PC87310 datasheet(35/52 Pages) NSC

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40 Functional Description (Continued)

SEEK

There are two ways to move the disk drive head to the

desired track number Method One is to enable the Implied

Seek Mode This way each individual Read or Write com-

mand will automatically move the head to the track speci-

fied in the command

Method Two is using the Seek Command During the Execu-

tion Phase of the Seek Command the track number to seek

to is compared with the present track number and a step

pulse is produced to move the head one track closer to the

desired track number This is repeated at the rate specified

by the Specify Command until the head reaches the correct

track At this point an interrupt is generated and a Sense

Interrupt Command is required to clear the interrupt

During the Execution Phase of the Seek Command the only

indication via software that a Seek Command is in progress

is bits 0 – 3 (Drive Busy) of the Main Status Register Bit 4 of

the Main Status Register (Command in Progress) is not set

The internal microengine is capable of executing seeks on

more than one drive at a time However since the active

drive is selected by the system software and there is no way

to coordinate this selection with the microengine step puls-

es only one seek command should be issued by the soft-

ware at a time No other command except the Sense Inter-

rupt Command should be issued while a Seek Command is

in progress

If the extended track range mode is enabled a fourth byte

should be written in the Command Phase to indicate the

four most significant bits of the desired track number Other-

wise only three bytes should be written

RECALIBRATE

The Recalibrate Command is very similar to the Seek Com-

mand It is used to step a drive head out to track zero Step

pulses will be produced until the track zero signal from the

drive becomes true If the track zero signal does not go true

before 77 step pulses are issued an error is generated If

the extended track range mode is enabled an error is not

generated until 3917 pulses are issued

Recalibrations on more than one drive at a time should not

be issued for the same reason as explained in the Seek

Command No other command except the Sense Interrupt

Command should be issued while a Recalibrate Command

is in progress

SENSE INTERRUPT STATUS

An interrupt is generated by the controller when any of the

following conditions occur

1 Upon entering the Result Phase of

a Read Data Command

b Read Deleted Data Command

c Write Data Command

d Write Deleted Data Command

e Read a Track Command

f Read ID Command

g Format Command

h Scan Commands

2 During data transfers in the Execution Phase while in the

Non-DMA mode

3 Internal Ready signal changes state (only occurs imme-

diately after a hardware or software reset)

4 Seek or Recalibrate Command termination

An interrupt generated for reasons 1 and 2 above occurs

during normal command operations and are easily discern-

ible by the mP During an execution phase in Non-DMA

Mode bit 5 (Execution Mode) in the Main Status Register is

set to 1 Upon entering Result Phase this bit is set to 0

Reasons 1 and 2 do not require the Sense Interrupt Status

command The interrupt is cleared by reading or writing in-

formation to the data register

Interrupts caused by reasons 3 and 4 are identified with the

aid of the Sense Interrupt Status Command This command

resets the interrupt when the command byte is written Use

bits 5 6 and 7 of ST0 to identify the cause of the interrupt as

shown in Table 4-19

TABLE 4-19 Status Register 0 Termination Codes

Status Register 0

Interrupt

Seek

Cause

Code

End

Internal Ready Went True

Normal Seek Termination

Abnormal Seek Termination

TABLE 4-20 Step Head Load and Unload Timer

Definitions (500 kbs MFM)

Timer

Mode 1

Mode 2

Unit

Value

Range

Value

Range

Step Rate

(16 b N)

1 – 16

(16 b N)

1–16

Head Unload

N c 16

0 – 240 N c 512 0 – 7680

Head Load

N c 2

0 – 254

N c 32

0 – 4064

Issuing a Sense Interrupt Status Command without an inter-

rupt pending is treated as an invalid command

If the extended track range mode is enabled a third byte

should be read in the Result Phase which will indicate the

four most significant bits of the Present Track Number Oth-

erwise only two bytes should be read

SPECIFY

The Specify Command sets the initial values for each of the

three internal timers The timer programming values are

shown in Table 4-20

The Head Load and Head Unload timers are artifacts of the

PD765A These timers determine the delay from loading

the head until a read or write command is started and un-

loading the head sometime after the command was com-

pleted Since the FDC’s head load signal is now from the

software controlled Motor lines in the Drive Control Regis-

ter these timers only provide some delay from the initiation

of a command until it is actually started These times can be

extended by setting the TMR bit in the Mode Command

The Step Rate Time defines the time interval between adja-

cent step pulses during a Seek Implied Seek or Recali-

brate Command

The times stated in the table are affected by the Data Rate

The values in the table are for 500 kbs MFM (250 kbs FM)

and 1 Mbs MFM (500 kbs FM) For a 300 kbs MFM data

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