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參數資料
| 型號： | 37C669 |
| 廠商： | SMSC Corporation |
| 英文描述： | DIODE SCHOTTKY SINGLE 25V 200mW 0.5V-vf 200mA-IFM 30mA-IF 2uA-IR SOD-323 3K/REEL |
| 中文描述： | 電腦98/99順從超級I / O軟盤控制器與紅外線支持 |
| 文件頁數： | 129/164頁 |
| 文件大?。?/td> | 621K |
| 代理商： | 37C669 |
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67
PRETRK
-
Pre-Compensation
Start
Track
Number. Programmable from track 0 to 255.
Defaults to track 0. A "00" selects track 0; "FF"
selects track 255.
Version
The Version command checks to see if the
controller is an enhanced type or the older type
(765A). A value of 90 H is returned as the result
byte.
Relative Seek
The command is coded the same as for Seek,
except for the MSB of the first byte and the DIR
bit.
DIR
Head Step Direction Control
RCN
Relative
Cylinder
Number
that
determines how many tracks to step the
head in or out from the current track
number.
The Relative Seek command differs from the
Seek command in that it steps the head the
absolute number of tracks specified in the
command instead of making a comparison
against
an
internal
register.
The
Seek
command is good for drives that support a
maximum of 256 tracks. Relative Seeks cannot
be overlapped with other Relative Seeks. Only
one Relative Seek can be active at a time.
Relative Seeks may be overlapped with Seeks
and Recalibrates.
Bit 4 of Status Register 0
(EC) will be set if Relative Seek attempts to step
outward beyond Track 0.
As an example, assume that a floppy drive has
300 useable tracks.
The host needs to read
track 300 and the head is on any track (0-255).
If a Seek command is issued, the head will stop
at track 255.
If a Relative Seek command is
issued, the FDC will move the head the
specified number of tracks, regardless of the
internal
cylinder
position
register
(but
will
increment the register). If the head was on track
40 (d), the maximum track that the FDC could
position the head on using Relative Seek will be
295 (D), the initial track + 255 (D). The
maximum count that the head can be moved
with a single Relative Seek command is 255
(D).
The internal register, PCN, will overflow as the
cylinder number crosses track 255 and will
contain 39 (D). The resulting PCN value is thus
(RCN + PCN) mod 256. Functionally, the FDC
starts counting from 0 again as the track
number goes above 255 (D).
It is the user's
responsibility to compensate FDC functions
(precompensation
track
number)
when
accessing tracks greater than 255.
The FDC
does not keep track that it is working in an
"extended track area" (greater than 255).
Any
command issued will use the current PCN value
except for the Recalibrate command, which only
looks for the TRACK0 signal.
Recalibrate will
return an error if the head is farther than 79 due
to its limitation of issuing a maximum of 80 step
pulses.
The user simply needs to issue a
second
Recalibrate
command.
The
Seek
command
and
implied
seeks
will
function
correctly within the 44 (D) track (299-255) area
of the "extended track area".
It is the user's
responsibility not to issue a new track position
that will exceed the maximum track that is
present in the extended area.
To return to the standard floppy range (0-255) of
tracks, a Relative Seek should be issued to
cross the track 255 boundary.
A Relative Seek can be used instead of the
normal Seek, but the host is required to
calculate the difference between the current
head
location
and
the
new
(target)
head
location. This may require the host to issue a
Read ID command to ensure that the head is
physically on the track that software assumes it
to be.
Different FDC commands will return
different cylinder results which may be difficult
DIR
ACTION
0
1
Step Head Out
Step Head In
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