Maintaining consistent location for spindle interrupts when spindle motor switches modes

A data storage device, comprising:

• one or more disks;
• a spindle motor for rotating the one or more disks, wherein the spindle motor comprises a plurality of magnetic pole-pairs, each magnetic pole-pair corresponding to one electrical revolution, and wherein the spindle motor is configured to operate in a plurality of modes, including at least a first mode and a second mode, wherein the first mode is associated with a first number of back electromotive force (BEMF) windows for each spindle interrupt, and wherein the second mode is associated with a second number of BEMF windows for each spindle interrupt;
• and
• one or more processing devices configured to:
• select a reference point for spindle motor interrupt service routines (ISRs);
• determine a target number of BEMF windows, wherein determining the target number of BEMF windows is based at least in part on one or more of the first number of BEMF windows and a number of magnetic pole-pairs;
• start a counter at a first ISR;
• increment, for each BEMF window detected, the counter;
• and
• when the counter is equal to the target number of BEMF windows, switch operation of the spindle motor from the first mode to the second mode.

A method of operating a data storage device, comprising:

• selecting a reference point for interrupt service routines (ISRs) for a spindle motor of the data storage device, wherein the spindle motor comprises a plurality of magnetic pole-pairs, and wherein the spindle motor is configured to operate in a plurality of modes, including at least a first mode and a second mode;
• determining a delay time, wherein the delay time is based at least in part on a number of magnetic pole-pairs and a mechanical revolution period of the spindle motor;
• starting a timer at a first ISR;
• and
• when the timer is equal to the delay time, switching operation of the spindle motor from the first mode to the second mode.

A method of operating a data storage device, comprising:

• selecting a reference point for interrupt service routines (ISRs) for a spindle motor of the data storage device, wherein the spindle motor comprises a plurality of magnetic pole-pairs, each magnetic pole-pair corresponding to one electrical revolution, and wherein the spindle motor is configured to operate in a plurality of modes, including at least a first mode and a second mode, wherein the first mode is associated with a first number of back electromotive force (BEMF) windows for each spindle interrupt, and wherein the second mode is associated with a second number of BEMF windows for each spindle interrupt;
• determining a target number of BEMF windows, wherein determining the target number of BEMF windows is based at least in part on one or more of the first number of BEMF windows and a number of magnetic pole-pairs;
• starting a counter at a first ISR;
• incrementing, for each BEMF window detected, the counter;
• and
• when the counter is equal to the target number of BEMF windows, switching operation of the spindle motor from the first mode to the second mode.