DP8496 datasheet(39/92 Pages) NSC

39 / 92 page

40 Functional Description (Continued)

44 SCSI INTERFACE

Primary among the objectives for the SCSI interface is to

simplify

the

microprocessor’s

interaction

with

the

DP84967 The most complex portion of SCSI related code

is usually dedicated to interrupt handling whether vectored

or polled In the DP84967 the multi-phase commands and

combination commands help to reduce the number of inter-

rupts and to pre-define the reason for those interrupts which

do occur An interrupt mask register enables the software to

control interrupt flow Also a physically separate pin dedi-

cated for SCSI interrupts (SINT) can shorten the processor

response time The chip is also capable of operating in ei-

ther the target mode for which it is optimized or the initiator

mode

The SCSI Port can be controlled by the processor by utiliz-

ing the DP84967 in one of two different modes Automatic

or Manual Automatic mode enables the SCSI Command

protocol engine to interpret and perform multi-phase or

combination commands

Manual mode disables the SCSI Command register and

SCSI protocol engine The processor must then manipulate

both the SCSI control and data bus through programmed

IO The SCSI Control register and the SCSI Data register

are the windows into the SCSI bus which allow this manipu-

lation

The DP84967 may be switched between the Automatic

and Manual modes of operation There are a few restric-

tions however and these are detailed in the SCSI Opera-

tion register description in Section 442

The DP8496 provides 48 mA single-ended transceivers on

all SCSI bus signal pins so the bus lines can be driven

directly The DP8497 version is designed to interface with

differential transceivers external to the chip for Fast SCSI

applications Therefore on the DP8497 all SCSI outputs are

the standard 2 mA type Fourteen additional pins on the

DP8497 provide direction control for the external differential

transceivers In the manual mode of operation the user

controls the transceiver direction by programming two regis-

ters SDIF1 and SDIF2 (4Eh and 4Fh) Also note that on the

DP8497 all SCSI signals are active high

441 Simple SCSI Operations

The flowchart in

Figure 411 describes how to implement

simple SCSI operations such as reading data from the disk

and writing data to the disk This flowchart has been kept

quite simple The next section describes the various regis-

ters that the user must program in order to configure and

control the SCSI Controller’s operation More advanced

SCSI operations are then described in Section 443

442 SCSI Registers

SCSI Command (SCMD)

40h

W Only

76543210

SCSI Command Opcode

While in Automatic mode writing an opcode to this register

will initiate a SCSI command Normally other SCSI registers

are configured before writing to this register After the prop-

er registers are loded writing to this register will initiate the

operation Invalid opcodes should NEVER be issued After

any invalid opcode is written to this register the DP84967

should be reset with the CRST pin

In general SCSI commands cannot be pipelined The combi-

nation commands eliminate the need for most pipelining

New commands should be written to the SCSI Command

operation as indicated by its completion interrupt See Sec-

tion 443 for a description of certain commands that may be

pipelined

SCSI commands which transfer information blocks may be

extended however If a new value is written to the SCSI

Block Count register the command will continue by down-

loading the new holding register value to the active SCSI

Block Count register when the current active contents

reaches zero This is described in Section 443

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