AM79C974 datasheet(86/153 Pages) AMD

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P R E L I M I N A R Y

Am79C974

into the FIFO before issuing this command. This com-

mand will be terminated early in the following situations:

The SCSI Timeout Register times out

The Target does not go to the Message Out Phase

following the Selection Phase

The Target exits the Message Phase early

The Target does not go to the Command Phase

following the Message Out Phase

The Target exits the Command Phase early

A Successful Operation/Service Request interrupt is

generated

when

this

command

completed

successfully.

Select With ATN and Stop Steps Command

(Command Code 43h/C3h)

The Select with

ATN and Stop Steps Command is used

by the Initiator to send messages with lengths other than

1 or 3 bytes. When this command is issued, the device

executes the Selection process, transfers the first mes-

sage byte, then STOPS the sequence.

ATN is not de-

asserted at this time, allowing the Initiator to send

additional message bytes after the ID message. To

send these additional bytes, the Initiator must write the

transfer counter with the number of bytes which will fol-

low, then issue a Transfer Information Command.

(Note: the Target is still in the Message Out phase when

this command is issued).

ATN will remain asserted until

the Current Transfer Count Register decrements

to zero.

The SCSI Timeout Register (STIMREG), Control Regis-

ter One (CNTLREG1), and the SCSI Destination ID

before the Initiator issues this command. This command

will be terminated early if the STIMREG times out or if

the Target does not go to the Message Out Phase fol-

lowing the Selection Phase.

Enable Selection/Reselection Command

(Command Code 44H/C4H)

The Enable Selection/Reselection Command is used to

respond to a bus-initiated Selection or Reselection.

Upon disconnecting from the bus the Selection/

Reselection circuit is automatically disabled by the de-

vice. This circuit must be enabled for the Am79C974 to

respond to subsequent reselection attempts and the En-

able Selection/Reselection Command is issued to do

that. This command is normally issued within 250 ms

(select/reselect timeout) after the device disconnects

from the bus. If DMA is enabled, the device loads the

received data to the buffer memory. If the DMA is dis-

abled, the received data stays in the FIFO.

Disable Selection/Reselection Command

(Command Code 45H)

The Disable Selection/Reselection Command is used

by the Target to disable response to a bus-initiated

Reselection. When this command is issued before a

bus-initiated Selection or Reselection is in progress, it

resets the internal state bits previously set by the Enable

Selection/Reselection Command. The device also gen-

erates a Successful Operation interrupt to the proces-

sor. If however, this command is issued after a

bus-initiated Selection/Reselection has begun, this

command and all incoming commands are ignored

since the Command Register (CMDREG) is held reset.

The

Am79C974

also

generates

Selected

Reselected interrupt when the sequence is complete.

Select With ATN3 Steps Command

(Command Code 46h/C6h)

The Select with

ATN3 Steps Command is used by the

Initiator to select a Target. This command is similar to

the Select with

ATN Steps Command, except that it

sends exactly three message bytes. When this com-

mand is issued the Am79C974 arbitrates for control of

the SCSI bus. When the device wins arbitration, it se-

lects the Target device with the

ATN signal asserted and

transfers the Command Descriptor Block (CDB) and

three message bytes. Before issuing this command the

SCSI Timeout Register (STIMREG), Control Register

One (CNTLREG1), and the SCSI Destination ID Regis-

ter (SDIDREG) must be set to the proper values. If DMA

enabled,

the

Start

Transfer

Count

(STCREG) must be set to the total length of the com-

mand. If DMA is not enabled, the data must be loaded

into the FIFO before issuing this command. This com-

mand will be terminated early in the following situations:

The SCSI Timeout Register times out

The Target does not go to the Message Out Phase

following the Selection Phase

The Target removes Command Phase early

The Target does not go to the Command Phase

following the Message Out Phase

The Target exits the Command Out Phase early

A Successful Operation/Service Request interrupt is

generated

when

this

command

executed

successfully.

General Commands

No Operation Command

(Command Code 00h/80h)

The No Operation Command administers no operation,

therefore an interrupt is not generated upon completion.

This command is issued following the Reset Device

Command to clear the Command Register (CMDREG).

A No Operation Command in the DMA mode may be

used to verify the contents of the Start Transfer Count

the transfer count and a DMA No Operation Command

is issued, reading the Current Transfer Count Register

(CTCREG) returns the transfer count value.

AM79C974 Datasheet(PDF) 86 Page - Advanced Micro Devices

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