Constant sensing current for reading resistive memory

1. A method of reading a resistive memory bit cell, the method comprising:

generating a load voltage at a gate of a mirror transistor, wherein the mirror transistor is diode-connected and coupled to a current mirror, wherein the current mirror supplies a constant current that is invariant with process-voltage-temperature variations;

generating a data voltage by connecting the gate of the mirror transistor to a data load transistor coupled to the resistive memory bit cell, to provide the load voltage to the data load transistor such that the constant current is mirrored as sensing current flowing through the resistive memory bit cell;

connecting the gate of the mirror transistor to reference load transistors coupled to reference cells, to provide the load voltage to the reference load transistors such that the constant current is mirrored as reference currents flowing through the reference cells;

deriving a reference voltage at a common node which is coupled to first terminals of diode-connected reference clamp transistors coupled to the reference cells, wherein second terminals of the reference clamp transistors are connected to each other; and

coupling the common node to a gate of a data clamp transistor coupled to the resistive memory bit cell.

2. The method of claim 1 , comprising coupling the mirror transistor to a voltage generator load transistor, and deriving a degeneration voltage from a gate of the voltage generator load transistor.

3. The method of claim 2 , further comprising driving gates of a degeneration data transistor coupled to the data load transistor and degeneration reference transistors coupled to the reference load transistors with the degeneration voltage.

4. The method of claim 1 , further comprising determining a resistance value or logical value of the resistive memory bit cell based on a comparison of the data voltage and the reference voltage.

5. The method of claim 4 , wherein a result of the comparison is invariant of process-voltage-temperature variations.

6. The method of claim 1 , wherein the resistive memory bit cell is a magnetoresistive random access memory (MRAM) bit cell or a magnetic tunnel junction (MTJ).

7. An apparatus comprising:

a resistive memory bit cell;

a load voltage generator configured to generate a load voltage, the load voltage generator comprising a current mirror and a mirror transistor coupled to the current mirror, wherein the mirror transistor is diode-connected and the load voltage is generated at a gate of the mirror transistor, wherein the current mirror is configured to supply a constant current that is invariant with process-voltage-temperature variations;

a data load transistor coupled to the resistive memory bit cell, wherein a gate of the data load transistor is connected to the gate of the mirror transistor to provide the load voltage to the data load transistor, and wherein the data load transistor is configured to generate a data voltage based on the constant current mirrored as sensing current passed through the resistive memory bit cell; and

reference load transistors coupled to reference cells, wherein gates of the reference load transistors are connected to the gate of the mirror transistor to provide the load voltage to the reference load transistors such that the constant current is mirrored as reference currents flowing through the reference cells;

diode-connected reference clamp transistors coupled to the reference cells, wherein first terminals of the reference clamp transistors are coupled to a common node configured to provide a reference voltage, and wherein second terminals of the reference clamp transistors are connected to each other; and

a data clamp transistor coupled to the resistive memory bit cell, wherein the common node is coupled to a gate of the data clamp transistor.

8. The apparatus of claim 7 , comprising a voltage generator load transistor coupled to the mirror transistor, wherein a gate of the voltage generator load transistor is configured to provide a degeneration voltage.

9. The apparatus of claim 8 , further comprising a degeneration data transistor coupled to the data load transistor and degeneration reference transistors coupled to the reference load transistors, wherein gates of the degeneration data transistor and the degeneration reference transistors are coupled to the degeneration voltage.

10. The apparatus of claim 7 , further comprising logic configured to determine a resistance value or logical value of the resistive memory bit cell based on a comparison of the data voltage and the reference voltage.

11. The apparatus of claim 10 , wherein a result of the comparison is invariant of process-voltage-temperature variations.

12. The apparatus of claim 7 , wherein the resistive memory bit cell is a magnetoresistive random access memory (MRAM) bit cell or a magnetic tunnel junction (MTJ).

13. The apparatus of claim 7 , integrated in a semiconductor die.

14. The apparatus of claim 7 , integrated in a device, selected from the group consisting of a set-top box, music player, video player, entertainment unit, navigation device, communications device, personal digital assistant (PDA), fixed location data unit, and a computer.

15. A system comprising:

a resistive memory bit cell;

means for generating a load voltage at a gate of a mirror transistor, based on a constant current that is invariant with process-voltage-temperature variations;

means for generating a data voltage at a data load transistor of the resistive memory bit cell, the data load transistor connected to the gate of the mirror transistor, based on the load voltage and the constant current mirrored as sensing current passed through the resistive memory bit cell; and

means for generating reference currents from the constant current, to be passed through reference cells, wherein reference load transistors are coupled to the reference cells, and the reference load transistor are connected to the gate of the mirror transistor;

means for deriving a reference voltage at a common node which is coupled to first terminals of diode-connected reference clamp transistors coupled to the reference cells, wherein second terminals of the reference clamp transistors are connected to each other; and

means for providing the reference voltage to a gate of a data clamp transistor coupled to the resistive memory bit cell.

16. The system of claim 15 , further comprising means for determining a resistance value or logical value of the resistive memory bit cell based on a comparison of the data voltage and the reference voltage.

17. The system of claim 16 , wherein a result of the comparison is invariant of process-voltage-temperature variations.

18. The system of claim 15 , wherein the resistive memory bit cell is a magnetoresistive random access memory (MRAM) bit cell or a magnetic tunnel junction (MTJ).