DP8496 datasheet(61/92 Pages) NSC

61 / 92 page

40 Functional Description (Continued)

DCBSY

Same as above Affects the DBSY pin In effect only during

manual mode

DCSEL

Same as above Affects the SEL pin In effect only during

manual mode

DCTARG

Same as above Affects the DTARG pin which controls the

direction of target signals CD IO REQ MSG In effect

only during manual mode

DCINIT

Same as above Affects the DINIT pin which controls the

direction of initiator signals ACK ATN In effect only during

manual mode

DCRST

Same as above Affects the DRST pin In effect only during

manual mode

SCK(10)

By programming these two bits the user can adjust the set-

up time delay of REQ and ACK signals with respect to the

SCSI Data signals Delay values of approximately 1 15 or

2 Bus Clock periods are possible depending on the values

of these bits and the CLK(10) bits of the Setup 1 register

Default values for these bits are 00

TABLE 428 SCSI Strobe Time Settings

SCSI Clock Bits (10)

BCLK

Freq

(10)

Where

10 is defined as bcp b k k is a characterization constant

15 is defined as bcp a bc b k bc is either bcl or bch

20 is defined as 2 bcp b k k is a characterization constant

bcp bch and bcl refer to the BCLK period high pulse width and low pulse

width respectively

443 SCSI Operations

Extending the Current SCSI Command

SCSI commands may be programmed to transfer more

blocks than originaly programmed If a new value is written

to the Block Count holding register while a SCSI command

is executing the new value will be downloaded from the

holding register to the active register when the active Block

Count reaches zero This new Block Count must be written

to the Block Count holding register any time before the ac-

tive Block Count reaches zero The Block Count register

should not be updated again (after the first pipelined value)

until the previous pipelined value has been downloaded

The ‘‘Group Complete’’ interrupt indicates when the pipe-

lined Block Count is downloaded

The SCSI Pointer can also be modified when extending the

current executing SCSI commandallowing for noncontigu-

ous buffer memory transfers If the current executing SCSI

command is a single phase information transfer command

the SCSI Pointer can be modified after 10 BCLK periods

have passed since the SCSI command was issued If the

current executing SCSI command is a reselect combination

command the SCSI Pointer can be modified after detecting

the SCSI data transfer phase The SCSI pointer should not

be updated again (after the first pipelined value) until the

previous pipelined value has been downloaded If the SCSI

pointer have been modified the ‘‘Group Complete’’ interrupt

indicates when the pipelined SCSI pointer is downloaded

If the SCSI pointer has not been modified when extending

the current executing SCSI command a SCSI pointer down-

load will not occur and contiguous buffer memory transfers

will result Although ‘‘Group Complete’’ interrupts will still

occur at the completion of each transfer

The ‘‘Group Complete’’ interrupt indicates when the pipe-

lined Block Count and the SCSI Pointer (if modified) are

downloaded After this interrupt is received the Block Count

A SCSI command may be extended multiple times if de-

sired Simply wait for the ‘‘Group Complete’’ interrupt be-

tween each update of the Block Count register

Table 429 summarizes these interrupts

Pipelining SCSI Commands

There are several combinations of SCSI commands that

can be pipelined

Receive Data followed by Send Disconnect

Receive Data followed by Send End

Send Data followed by Send Disconnect

Send Data followed by Send End

Reselect-Receive Data followed by Send Disconnect

Reselect-Receive Data followed by Send End

Reselect-Send Data followed by Send Disconnect

Reselect-Send Data followed by Send End

Except for the Clear Pending command only these com-

mand pairs can be pipelined The Send End Send Discon-

nect and Clear Pending commands are the only commands

allowed to be pipelined No additional commands can be

written to the SCSI Command register until the pipelined

command has completed its operation

The Clear Pending Command can be issued to clear the

pipelined command if the first command is still executing

and the pipelined command has not yet begun

In addition changing the SCSI pointer or the Block Count

Disconnect command If the Block Count register is modi-

fied after the Send End or Send Disconnect command has

been pipelined the current executing SCSI command may

possibly be extended

A pipelined command should not be issued before 10 BCLK

periods have passed since the first command was issued

It is important to understand how the DP84967 determines

completion interrupts when pipelining commands It is pos-

sible to completely miss a completion interrupt if errors do

not occur for either of the pipelined commands Refer to

Table 427 for a description of when completion interrupts

are generated and how they are buffered

DP8496 Datasheet(PDF) 61 Page - National Semiconductor (TI)

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