DP8496 datasheet(63/92 Pages) NSC

63 / 92 page

40 Functional Description (Continued)

signals in manual mode can be accomplished by writing to

the Differential SCSI 1 and 2 SDIF1 and SDIF2 (4Eh and

4Fh) registers

SCSI Bus Reset

Normally if the RST signal on the SCSI bus becomes as-

serted the DP84967 will deassert all the SCSI bus signals

A ‘‘SCSI Bus Reset Received’’ interrupt will be generated If

a SCSI command is executing it will be terminated immedi-

ately with a ‘‘SCSI Bus Reset Received’’ interrupt This in-

terrupt may be buffered after other pending interrupts as

defined in Table 427

The RST signal on the SCSI bus will be ignored by the

DP84967 for only one condition This condition is while in

the Manual Mode with the HE (Handshake Enable) bit in the

Synchronous Transfer register is set to ‘‘0’’ In this situation

the DP84967 will not be affected by the RST signal This is

a dangerous mode to be in on a normally operating SCSI

bus The intended use of this mode is to allow manipulation

of SCSI bus signals while SCSI Bus Reset shuts down all

other devices on the SCSI bus

Target and Initiator Modes

The DP84967 has commands in the Automatic Mode for

both the Initiator and Target roles However since the chip

will primarily be used in the Target Mode there are combi-

nation commands which optimize performance for this role

Single- and multi-phase commands are available for the Ini-

tiator role for example Select Transfer Info and Transfer

Pad Single- and multi-phase commands are also available

for the Target role But combination commands are also

available in the Target mode which can minimize processor

overhead Manual mode can be used with either Target or

Initiator modes

SCSI Parity

Odd or even parity generation and checking on SCSI trans-

fers is offered The user enables parity and selects parity

polarity by setting the SPE and SPP bits of the Setup 2

tion 32 and Table 410 in Section 429 for more details

When in Initiator mode the DP84967 will assert ATN

whenever it detects a parity error on the received data if

SCSI parity is enabled

Combination Commands

Combination commands bring together most of the com-

monly used sequences for Target operation Their primary

purpose is to reduce the number of interrupts and thus the

processor overhead required in SCSI transactions

Most combination commands may be issued before or after

a SCSI connection However two commands (Send Discon-

nect and Send End) can only be issued after a SCSI con-

nection has been established

The other combination commands are of two classes those

which wait to be selected and those which reselect Both of

these classes can be issued before or after a SCSI connec-

tion has been established For example an Initiator may

have selected the Target before any ‘‘Wait for Select’’ com-

mand was issued by the local processor This ‘‘Wait for Se-

lect’’ command can still be issued after the selection is com-

plete and the specified transfer phases will be accom-

plished Likewise the Target may have used the stand-

alone Reselect command to re-establish a connection with

some Initiator and now wish an Identify message and data

transfer sequence Simply issue the appropriate Reselect

combination command and the Reselection sequence will

automatically be skipped The command will start by send-

ing the Idenfity message

The ‘‘Wait-for-Select’’ type commands are terminated either

by completing their execution being overwritten by another

command or aborted by an unexpected condition

However if a ‘‘Wait for Select’’ command is overwritten by

another command after it has started responding to a selec-

tion but before an interrupt has been issued a ‘‘Last Com-

mand Ignored’’ interrupt will be generated through the SCSI

Interrupt register

It is important to consider the SCSI Pointer while overwriting

a ‘‘Wait for Select’’ with another command such as a Rese-

lect combination command Typically two different pointer

locations will be necessary one for the Command Descrip-

tor Block of the ‘‘Wait for select’’ command and one for the

data block(s) of the Reselect command The software must

make sure there is no chance the ‘‘Wait for Select’’ com-

mand is executing before changing the SCSI Pointer The

following procedure will ensure proper SCSI Pointer modifi-

cation

1 Issue a Clear Pending Command to overwrite the ‘‘Wait

for Select’’ command

2 Read the SCSI Status register and check for the Selec-

tion in Process code

3 If the SCSI Bus is free (Status Code e 0000 or 0001)

then the ‘‘Wait for Select’’ command has been success-

fully aborted You are free to update the SCSI Pointer

4 If any other Status Code wait for an interrupt to indicate

the completion of the ‘‘Wait for Select’’ command (or

perhaps an unexpected reselection) Then update the

SCSI Pointer

Unexpected (Re)Selection

Even if no command is currently executing the DP84967

will still respond to a (Re)Selection from an Initiator or Tar-

get broadcasting the proper ID while in Automatic Mode

This will generate a ‘‘(Re)Selected’’ interrupt

Asserting CRST or issuing a SCSI Reset command will set

the Manual Mode to avoid a Selection response before the

system is initialized

A (re)selection is usually asynchronous to normal opera-

tions Therefore new commands may be written to the SCSI

Command register at critical times related to a (re)selection

Here is a summary of responses by the DP84967 based on

the relationship between writing to the SCSI Command reg-

ister and a (re)Selection attempt This assumes that the

DP84967 is not connected and there is no command cur-

rently executing

1 A new command is written just before (re)selection As-

suming the DP84967 losses arbitration and the winning

initiator

(or

Target)

selects

the

DP84967

‘‘(Re)Selected’’ interrupt will be generated and a ‘‘Last

Command Ignored’’ status will be reported

2 A new comand is written during a (re)selection In this

case the DP84967 will generate a ‘‘(Re)Selected’’ inter-

rupt and ‘‘Last Command Ignored’’ interrupt

3 A new command is written after a (re)selection Since the

(re)selection is complete a ‘‘(Re)Selected’’ interrupt will

be generated and a ‘‘Last Command Ignored’’ status will

be reported

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