GM82C803CN Datasheet, PDF (57/84 Pages) List of Unclassifed Manufacturers – 2.88 MB FDC/ Dual UARTs with FIFO/PIO(EPP/ ECP)/ IDE Interface/ S-IR/ PnP

GM82C803CN
Table 4-29. Effects of WGATE and GAP Bites on Format and Write commands
GAP WG
Mode
GAP2 Length Written
During Format
Portion of GAP2 Rewritten
by Write Data Command
0
0
Conventional
22bytes
0 bytes
0
1
1
0
1
1
Perpendicular
( 500Kbps )
Reserved
( Conventional )
Perpendicular
(1Mbps)
22bytes
22bytes
41bytes
19bytes
0 bytes
38bytes
Looking at the second-command byte, DC3-0 correspond to the four logical drives. A 0 written to DCn sets drive n to
conventional mode, and a 1 sets drive n to perpendicular mode. Also, the OW(Overwrite) bit offers additional control.
When OW = 1, changing the values of DC3-0 (drive configuration bits) is enabled. When OW=0, the internal values of
DC3-0 are unaffected, regardless of what is written to DC3-0.
The function of the DCn bits must also be qualified by setting both WG and GAP to 0. If WG and GAP are used i.e. n-
ot set to 00 they will override whatever is programmed in the DCn bits. Table 4-26 indicates the operation of the FDC
based on the values of GAP and WG. Note that when GAP and WG are both 0, the DCn bits are used to configure each
logical drive as conventional or perpendicular. DC3-0 are unaffected by a software reset, but WG and GAP are both cl-
eared to 0 after a software reset. A hardware reset will reset all the bits to zero (conventional mode for all drives). The
Perpendicular Mode command bits may be rewritten at any time.
Perpendicular Recording type disk drives have a Pre-Erase Head Which leads The Read/Write Head by 200 m, wh-
ich translates to 38 bytes at the 1 Mbps data transfer rate(19 bytes at 500 Kbps). The increased spacing between the two
heads requires a larger GAP2 between the Address Field and Data Field of a sector at 1Mbps. This GAP2 length of 41
bytes(at 1Mbps) will ensure that the Preamble in the Data Field is completely pre-erased by the Pre-Erase Head. Also,
during Write Data operations to a perpendicular drive a portion of GAP2 must be rewritten by the controller to guara-
ntee that the Data Field Preamble has been pre-erased..
Configure
This command will control some operation modes of the controller. It should be issued during the initialization of the
FDC after power up. The function of the bits in the Configure registers are described below. These bits are set to their
default values after a hardware reset. The value of each bit after a software reset is explained. A Configure command
need not be issued if the default values of the FDC meet the system requirements
Configure Default Values:
EIS
: No Implied Seeks
EFIFO : FIFO Disabled
POLL
: Polling Enabled
FIFOTHR : FIFO Threshold Set to 1 Byte
PRETRK : Pre-Compensation Set to Track 0