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The dynamic window margin performance curve also

indicates how much bit jitter (or window margin) can

be tolerated by the data separator. This parameter is

shown on the y-axis of the graph. Bit jitter is caused

by the magnetic interaction of adjacent data pulses on

the disk, which effectively shifts the bits away from

their nominal positions in the middle of the bit window.

Window margin is commonly measured as a percent-

age. This percentage indicates how far a data bit can

be shifted early or late with respect to its nominal bit

position, and still be read correctly by the data sepa-

rator. If the data separator cannot correctly decode a

shifted bit, then the data is misread and a CRC error

results.

The dynamic window margin performance curve sup-

plies two pieces of information:

• The maximum range of MSV (also called “lock

range”) that the data separator can handle with no

read errors.

• The maximum percentage of window margin (or

bit jitter) that the data separator can handle with no

read errors.

Thus, the area under the dynamic window margin

curves in Figure 42 is the range of MSV and bit jitter

that the FDC can handle with no read errors. The in-

ternal digital data separator of the FDC performs

much better than comparable digital data separator

designs, and does not require any external compo-

nents.

The controller maximizes the internal digital data sep-

arator by implementing a read algorithm that enhanc-

es the lock characteristics of the fully digital Phase-

Locked Loop (PLL). The algorithm minimizes the ef-

fect of bad data on the synchronization between the

PLL and the data.

It does this by forcing the fully digital PLL to re-lock to

the clock reference frequency any time the data sep-

arator attempts to lock to a non-preamble pattern.

See the state diagram of this read algorithm in Figure

43.

FIGURE 43. Read Algorithm State Diagram

3.2.3

Perpendicular Recording Mode

Support

The FDC is fully compatible with perpendicular re-

cording mode disk drives at all data transfer rates.

These perpendicular drives are also called 4 Mbyte

(unformatted) or 2.88 Mbyte (formatted) drives. This

refers to their maximum storage capacity.

Perpendicular recording orients the magnetic flux

changes (which represent bits) vertically on the disk

surface, allowing for a higher recording density than

conventional longitudinal recording methods. This in-

creased recording density increases data rate by up

to 1 Mbps, thereby doubling the storage capacity. In

addition, the perpendicular 2.88 MB drive is read/write

compatible with 1.44 MB and 720 KB diskettes (500

Kbps and 250 Kbps respectively).

The 2.88 MB drive has unique format and write data

timing requirements due to its read/write head and

pre-erase head design. This is illustrated in Figure 44.

Not

6th Bit

Read Gate = 1

Read Gate = 0

PLL

to data,

wait 6 bits.

3 Address

Not Found

Wait for

that is not a

bit.

Bit is

Preamble

Bit is Not

Preamble

3 Address

Marks

Check for

3 address

mark bytes.

Not 3rd

Address Mark

PLL idle

locked

to clock.

Operation

Completed

Read ID

field or

data field.

Found

locking

1st bit

preamble

Marks

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