am79c974 Advanced Micro Devices, am79c974 Datasheet - Page 87

• Manufacturer Part Number: am79c974
• Description: Pcnettm-scsi Combination Ethernet And Scsi Controller For Pci Systems
• Manufacturer: Advanced Micro Devices

Clear FIFO Command
(Command Code 01h)
The Clear FIFO Command is used to initialize the SCSI
FIFO to the empty condition. The Current FIFO Register
(CFISREG) reflects the empty FIFO status and the bot-
tom of the FIFO is set to zero. No interrupt is generated
at the end of this command.
Reset Device Command
(Command Code 02h)
The Reset Device Command immediately stops any de-
vice operation and resets all the functions of the device.
Additionally, it returns the device to the disconnected
state and it generates a hard reset. The Reset Device
Command remains on the top of the Command Register
FIFO holding the device in the reset state until the No
Operation Command is loaded. Once loaded, the No
Operation command serves to re-enable the Command
Register.
Reset SCSI Bus Command
(Command Code 03h)
The Reset SCSI Bus Command forces the SCSI^RST
signal active for a period of 25 ms, and drives the chip to
the Disconnected state. An interrupt is not generated
upon command completion, however, if bit 6 is set to ‘0’
in Control Register One (CNTLREG1), a SCSI Reset in-
terrupt will be issued.
SCSI Power Management Features
As a leader in low-voltage technology, AMD has incor-
porated power-saving features into the Am79C974.
Through hardware and software, the Am79C974 can be
powered down to reduce consumption during chip
inactivity.
SCSI Activity Pin
The SCSI Bus activity is reflected by the BUSY output
line. This signal, when active, indicates that the SCSI
Bus is in use, therefore the Am79C974 should not be
powered down. This signal is the logical equivalent to
the SCSI bus signal BSY, however, it is not physically
connected to the BSY signal on the bus. To correctly
identify the Bus Free State on the SCSI bus, this BUSY
output line must be inactive for at least 250 ms (Selec-
tion Timeout period).
Reduced Power Mode
When there is no activity on the SCSI Bus, and there are
no pending commands, the Am79C974 may be pow-
P R E L I M I N A R Y
Am79C974
ered down by turning off the input buffers on the SCSI
Bus lines (set bit 5 in Control Register Four (B)+34h).
For further power reduction, the internal registers may
be programmed to a predetermined state, and the input
clock disconnected via external logic.
Power Down Pin (PWDN Pin)
When PWDN is driven active, it sets the PWDN bit in the
Status Register (Bit 0, DMA Status Register (B)+54h),
signaling the Am79C974 that the host would like to
power down the SCSI interface. An interrupt is gener-
ated when this bit is first set.
Software Disk Spin-Down
Incorporated into the SCSI ROM BIOS and certain de-
vice driver’s of AMD’s software solution is a module
which physically spins down SCSI fixed disks when the
host system elects to enact power management on the
SCSI system. The software module is activated upon
the ROM BIOS and/or other device driver’s receipt of an
interrupt caused by the PWDN pin being driven active.
Upon receipt of this interrupt, the current software proc-
ess is suspended and software control is given to the
power management module.
When the power management module is activated, it
checks the status of all SCSI fixed disks on the system
under its control. The SCSI fixed disks that are idle are
issued a command to spin down their media. All fixed
disks that are active at the time will be scheduled for spin
down upon completion of their pending commands.
Once the BIOS and/or drivers have detected the com-
pletion of each fixed disk’s final pending commands,
they are issued the command to spin down as well.
To spin down the disk drives, the power management
module issues a SCSI command (1B– Start/Stop unit).
When the command is received by the drive, it spins
down and waits in an idle state. For multiple drives on
the SCSI bus, the power management driver spins
down each drive individually. The drives remain in the
idle state until the BIOS and/or driver receives a com-
mand for the particular fixed disk. Once this occurs, the
drive is issued the command to spin up. When the drive
has completely spun up and is ready, the pending com-
mand is issued to the drive. Only the particular fixed disk
issued the command is instructed to spin up. All other
drives will remain in the spun down state until a com-
mand is issued to them.
Note: This sequence does not turn off the input buffers
as described in the previous section.
AMD
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