreceive rate = (QCLK/SCF) /4 =
(80/1)/4 = 20 Mbytes/s
SCLK= 20 MHz
Note:
Synchronous send rate must not exceed 20 Mbytes/s because the
LSI53C875A is an Ultra SCSIdevice.
Although maximum synchronous receive rate is 40 Mbytes/s the
maximum transferrate is 20 Mbytes/s because the LS I53C875A isan
Ultra SCSI device.