pc8477b National Semiconductor Corporation, pc8477b Datasheet - Page 47

• Manufacturer Part Number: pc8477b
• Description: Advanced Floppy Disk Controller
• Manufacturer: National Semiconductor Corporation

7 0 Reference Section
7 2 PC8477B ENHANCEMENTS VS 82077AA
The enhancements listed below are additional functions of
the PC8477B that the 82077AA does not have and do not
affect the compatibility between the two floppy controllers
Commands
The following are PC8477B commands not supported by
the 82077AA
FIFO Operation
The PC8477B FIFO is compatible with the 82077AA FIFO
with the addition of a Non-Burst mode The default setting
when the FIFO is enabled is the 82077AA compatible Burst
mode The Non-Burst mode is enabled via the Mode com-
mand The Non-Burst mode will pulse the DRQ or INT sig-
nals during a burst transfer to or from the FIFO
For both the Burst and Non-Burst modes with the FIFO en-
abled no external circuitry is required with the PC8477B
during DMA verify transfers During verify operations the
DMA controller will assert the DACK signal without a RD
signal in response to a DRQ from the floppy controller The
82077AA however requires external circuitry to create the
RD signal during DMA verify operations with its FIFO en-
abled in order to work successfully without an overrun error
The published Intel bug fix for the 82077AA can only be
used for motherboard applications and not for add-in
boards The PC8477B does not have this problem
Also because of the byte counter in the PC8477B design
the DRQ or INT signal will be deasserted when the last byte
of a sector is written to the FIFO during the execution phase
of a write or format operation The 82077AA does not deas-
sert DRQ or INT until the last byte has been read out of the
FIFO This will cause a delay in the deassertion of DRQ or
INT of up to 16 byte times resulting in extra bytes trans-
ferred to the floppy controller The PC8477B does not have
this problem
Data Separator
The PC8477B data separator’s performance meets that of
the 82077AA’s However there are no dual modes in the
PC8477B data separator whereas the 82077AA data sepa-
rator has an internal floppy drive mode and an internal tape
drive mode This singular mode design of the PC8477B data
separator eliminates the need for hardware or software con-
trol and provides for more consistent performance The
Mode Command Controls several enhanced fea-
tures of the PC8477B such as Implied Seeks Low
Power mode additional FIFO modes and DENSEL
encoding The Mode command parameters are de-
fault to 82077AA compatible states and will be un-
affected by 82077AA-based software that does not
recognize the existence of a Mode command See
the PC8477B data sheet for more details
NSC Command This one byte command is used
to identify the PC8477B in the system Other floppy
controllers will return an 80 hex (invalid command)
while the PC8477B will return a value of 73 hex (the
lower four bits are reserved to indicate revision up-
dates in the part)
Set Track Command This command allows the
user to program the value of any of the four Present
Track Registers corresponding to the four logical
drives
(Continued)
47
PC8477B data separator is designed to work with the strict-
est motor speed and bit jitter requirements of both floppy
and tape drives
Low Power Mode
The typical measured low power current for the PC8477B
(analog and digital) is 1 mA The typical measured low pow-
er current for the 82077AA is 2 mA – 3 mA
The PC8477A supports the 82077AA manual low power
mode by writing to the Low Power bit (D6) in the Data Rate
Select register The low power mode is turned off by issuing
a reset to the chip whereupon re-initialization is necessary
In addition the PC8477B supports a manual low power AND
automatic low power mode via the Mode command Manual
low power must be invoked every time the low power mode
is desired Automatic low power mode need only be invoked
once during initialization and then low power is entered
whenever the floppy controller is idle
As mentioned the 82077AA and PC8477B will exit the low
power mode after a reset The PC8477B will also exit the
low power mode after any read or write to the Main Status
Register or Data Register In this way the part can exit low
power cleanly without requiring additional software initializa-
tion This feature gives the PC8477B an advantage in that
once software has initialized it for automatic low power no
additional software modifications are necessary and the
chip will power down whenever it is idle Even for manual
low power mode via the DSR or Mode command the
PC8477B can return to normal mode without re-initialization
as required for the 82077
Reset Pulse Width
The PC8477B software reset pulse width is 100 ns mini-
mum This means that software can issue two consecutive
writes to the Digital Output Register of the PC8477B to tog-
gle the Reset Controller bit (D2) without intervening delay
This specification is significantly better than the 82077AA
minimum software reset pulse width which is specified as
3 5 ms (worst case at the 250 kb s data rate)
When using an external pull-up or pull-down 10 kX resistor
on the MFM pin the hardware reset pulse width is also
100 ns minimum for the PC8477B The minimum hardware
reset pulse width for the 82077AA is 7 1 ms Again the
PC8477B specification is much better allowing the system
reset pulse to be very short
Tape Drive Register
The PC8477B will support reads and writes to this register
just as the 82077AA does However the PC8477B will not
use the information written to the Tape Drive Register to
alter the state of the Data Separator That is there is only
one mode of the internal PC8477B data separator a high
performance mode that will support the requirements for all
floppy and tape drives
Implied Seeks
The PC8477B supports our popular DP8473 method as well
as the 82077AA method of implementing Implied Seeks
The DP8473 method is to set a bit in the Mode command
for enabling Implied Seeks and then set the Implied Seek
bit if desired in the Read Write or Scan commands The
82077AA method is to set the EIS bit (enable implied seeks)
in the Configure command and then Implied Seeks will al-
ways be enabled for Read Write and Verify commands