am53c94 Advanced Micro Devices, am53c94 Datasheet - Page 38

• Manufacturer Part Number: am53c94
• Description: High Performance Scsi Controller
• Manufacturer: Advanced Micro Devices

deasserted at the end of the command which allows the
initiator to send other messages after the ID message is
sent out. Before issuing this command the SCSI
Timeout Register (STIMREG) 05H, the Control Register
One (CNTLREG1) 08H and the SCSI Destination ID
Register (SDIDREG) 04H must be set to the proper val-
ues. This command will be terminated early if the SCSI
Timeout Register times out or if the target does not go to
the Message Out Phase following the Selection Phase.
Enable Selection/Reselection Command
(Command Code 44H/C4H)
The Enable Selection/Reselection Command is used by
the target to respond to a bus-initiated reselection. Upon
disconnecting from the bus the Selection/Reselection
circuit is automatically disabled by device. This circuit
has to be enabled for the device to respond to subse-
quent reselection attempts and the Enable Selection/
Reselection Command is issued to do that. This com-
mand 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, but if the DMA is disabled, the re-
ceived data stays in the FIFO.
Disable Selection/Reselection Command
(Command Code 45H/C5H)
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 initiated, it resets the
internal mode bits previously set by the Enable Selec-
tion/Reselection Command. The device also generates
a function complete interrupt to the processor. If how-
ever, this command is issued after a bus initiated selec-
tion/reselection has already begun the command is ig-
nored since the Command Register is held reset and all
incoming commands are ignored. The device generates
a selected or 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 command is
issued the device arbitrates for the control of the SCSI
bus. When the device wins arbitration, it selects 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) 05H, the Control Register
One (CNTLREG1) 08H and the SCSI Destination ID
Register (SDIDREG) 04H must be set to the proper
values. If DMA is enabled, the Start Transfer Count
Register (STCREG) 00H–01H must be set to the total
length of the command. If DMA is not enabled, the data
must be loaded into the FIFO before issuing this
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AMD
P R E L I M I N A R Y
Am53C94/Am53C96
command. This command will be terminated early in the
following situations:
General Commands
No Operation Command
(Command Code 00H/80H)
The No Operation Command is used to perform no op-
eration and no interrupt is generated at the end of this
command. This command is issued after the Reset De-
vice Command to enable the Command Register. A No
Operation Command in the DMA mode may be used to
verify the contents of the Start Transfer Count Register
(STCREG) 00H – 01H. After the STCREG is loaded with
the transfer count and a No Operation Command is is-
sued, reading the Current Transfer Count Register
(CTCREG) 00H–01H will give the transfer count value.
Clear FIFO Command
(Command Code 01H/81H)
The Clear FIFO Command is used to initialize the FIFO
to the empty condition. The Current FIFO Register
(CFISREG) 07H reflects the empty FIFO status and the
bottom of the FIFO is set to zero. No interrupt is gener-
ated at the end of this command.
Reset Device Command
(Command Code 02H/82H)
The Reset Device Command immediately stops any de-
vice operation and resets all the functions of the device.
It returns the device to the disconnected state and it also
generates a hard reset. The Reset Device Command re-
mains on the top of the Command Register FIFO hold-
ing the device in the reset state until the No Operation
Command is loaded. The No Operation command
serves to enable the Command Register.
Reset SCSI Bus Command
(Command Code 03H/83H)
The Reset SCSI Bus Command is used to assert the
RSTC signal for approximately 25 ms. This command
causes the device to go to the disconnected state. No
interrupt is generated upon command completion. A
SCSI reset interrupt is however generated upon com-
mand completion if the interrupt is not disabled in the
Control Register One (CNTLREG1) 08H.
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 Selection Phase
The target exits the Command Phase early.