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Figure 13. VCM Bemf Offset CALIBRATION Routine

At the end of the calibration routine the analog value measured at pin 31 is rapresenting the VCM BEMF value

at the zero motion (BEMF zero value). With the ADC it is possible to operate a new convertion in order to mem-

orize this value and to take in account of it during the load/unload procedure.

3.8.4 Power Off Unload - Active brake and constant voltage unload operation

In case of power shut down, an unload procedure start automatically in order to take the heads over the ramp

in the parking position (the same procedure can be also enabled, when the power is on, via serial port program-

ming the unload/retract status of the VCM -> reg. 03H. In this case at the end of the unload phase the spindle

motor is driven in tri-state condition).

The unload procedure doesn’t start at power off if the VCM status bit are set to 000 because the system is con-

sidering the heads already in park position. No entering the unload procedure also the spindle brake is not ac-

tivated.

The unload procedure is done in two step:

- active brake

- constant voltage unload operation

The unload procedure take place only if the VCM status bit have moved from the 000 configuration. Otherwise

the unload procedure doesn't start and in case of power shut down the spindle motor enter the brake condition.

Active Brake : it is used to have a fast recovery of the VCM velocity down to the unload programmed velocity.

If just before a power shut down a fast seek was commanded, it is necessary to recover the VCM velocity in

Set VCM in T ristate

Write Reg.03H

VCMState[2:0] = 001

START

VCM Bemf offset calibration

Routine

Set ADC Clock

Write Reg.06H

PREADC[1:0] = 01

START 10Bit ADC Conversion

of the BEMF Channel

Write Reg.0CH

ADC_CH_ADDR[1:0] =10

ADC_ST ART =1

EXIT

VCM Bemf offset calibration

W ait End of

Conversion

Read 10Bit ADC

Read Reg.0BH

ADC_DATA[9:2]

Read Reg.0CH

ADC_DATA[1:0]

YES

NO

OPTIONAL

Latch O ffset Compensation

W rite Reg.03H

BemfOffCal = 1

then

BemfOffCal = 0

To restart this routine is mandatory to start

First the clear routine (see Fig. 10)