Independent Bi-Directional Margin Control Per Level and Independently Expandable Reference Cell Levels for Voltage Mode Sensing

Abstract

A memory system includes reference level generators that may provide programmable margins, and programmable verify voltage levels. The reference levels may be shifted within a range of voltages with varying differences between reference voltage levels and with different margins and verify levels.

1 . A method for generating read windows in a read voltage range, the method comprising:
 setting a plurality of read voltage levels;   selecting one of a plurality of selectable positive margin voltage levels for each of the plurality of read voltage levels;   selecting one of a plurality of selectable negative margin voltage levels for each of the plurality of read voltage levels;   setting program verify levels to form read windows between said program verify levels in response to the plurality of read voltage levels, the selected positive margin voltage levels, and the selected negative margin voltage levels.   
   
   
       2 . The method of  claim 1  wherein the positive and negative margin voltages levels have equal magnitudes. 
   
   
       3 . The method of  claim 1  further comprising:
 shifting each of the plurality of read voltage levels a predetermined level.   
   
   
       4 . The method of  claim 1  further comprising:
 shifting at least two of the plurality of read voltage levels a corresponding one of a plurality of predetermined levels, the differences between said read voltage levels being equal while maintaining read margin levels at a same level.   
   
   
       5 . The method of  claim 1  further comprising:
 shifting each of the plurality of read voltage levels, the differences between said read voltage levels being unequal.