System and method for using a stepper motor as a rotary sensor

1. A method, comprising:

driving a stepper motor; and

using the stepper motor as a rotary sensor when the stepper motor is not being driven.

2. The method of claim 1 , further comprising:

detecting a first control signal from an external control circuit;

allowing power to flow to the stepper motor when the first control signal is detected; and

preventing power from flowing to the stepper motor when the first control signal is not detected.

3. The method of claim 2 , wherein driving the stepper motor comprises applying the first control signal from the external control circuit.

4. The method of claim 1 , wherein the stepper motor comprises two or more coils and a rotor.

5. The method of claim 4 , wherein using the stepper motor as a rotary sensor comprises sensing a change in voltage across the two or more coils caused by a change in magnetic flux resulting from movement of the rotor.

6. The method of claim 5 , wherein sensing a change in voltage across the two or more coils comprises activating a coil reference tie down by coupling a first end of a first coil to a low potential node and sensing a voltage level at a second end of the first coil.

7. The method of claim 6 , wherein the two or more coils are oriented perpendicular to each other.

8. The method of claim 7 , further comprising:

interpreting the change in voltage across the two or more coils to determine a number of steps the rotor was turned through and a direction of rotation of the rotor.

9. The method of claim 8 , further comprising:

deactivating the coil reference tie down by decoupling the first end of the first coil from the low potential node;

allowing power to flow to the stepper motor; and

applying a second control signal from the external control circuit.

10. An apparatus, comprising:

a first stepper motor drive circuit, wherein the first stepper motor drive circuit comprises

a first transistor to supply power to the first stepper motor drive circuit,

a second transistor to enable operation of the first stepper motor drive circuit,

a third transistor to disable operation of the first stepper motor drive circuit, and

a fourth transistor,

wherein the first transistor and the second transistor have a same first source node coupled to a high power supply, the third transistor and the fourth transistor have a same second source node coupled to a low power supply, the first transistor and the third transistor have a same first drain node, and the second transistor and the fourth transistor have a same second drain node;

logic to regulate power flow to the first stepper motor drive circuit; and

a reference transistor coupled to the second drain node, the reference transistor to apply a low voltage potential to the second drain node when power is not flowing to the first stepper motor drive circuit.

11. The apparatus of claim 10 , wherein the logic comprises an OR gate having a first input, a second input and an output, the first input and the second input configured to receive a control signal from an external control circuit and the output to control operation of a first power transistor, a second power transistor and a third power transistor.

12. The apparatus of claim 10 , further comprising:

a first magnetic coil, the first magnetic coil having a first end and a second end, the first end coupled to the first drain node, and the second end coupled to the second drain node;

a second magnetic coil; and

a second stepper motor drive circuit coupled to the second magnetic coil.

13. The apparatus of claim 12 , wherein the first magnetic coil and the second magnetic coil are oriented perpendicular to each other.

14. An apparatus, comprising:

a stepper motor; and

means for using the stepper motor as a rotary sensor when the stepper motor is not being driven.

15. The apparatus of claim 14 , wherein the means for using the stepper motor as a rotary sensor comprises a logic gate to disconnect a power supply to a circuit, the circuit to sense a change in voltage across two or more coils in the stepper motor resulting from physical movement of a rotor in the stepper motor.