Skewed SRAM cell
View Patent ↗A memory cell including a cross-coupled latch with corresponding storage nodes, and further including first and second write pass gate transistors and first and second read pass gate transistors. The write pass gate transistors are controlled by a write word line and the read pass transistors are controlled by a read word line. Each read and write pass gate transistor is coupled between a storage node and either a bit line or a complementary bit line. The write pass gate transistors are implemented at a first strength level and the read pass gate transistors are implemented at a second strength level which is less than the first strength level. In this manner, the read and write margins are independently configurable without negatively impacting each other.
1. A memory cell, comprising:
a cross-coupled latch comprising first and second storage nodes;
a first write pass gate transistor having a pair of current terminals coupled between said first storage node and one of at least one bit line and having a control terminal coupled to a write word line;
a second write pass gate transistor having a pair of current terminals coupled between said second storage node and one of at least one complementary bit line and having a control terminal coupled to said write word line;
a first read pass gate transistor having a pair of current terminals coupled between said first storage node and one of said at least one bit line and having a control terminal coupled to a read word line;
a second read pass gate transistor having a pair of current terminals coupled between said second storage node and one of said at least one complementary bit line and having a control terminal coupled to said read word line; and
wherein said first and second write pass gate transistors are implemented at a first strength level and wherein said first and second read pass gate transistors are implemented at a second strength level which is less than said first strength level.
2. The memory cell of claim 1 , wherein said at least one bit line comprises a first bit line, wherein said at least one complementary bit line comprises a first complementary bit line, wherein said first write pass gate transistor and said first read pass gate transistor are both coupled to said first bit line, and wherein said second write pass gate transistor and said second read pass gate transistor are both coupled to said first complementary bit line.
3. The memory cell of claim 1 , wherein said at least one bit line comprises a read bit line and a write bit line, wherein said at least one complementary bit line comprises a complementary read bit line and a complementary write bit line, wherein said first write pass gate transistor is coupled to said write bit line, wherein said second write pass gate transistor is coupled to said complementary write bit line, wherein said first read pass gate transistor is coupled to said read bit line, and wherein said second read pass gate transistor is coupled to said complementary read bit line.
4. The memory cell of claim 1 , wherein said cross-coupled latch is implemented with a pull-up strength and a pull-down strength, wherein said first strength level is configured relative to said pull-up strength to determine a write margin, and wherein said second strength level is configured relative to said pull-down strength to determine a read margin.
5. The memory cell of claim 4 , wherein said pull-down strength is about three times said pull-up strength, wherein said first strength level is greater than twice said pull-up strength, and wherein said second strength level is less than twice said pull-up strength.
6. The memory cell of claim 1 , wherein said cross-coupled latch comprises pull-up PMOS transistors and pull-down NMOS transistors, and wherein said first and second write pass gate transistors and said first and second read pass gate transistors comprise NMOS transistors.
7. The memory cell of claim 1 , wherein said first and second strength levels are measured by relative drive current.
8. The memory cell of claim 1 , wherein said first and second strength levels are determined by transistor size and threshold voltage.
9. A memory array, comprising:
a read word line;
a write word line;
a plurality of bit lines;
a plurality of complementary bit lines;
a plurality of SRAM cells, each comprising:
a cross-coupled latch comprising first and second storage nodes;
a first write pass gate transistor having a pair of current terminals coupled between said first storage node and one of said plurality of bit lines and having a control terminal coupled to said write word line;
a second write pass gate transistor having a pair of current terminals coupled between said second storage node and one of said plurality of complementary bit lines and having a control terminal coupled to said write word line;
a first read pass gate transistor having a pair of current terminals coupled between said first storage node and one of said plurality of bit lines and having a control terminal coupled to said read word line;
a second read pass gate transistor having a pair of current terminals coupled between said second storage node and one of said plurality of complementary bit lines and having a control terminal coupled to said read word line; and
wherein said first and second write pass gate transistors are implemented at a first strength level and wherein said first and second read pass gate transistors are implemented at a second strength level which is less than said first strength level.
10. The memory array of claim 9 , wherein said first write pass gate transistor and said first read pass gate transistor each have a current terminal coupled to a first bit line, and wherein said second write pass gate transistor and said second read pass gate transistor each have a current terminal coupled to a first complementary bit line.
11. The memory array of claim 9 , wherein said plurality of bit lines comprises a plurality of read bit lines and a plurality of write bit lines, wherein said plurality of complementary bit lines comprises a plurality of complementary read bit lines and a plurality of complementary write bit lines, wherein said first write pass gate transistor is coupled to a first write bit line, wherein said second write pass gate transistor is coupled to a first complementary write bit line, wherein said first read pass gate transistor is coupled to a first read bit line, and wherein said second read pass gate transistor is coupled to a first complementary read bit line.
12. The memory array of claim 9 , wherein said cross-coupled latch is implemented with a pull-up strength and a pull-down strength, wherein said first strength level is configured relative to said pull-up strength to determine a write margin, and wherein said second strength level is configured relative to said pull-down strength to determine a read margin.
13. The memory array of claim 12 , wherein said pull-down strength is about three times said pull-up strength, wherein said first strength level is greater than twice said pull-up strength, and wherein said second strength level is less than twice said pull-up strength.
14. The memory array of claim 9 , wherein said cross-coupled latch comprises pull-up PMOS transistors and pull-down NMOS transistors, and wherein said first and second write pass gate transistors and said first and second read pass gate transistors comprise NMOS transistors.
15. The memory array of claim 9 , wherein said first and second strength levels are measured by relative drive current.
16. The memory array of claim 14 , wherein said first and second strength levels are determined by transistor size and threshold voltage.
17. A method of fabricating a memory cell, comprising:
implementing a cross-coupled latch with first and second storage nodes;
implementing a first write pass gate transistor at a first strength level having a pair of current terminals coupled between the first storage node and one of at least one bit line and having a control terminal coupled to a write word line;
implementing a second write pass gate transistor at the first strength level having a pair of current terminals coupled between the second storage node and one of at least one complementary bit line having a control terminal coupled to the write word line;
implementing a first read pass gate transistor at a second strength level which is less than the first strength level, wherein the first read pass gate transistor has a pair of current terminals coupled between the first storage node and one of the at least one bit line and has a control terminal coupled to a read word line; and
implementing a second read pass gate transistor at the second strength level having a pair of current terminals coupled between the second storage node and one of the at least one complementary bit line and having a control terminal coupled to the read word line.
18. The method of claim 17 , further comprising:
implementing the cross-coupled latch with a pull-up strength and a pull-down strength;
implementing the first and second write pass gate transistors at the first strength level which is configured relative to the pull-up strength to determine a write margin; and
implementing the first and second read pass gate transistors at the second strength level which is configured relative to the pull-down strength to determine a read margin.
19. The method of claim 17 , wherein said implementing the first and second write pass gate transistors at the first strength level and wherein said implementing the first and second read pass gate transistors at the second strength level comprises fabricating each transistor with a selected size and a selected threshold voltage.
20. The method of claim 17 , wherein said implementing the first and second write pass gate transistors at the first strength level and wherein said implementing the first and second read pass gate transistors at the second strength level comprises fabricating each transistor with a target drive current.