CAM storage schemes and CAM read operations for detecting matching keys with bit errors
A memory array includes strings that are configured to store keywords and inverse keywords corresponding to keys according to content addressable memory (CAM) storages schemes. A read circuit performs a CAM read operation over a plurality of iterations to determine which of the keywords are matching keywords that match a target keyword. During the iterations, a read controller biases word lines according to a plurality of modified word line bias setting that are each modified from an initial word line bias setting corresponding to the target keyword. At the end of the CAM read operation, the read controller detects which of the keywords are matching keywords, even if the strings are storing the keywords or inverse keywords with up a certain number of bit errors.
1. A circuit comprising:
a memory array comprising a plurality of strings of memory cells configured to store a plurality of keywords according to a content addressable memory (CAM) storage scheme; and
a read controller configured to:
bias one or more word lines, from among a plurality of word lines coupled to a set of the memory cells of the memory array storing at least a portion of a keyword, according to a plurality of modified word line bias settings during a set of sense operations, each of the plurality of modified word line bias settings modified from a target word line bias setting corresponding to a target keyword;
wherein the one or more word lines are identified based on a bit error number associated with storage of the keyword; and
determine that the keyword matches the target keyword based on a set of sense results identified from the set of sense operations.
2. The circuit of claim 1 , wherein the one or more word lines comprises a first word line group, the set of memory cells comprises a first set of memory cells, the plurality modified word line bias settings comprises a first plurality of modified word line bias settings, the set of sense operations comprises a first set of sense operations, the set of sense results comprises a first set of sense results, and wherein the read controller is further configured to:
bias a second word line group comprising one or more word lines, from among a plurality of word lines coupled to a second set of memory cells storing at least a portion of an inverse keyword, according to a second plurality of modified word line bias settings during a second set of sense operations, each of the second plurality of modified bias settings modified from an inverse target word line bias setting corresponding to an inverse target keyword,
wherein the one or more word lines of the second word line group are identified based on the bit error number; and
determine that the keyword matches the target keyword based further on a second set of sense results identified from the second set of sense operations.
3. The circuit of claim 1 , wherein the word line bias setting is
a low voltage level and the modified word line bias settings are high voltage levels, higher than the low voltage level, wherein the low voltage level is configured to cause memory cells in an erased state to conduct but not in a program state, and the high voltage level is configured to cause memory cells in both the erased state and the program state to conduct.
4. The circuit of claim 1 , wherein the bit error number is a maximum number of bit errors with which the plurality strings stores each of the plurality of keywords.
5. The circuit of claim 1 , wherein the memory array is configured to store the keyword as a plurality of keyword portions, the set of memory cells storing one of the keyword portions.
6. A circuit comprising:
a memory array comprising a plurality of strings of memory cells configured to store a plurality of keywords according to a content addressable memory (CAM) storage scheme; and
a read controller configured to:
bias a word line group coupled to a set of the memory cells according to a plurality of modified word line bias settings during a set of sense operations, the set of memory cells storing at least a portion of a keyword, each of the plurality of modified word line bias settings modified from a target word line bias setting corresponding to a target keyword based on a bit error number associated with storage of the keyword; and
determine that the keyword matches the target keyword based on a set of sense results identified from the set of sense operations; and
a keyword portion generation circuit configured to generate a plurality of keyword portions such that any two different keyword portions of the plurality of keyword portions has a minimum distance of 2*t+1, wherein t is a maximum number of bit errors with which the memory array stores each of the two different keyword portions.
7. The circuit of claim 1 , wherein the read controller is further configured to:
increase a low voltage level applied to one or more word lines during a CAM read operation; and
determine that the keyword matches the target keyword based on the increase of the low voltage level.
8. The circuit of claim 1 , further comprising a sense circuit configured to determine a plurality of sense results over a plurality of iterations, wherein the read controller is further configured to:
bias the one or more word lines at a plurality of low voltage levels and at a high voltage level over the set of sense operations, wherein a first low voltage level of the plurality of low voltage levels is a read pulse level associated with a program state of a single level cell storage scheme, a second low voltage level is a first predetermined amount above the read pulse level, and a third low voltage level is a second predetermined amount below the read pulse level.
9. A system comprising:
a block comprising a plurality of strings of memory cells;
a program circuit configured to program a plurality of keywords in the plurality of strings according to a content addressable memory (CAM) storage scheme; and
a read circuit configured to:
bias one or more word lines, from among a plurality of word lines coupled to the plurality of strings according to an initial word line bias setting that corresponds to a target keyword, wherein the one or more word lines, from among the plurality of word lines, are identified based on a bit error number associated with a storage of the plurality of keywords;
in response to an initial bias according to the initial word line bias setting, determine that a number of matching keywords is below a threshold level;
in response to the a of matching keywords being below the threshold level, iterate through a plurality of sense operations for the plurality of strings, wherein for each of the sense operations, the read controller is configured to bias the plurality of word lines according to one of a plurality of modified word line bias settings; and
determine one or more matching key words from among the plurality of keywords that match the target keyword in response to the plurality of sense operations.
10. The system of claim 9 , wherein the program circuit is configured to program the plurality of keywords as a plurality of keyword portions.
11. The system of claim 9 , wherein a first one of the plurality of modified word line bias settings maps at least a first word line of the one or more word lines to a first voltage level, and a second one of the plurality of modified word line bias settings maps at least a second word line of the one or more word lines to a second voltage level.
12. The system of claim 9 , wherein each of the plurality of modified word line bias settings maps a different combination of the one or more word lines of the plurality of word lines to a voltage level different than a voltage level defined by the initial word line bias setting.
13. The system of claim 9 , wherein each of the plurality of modified word line bias settings maps a combination of the one or more word lines of the plurality of word lines different than a combination of the one or more word lines mapped by the initial word line bias setting.