Managing threshold voltage shift in nonvolatile memory
Apparatus, systems, and methods to correct for threshold voltage drift in non-volatile memory devices are disclosed and described. In one example, a compensated demarcation voltage is generated by either a time-based drift compensation scheme or a disturb-based drift compensation scheme, and read and write operations to the non-volatile memory are carried out using the compensated voltage threshold.
1. A memory controller, comprising circuitry configured to:
receive an input/output (I/O) request referenced to a memory location of a non-volatile memory (NVM) device;
obtain a compensated demarcation voltage (Vdm) by either a time-based drift compensation mode or a disturb-based drift compensation mode;
determine if the I/O request is a read request or a write request;
if the I/O request is a read request, apply the compensated Vdm to a memory cell of the memory location; and
if the I/O request is a write request, apply a programming pulse to a memory cell of the memory location, the programming pulse having a voltage greater than the compensated Vdm.
2. The memory controller of claim 1 , wherein to obtain the compensated Vdm by the time-based drift compensation mode, the circuitry is further configured to:
determine a time elapsed since a last write operation to the memory cell of the memory location; and
determine the compensated Vdm using the time elapsed.
3. The memory controller of claim 2 , wherein to determine the compensated Vdm, the circuitry is further configured to:
compare the time elapsed to a plurality of elapsed time subranges correlated with a plurality of potential compensated Vdms;
select an elapsed time subrange from the plurality of elapsed time subranges that includes the time elapsed; and
identify the potential compensated Vdm associated with the selected elapsed time subrange as the compensated Vdms.
4. The memory controller of claim 1 , wherein to obtain the compensated Vdm by the disturb-based drift compensation mode, the circuitry is further configured to:
receive a read or a write operation referenced to the memory location;
determine a number of disturbs to the memory cell since a last write to the memory cell; and
determine the compensated Vdm using the number of disturbs.
5. The memory controller of claim 4 , wherein to determine the compensated Vdm, the circuitry is further configured to:
compare the number of disturbs to a plurality of disturb number subranges correlated with a plurality of potential compensated Vdms;
select a disturb number subrange from the plurality of disturb number subranges that includes the number of disturbances to the memory cell; and
identify the potential compensated Vdm associated with the selected disturb number subrange as the compensated Vdms.
6. The memory controller of claim 4 , wherein the number of disturbs comprises a number of read operations to the memory cell.
7. The memory controller of claim 1 , wherein the circuitry further comprises:
a mode indicator configured to specify whether the compensated Vdm is obtained according to the time-based drift compensation mode or the disturb-based drift compensation mode.
8. The memory controller of claim 7 , wherein the mode indicator is a mode bit.
9. The memory controller of claim 7 , wherein the memory cell comprises a phase change memory (PCM) cell.
10. The memory controller of claim 1 , wherein the NVM device comprises a three-dimensional (3D) cross-point array.
11. A memory computation system, comprising:
at least one processor;
a non-volatile memory (NVM) device comprising an array of memory cells; and
a controller coupled to the NVM device and the at least one processor, the controller further comprising circuitry configured to:
receive an input/output (I/O) request referenced to a memory location of the NVM device;
determine whether a compensated demarcation voltage (Vdm) is to be obtained according to a time-based drift compensation mode or a disturb-based drift compensation mode;
obtain the compensated Vdm according to the determined mode;
determine if the I/O request is a read request or a write request;
if the I/O request is a read request, apply the compensated Vdm to a memory cell of the memory location of the array of memory cells; and
if the I/O request is a write request, apply a programming pulse to a memory cell of the memory location, the programming pulse having a voltage greater than the compensated Vdm.
12. The computation system of claim 11 , further comprising a mode indicator coupled to the controller and configured to specify whether the compensated Vdm is obtained according to the time-based drift compensation mode or the disturb-based drift compensation mode.
13. The computation system of claim 11 , wherein to obtain the compensated Vdm by the time-based drift compensation mode, the circuitry is further configured to:
receive a read or a write operation referenced to the memory location;
determine a time elapsed since a last write operation to the memory cell of the memory location; and
determine the compensated Vdm using the time elapsed.
14. The computation system of claim 13 , wherein to determine the compensated Vdm, the circuitry is further configured to:
compare the time elapsed to a plurality of elapsed time subranges correlated with a plurality of potential compensated Vdms;
select an elapsed time subrange from the plurality of elapsed time subranges that includes the time elapsed; and
identify the potential compensated Vdm associated with the selected elapsed time subrange as the compensated Vdms.
15. The computation system of claim 11 , wherein to obtain the compensated Vdm by the disturb-based drift compensation mode, the circuitry is further configured to:
receive a read or a write operation referenced to the memory location;
determine a number of disturbs to the memory cell since a last write to the memory cell; and
determine the compensated Vdm using the number of disturbs.
16. The computation system of claim 15 , wherein to determine the compensated Vdm, the circuitry is further configured to:
compare the number of disturbs to a plurality of disturb number subranges correlated with a plurality of potential compensated Vdms;
select a disturb number subrange from the plurality of disturb number subranges that includes the number of disturbs to the memory cell; and
identify the potential compensated Vdm associated with the selected disturb number subrange as the compensated Vdms.
17. The computation system of claim 11 , wherein the array of memory cells includes an array of phase change memory (PCM) cells.
18. The computation system of claim 17 , wherein the array of PCM cells is configured as a three-dimensional (3D) cross-point array.
19. The computation system of claim 11 , further comprising one or more of:
a network interface communicatively coupled to the at least one processor;
a display communicatively coupled to the at least one processor; or
a power supply communicatively coupled to the at least one processor.