Index signal recovery on modern HDDs

[shaun]

Hi there,
this is the beginning (hopefully) of an in-depth discussion of the generation of the index signal in recent hard drivers.
Long ago, disk drives used photo interrupters or inductive pickups to "know where they are".
Today I was studying data sheets of different motor combo chips from TI, Hitachi and ST.
Apart from chips which only contain the power drivers for the spindle motors I found two derivates when it comes to the treatment of the index signal.
Some can be configured to accept an external index signal in order to synchronize the commutation of the spindle motor, and the others do generate an index signal themselves from the evaluation of the back-EMF also used for commutation.
I wonder whether modern high density disks actually rely on the accuracy of a reconstructed index signal. This would, by the way, cause a new problem in plattern swapping: when the data on the platters must be in tight synchronicity with the motor drive signals, it won't be possible to do swapping. As it is in most cases, I am still asking myself (and you, of course) what the drive controller does with this motor-centric index signal.
Another question is whether the data slicing is synchronized to the data stream and thereby to the motor rotation or the spindle is tightly locked to the servo track.
Just some thoughts about speed accuracy, and you're invited to join in thinking

[N.C.]

Hi,

I think, the platter swap is this case is only depend on intelligence of the drive.
Right, the tracks are context with the index signal from the motor, but the shifting is only need one time measured, when the drive is recalibrated on power on. (spin up exactly)
Since it is measured, only need one + or - during works...

This is manufacturer's choice...

Regars,
Janos

[shaun]

Yep, as plattern swapping works in many cases (and most probably for other reasons than index mismatch in those cases it doesn't work), there's obviously no requirement for a permanent synchronicity. Even the jitter of the fastest comparators won't suffice for hitting exact bit positions in modern high density drives, especially not the low end circuitry used in common combo chips. So I'm still wondering why some drives actually do use the index signal from the motor driver. Just for short term bit synchronization?
All this may become important for a new solution...

[pepe]

Hi,

it is probably used to toggle that bit in the status register,

pepe