Read threshold voltage selection

1. An apparatus for read threshold voltage selection, comprising:

an array of memory cells; and

a controller coupled to the array of memory cells, the controller configured to:

set a first soft read threshold voltage based on mutual information of two adjacent threshold voltage distributions and

set a second soft read threshold voltage, wherein a difference between the first soft read threshold voltage and the hard read threshold voltage is the same as a difference between the second soft read threshold voltage and the hard read threshold voltage.

2. The apparatus of claim 1 , wherein the controller is configured to set the first soft read threshold voltage such that a read operation using the first soft read threshold voltage results in an amount of mutual information that is at least a particular percentage of an achievable peak mutual information value for the two adjacent threshold voltage distributions.

3. The apparatus of claim 1 , wherein the controller is configured to perform read operations using the first soft read threshold voltage and the second soft read threshold voltage and send results of the read operations to an error correction coding (ECC) component.

4. The apparatus of claim 1 , wherein the two adjacent threshold voltage distributions include a first threshold voltage distribution including memory cells programmed to a first state and a second threshold voltage distribution including memory cells programmed to a second state.

5. An apparatus for read threshold voltage selection, comprising:

an array of memory cells; and

a controller coupled to the array of memory cells, the controller configured to:

perform a first read operation using a hard read threshold voltage to determine a first number of memory cells that are read as being programmed to a first state;

perform a second read operation using a first threshold voltage to determine a second number of memory cells that are read as being programmed to the first state;

set a first soft read threshold voltage as the first threshold voltage based on mutual information between the first number of memory cells and the second number of memory cells;

set a second soft read threshold voltage, wherein a difference between the first soft read threshold voltage and the hard read threshold voltage is the same as a difference between the second soft read threshold voltage and the hard read threshold voltage.

6. The apparatus of claim 5 , wherein the controller is configured to set the first soft read threshold voltage such that a read operation using the first soft read threshold voltage results in an amount of mutual information that is at least a particular percentage of an achievable peak mutual information value for the first number of memory cells and the second number of memory cells.

7. The apparatus of claim 5 , wherein the controller is configured to perform read operations using the first soft read threshold voltage and the second soft read threshold voltage and send results of the read operations to an error correction coding (ECC) component.

8. The apparatus of claim 5 , wherein the first number of memory cells include a first portion of memory cells programmed to a first state and wherein the first number of memory cells include a second portion of memory cells programmed to a second state.

9. The apparatus of claim 8 , wherein the second number of memory cells include a first portion of memory cells programmed to the first state and wherein the second number of memory cells include a second portion of memory cells programmed to the second state.

10. The apparatus of claim 9 , wherein a first threshold voltage distribution includes the first portion of the first number of memory cells and the first portion of the second number of memory cells programmed to the first state.

11. The apparatus of claim 10 , wherein a second threshold voltage distribution includes the second portion of the first number of memory cells and the second portion of the second number of memory cells programmed to the second state.

12. The apparatus of claim 11 , wherein the first threshold voltage distribution corresponds to the first state and the second threshold voltage distribution corresponds to the second state and wherein the first state is adjacent to the second state.

13. A method for read threshold voltage selection, comprising:

setting a first soft read threshold voltage based on mutual information of two adjacent threshold voltage distributions, wherein the two adjacent threshold voltage distributions include a first threshold voltage distribution including memory cells programmed to a first state and a second threshold voltage distribution including memory cells programmed to a second state;

setting a second soft read threshold voltage as an offset from a hard read threshold voltage for the two adjacent threshold voltage distributions, wherein the offset from the hard read threshold voltage is equal for the first soft read threshold voltage and the second soft read threshold voltage; and

setting the second soft read threshold voltage at a third threshold voltage, wherein a difference between the first soft read threshold voltage and the second soft read threshold voltage is the same as a difference between the first soft read threshold voltage and a hard read threshold voltage.

14. The method of claim 13 , wherein the method includes setting the first soft read threshold voltage such that a read operation using the first soft read threshold voltage results in an amount of mutual information that is at least a particular percentage of an achievable peak mutual information value for the two adjacent threshold voltage distributions.

15. The method of claim 13 , the method includes setting the first soft read threshold voltage as an offset from a hard read threshold voltage for the two adjacent threshold voltage distributions.

16. The method of claim 13 , further comprising setting a hard read threshold voltage at the first soft read threshold.