82077AA Datasheet, PDF (43/62 Pages) Intel Corporation – CHMOS SINGLE-CHIP FLOPPY DISK CONTROLLER

82077AA
8 4 Read Write Data Operations
A read or write data operation requires several steps
to complete successfully The motor needs to be
turned on the head positioned to the correct cylin-
der the DMA controller initialized the read or write
command initiated and an error recovery scheme
implemented The flowchart in Figure 8-5 highlights
a recommended algorithm for performing a read or
write data operation
Before data can be transferred to or from the disk-
ette the disk drive motor must be brought up to
speed For most 3 disk drives the spin-up time is
300 ms while the 5 drive usually requires about
500 ms due to the increased moment of inertia asso-
ciated with the larger diameter diskette
One technique for minimizing the motor spin-up de-
lay in the read data case is to begin the read opera-
tion immediately after the motor is turned on When
the motor is not initially up to speed the internal
data separator will fail to lock onto the incoming data
stream and report a failure in the status registers
The read operation is then repeated until successful
status is obtained There is no risk of a data integrity
problem since the data field is CRC validated But it
is not recommended to use this technique for the
write data operation even though it requires suc-
cessful reading of the ID field before the write takes
place The data separator performance of the
82077AA is such that locking to the data stream
could take place while the motor speed variation is
still significant This could result in errors when an
attempt is made to read the disk media by other disk
controllers that have a narrower incoming data
stream frequency bandwidth
After the motor has been turned on the matching
data rate for the media inserted into the disk drive
should then be programmed to the 82077AA via the
Configuration Control Register (CCR) The 82077AA
is designed to allow a different data rate to be pro-
grammed arbitrarily without disrupting the integrity of
the device In some applications it is required to au-
tomatically determine the recorded data rate of the
inserted media One technique for doing this is to
perform a READ ID operation at each available data
rate until a successful status is returned in the result
phase
If implied seeks are not enabled the disk drive head
must be positioned over the correct cylinder by exe-
cuting a SEEK command After the seek is com-
plete a head settling time needs to be asserted be-
fore the read or write operation begins For most
drives this delay should be a minimum of 15 ms
When using implied seeks the minimum head set-
tling time can be enforced by the head load time
(HLT) parameter designated in the SPECIFY com-
mand For example a HLT value of 8 will yield an
effective head settling time of 16 ms for a pro-
grammed data rate of 500 Kbps Of course if the
head is already positioned over the correct cylinder
the head settling time does not need to be enforced
The DMA controller is then initialized for the data
transfer and the read or write command is executed
Typically the DMA controller will assert Terminal
Count (TC) when the data transfer is complete The
82077AA will then complete the current data trans-
fer and assert the INT signal signifying it has entered
the result phase The result phase can also be en-
tered by the 82077AA if an error is encountered or
the last sector number equals the End of Track
(EOT) parameter
Based on the algorithm in Figure 8-5 if an error is
encountered after reading the result bytes two more
retries are performed by reinitializing the DMA con-
troller and re-issuing the read or write data com-
mand A persisting failure could indicate the seek
operation did not achieve proper alignment between
the head and the track The disk head should then
be recalibrated and the seek repeated for a maxi-
mum of two more tries Unsuccessful operation after
this point should be reported as a disk failure to the
operating system
8 5 Formatting
The disk formatting procedure involves positioning
the head on each track and creating a fixed format
field used for organizing the data fields The flow-
chart in Figure 8-6 highlights the typical format pro-
cedure
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