Suppressing outlier drift coefficients while programming phase change memory synapses

1. A computer-implemented method for suppressing outlier drift of a phase change memory (PCM) device, the method comprising:

programming, by a controller, a conductance of the PCM device, the programming comprising:

configuring the conductance of the PCM device to a first conductance value at a first time-point, the first time-point being a programming time-point;

determining, at a first pre-compensation time-point, that the conductance of the PCM device has changed to a second conductance value that differs from a target conductance value by no more than a predetermined threshold; and

reprogramming the PCM device to the first conductance value at a second time-point, including measuring the conductance again, but at a second pre-compensation time-point.

2. The computer-implemented method of claim 1 , wherein the first pre-compensation time-point is at a predetermined time duration after the programming time-point.

3. The computer-implemented method of claim 2 , wherein the target conductance value at the first pre-compensation time-point is determined based on a target time window, which has a second predetermined duration after the programming time-point, wherein the conductance of the PCM device is desired to be in a particular range during the target time window.

4. The computer-implemented method of claim 2 , wherein programming is continued until the difference between (i) the second conductance value at the pre-compensation time-point and (ii) the target conductance value is less than the predetermined threshold.

5. The computer-implemented method of claim 1 , wherein the programming further comprises:

determining, at the first pre-compensation time-point, that the difference between the second conductance value at the pre-compensation time-point and the target conductance value is less than the predetermined threshold;

determining at a first checkpoint, that the conductance of the PCM device has changed to a third conductance value that differs from a second target conductance value by no more than the predetermined threshold; and

programming the PCM device to the first conductance value at a third time-point, including measuring said pre-compensation again.

6. The computer-implemented method of claim 5 , wherein the first checkpoint is at a second predetermined time duration after the programming time-point.

7. The computer-implemented method of claim 5 , wherein the second target conductance value at the first checkpoint is determined based on a target time window, which is at a second predetermined duration since the programming time-point, wherein the conductance of the PCM device is maintained within a particular range during the target time window.

8. The computer-implemented method of claim 1 , wherein the PCM device is being used as a synapse in an artificial neural network system, the conductance being a weight that is assigned to the synapse.

9. The computer-implemented method of claim 1 , wherein the PCM device comprises a plurality of PCM devices, each of the PCM devices being associated with a respective target conductance value.

10. The computer-implemented method of claim 1 , wherein the conductance of the PCM device changes to the second conductance value because of conductance drift.

11. A system comprising:

a controller; and

a co-processor comprising one or more cross-bar arrays; and wherein, the controller is configured to implement an artificial neural network using the co-processor by mapping a layer of the artificial neural network with a cross-bar array, wherein implementing the artificial neural network comprises a method comprising:

programming a conductance of a phase change memory (PCM) device from the cross-bar array, the programming comprising:

configuring the conductance of the PCM device to a first conductance value at a first time-point, the first time-point being a programming time-point;

determining, at a first pre-compensation time-point, that the conductance of the PCM device has changed to a second conductance value that differs from a target conductance value by no more than a predetermined threshold; and

reprogramming the PCM device to the first conductance value at a second time-point, including measuring the conductance again, but at a second pre-compensation time-point.

12. The system of claim 11 , wherein the first pre-compensation time-point is at a predetermined time duration after the programming time-point.

13. The system of claim 12 , wherein the target conductance value at the first pre-compensation time-point is determined based on a target time window, which has a second predetermined duration after the programming time-point, wherein the conductance of the PCM device is to be in a particular range during the target time window.

14. The system of claim 12 , wherein programming is continued until the difference between (i) the second conductance value at the pre-compensation time-point and (ii) the target conductance value is less than the predetermined threshold.

15. The system of claim 11 , wherein the programming further comprises:

determining, at the first pre-compensation time-point, that the difference between the second conductance value at the pre-compensation time-point and the target conductance value is less than the predetermined threshold;

determining at a first checkpoint, that the conductance of the PCM device has changed to a third conductance value that differs from a second target conductance value by no more than the predetermined threshold; and

reprogramming the PCM device to the first conductance value at a third time-point, including measuring said pre-compensation again.

16. The system of claim 15 , wherein the first checkpoint is at a second predetermined time duration after the programming time-point.

17. The system of claim 15 , wherein the second target conductance value at the first checkpoint is determined based on a target time window, which is at a second predetermined duration since the programming time-point, wherein the conductance of the PCM device is maintained within a particular range during the target time window.

18. The system of claim 15 , wherein the PCM device is being used as a synapse in an artificial neural network system, the conductance being a weight that is assigned to the synapse.

19. A computer program product comprising a computer-readable storage medium having program instructions embodied therewith, the program instructions executable by a processing circuit to perform a method to suppress outlier drift of a phase change memory (PCM) device, the method comprising:

programming a conductance of the PCM device, the programming comprising:

configuring the conductance of the PCM device to a first conductance value at a first time-point, the first time-point being a programming time-point;

determining, at a first pre-compensation time-point, that the conductance of the PCM device has changed to a second conductance value that differs from a target conductance value by no more than a predetermined threshold; and

reprogramming the PCM device to the first conductance value at a second time-point, including measuring the conductance again, but at a second pre-compensation time-point.

20. The computer program product of claim 19 , wherein the first pre-compensation time-point is at a predetermined time duration after the programming time-point.

21. The computer program product of claim 20 , wherein the target conductance value at the first pre-compensation time-point is determined based on a target time window, which has a second predetermined duration after the programming time-point, wherein the conductance of the PCM device is to be in a particular range during the target time window.

22. The computer program product of claim 20 , wherein programming is continued until the difference between (i) the second conductance value at the pre-compensation time-point and (ii) the target conductance value is less than the predetermined threshold.

23. A cross-bar array comprising:

a plurality of phase change memory (PCM) devices, a PCM being at each of a plurality of cross-points in the cross-bar array, wherein one or more PCM devices represents an analog synapse in an artificial neural network, wherein programming a conductance of a PCM device from the cross-bar array comprises:

receiving a signal to configure a conductance of the PCM device to a first conductance value at a first time-point, the first time-point being a programming time-point;

determining, at a pre-compensation time-point, that the conductance of the PCM device has changed to a second conductance value that differs from a target conductance value by no more than a predetermined threshold; and

receiving another signal to configure the conductance of the PCM device to the first conductance value at a second time-point, including measuring the conductance again, but at a second pre-compensation time-point.

24. The cross-bar array of claim 23 , wherein the pre-compensation time-point is at a predetermined time duration after the programming time-point, and wherein the target conductance value at the pre-compensation time-point is determined based on a target time window, which is at a second predetermined duration after the programming time-point, wherein the conductance of the PCM device is to be in a particular range during the target time window.

25. A computer-implemented method comprising:

configuring, from a plurality of phase change memory (PCM) devices in a cross-bar array, a conductance value of a PCM device by:

receiving a signal to configure the conductance value of the PCM device to a first conductance value at a first time-point, the first time-point being a programming time-point;

determining at a pre-compensation time-point, that the conductance value of the PCM device has changed to a second conductance value that differs from a target conductance value by no more than a predetermined threshold; and

receiving another signal to configure the conductance value of the PCM device to the first conductance value at a second time-point, including measuring the conductance value again, but at a second pre-compensation time-point.