pc8477b National Semiconductor Corporation, pc8477b Datasheet - Page 36

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

5 0 Functional Description
In 2 88M drives the pre-erase head leads the read write
head by 200 mm which translates to 38 bytes at 1 Mb s
(19 bytes at 500 kb s) For both conventional and perpen-
dicular drives WGATE is asserted with respect to the posi-
tion of the read write head With conventional drives this
means that WGATE is asserted when the read write head is
located at the beginning of the Data Field preamble With
the 2 88M drives since the preamble must be pre-erased
before it is rewritten WGATE should be asserted when the
pre-erase head is located at the beginning of the Data Field
preamble This means that WGATE should be asserted
when the read write head is at least 38 bytes (at 1 Mb s)
before the preamble See Table 4-4 for a description of the
WGATE timing for perpendicular drives at the various data
rates
Because of the 38 byte spacing between the read write
head and the pre-erase head at 1 Mb s the GAP2 length of
22 bytes used in the standard IBM disk format is not long
enough There is a new format standard for 2 88M drives at
1 Mb s called the Perpendicular Format which increases
the GAP2 length to 41 bytes (see Figure 4-1 )
The Perpendicular Mode command of the PC8477B will put
the floppy controller into perpendicular recording mode
which allows it to read and write perpendicular media Once
this command is invoked the read write and format com-
mands can be executed in the normal manner The perpen-
dicular mode of the floppy controller will work at all data
rates adjusting the format and write data parameters ac-
cordingly See Section 4 2 8 for more details
5 8 DATA RATE SELECTION
The data rate can be chosen two different ways with the
PC8477B For PC compatible software the Configuration
Control Register at address 3F7 (hex) is used to program
the data rate for the floppy controller The lower bits D1 and
D0 are used in the CCR to set the data rate The other bits
should be set to zero See Table 3-6 for the data rate select
encoding
The data rate can also be set using the Data Rate Select
Register at address 4 Again the lower two bits of
FIGURE 5-5 Perpendicular Recording Drive R W Head and Pre-Erase Head
(Continued)
36
the register are used to set the data rate The encoding of
these bits is exactly the same as those in the CCR The
remainder of the bits in the DSR are used for other func-
tions Consult the Register Description (Section 5 1) for
more details
The data rate is determined by the last value that is written
to either the CCR or the DSR In other words either the
CCR or the DSR can override the data rate selection of the
other register
When the data rate is selected the microengine and data
separator clocks are scaled appropriately
DRATE0 and DRATE1 output pins will reflect the state of
the data select bits that were last written to either the CCR
or the DSR
5 9 WRITE PRECOMPENSATION
Write precompensation is a way of preconditioning the
WDATA output signal to adjust for the effects of bit shift on
the data as it is written to the disk surface Bit shift is caused
by the magnetic interaction of data bits as they are written
to the disk surface and has the effect of shifting these data
bits away from their nominal position in the serial MFM or
FM data pattern Data that is subject to bit shift is much
harder to read by a data separator and can cause soft read
errors Write precompensation predicts where bit shift could
occur within a data pattern It then shifts the individual data
bits early late or not at all such that when they are written
to the disk the resultant shifted data bits will be back in their
nominal position
The PC8477B supports software programmable write pre-
compensation Upon power up the default write precomp
values will be used (see Table 3-5) The programmer can
choose a different value of write precomp with the DSR
register if desired (see Table 3-4) Also on power up the
default starting track number for write precomp is track zero
This starting track number for write precomp can be
changed with the Configure command
5 10 LOW POWER MODE LOGIC
The PC8477B supports a low power mode in which the
oscillator and data separator circuitry are turned off The
floppy controller will typically draw about 1 mA while in low
TL F 11332 – 13
Also
the