Non-volatile memory using bi-directional resistive elements
A memory cell includes a first storage node and a second storage node that is complementary to the first storage node. A first bidirectional resistive memory element (BRME) includes a first terminal, a second BRME includes a first terminal. A first access transistor couples the first storage node to a first bit line. A second access transistor couples the second storage node to a second bit line. A third transistor couples the first terminal of the first BRME to the second bit line. A fourth transistor couples the first terminal of the second BRME to the first bit line.
1. A memory cell comprising:
a first storage node;
a second storage node, the second storage node complementary to the first storage node;
a first bidirectional resistive memory element (BRME) including a first terminal;
a second BRME including a first terminal;
a first access transistor for coupling the first storage node to a first bit line;
a second access transistor for coupling the second storage node to a second bit line;
a third transistor for directly coupling the first terminal of the first BRME to the second bit line;
a fourth transistor for directly coupling the first terminal of the second BRME to the first bit line.
2. The memory cell of claim 1 wherein the third transistor couples the first terminal of the first BRME to the second bit line and the fourth transistor couples the first terminal of the second BRME to the first bit line during a resistive element write to the memory cell.
3. The memory cell of claim 2 wherein the during a resistive element write, a second terminal of the first BRME is coupled to the first bit line and a second terminal of the second BRME is coupled to the second bit line.
4. The memory of claim 1 wherein a second terminal of the first BRME is coupled to the first storage node and a second terminal of the second BRME is coupled to the second storage node.
5. The memory cell of claim 1 wherein during a resistive element write, a second terminal of the first BRME is coupled to the first bit line through the first access transistor that is conductive, the first terminal of the first BRME is coupled to the second bit line through the third transistor that is conductive, a second terminal of the second BRME is coupled to the second bit line through the second access transistor that is conductive, and the first terminal of the second BRME is coupled to the first bit line through the fourth transistor that is conductive.
6. The memory cell of claim 1 further comprising:
a fifth transistor including a first current terminal coupled to the first storage node, a second current terminal to receive a low supply voltage, and a control terminal coupled to the second storage node;
a sixth transistor including a first current terminal coupled to the second storage node, a second current terminal to receive the low supply voltage, and a control terminal coupled to the first storage node.
7. The memory of claim 6 further comprising:
a seventh transistor including a first current terminal coupled to the first storage node, a second current terminal to receive a high supply voltage, and a control terminal coupled to the second storage node;
an eighth transistor including a first current terminal coupled to the second storage node, a second current terminal to receive the high supply voltage, and a control terminal coupled to the first storage node.
8. The memory of claim 7 wherein during a resistive memory write to the cell, the second current terminal of the seventh transistor and the second current terminal of the eight transistor do not receive the high supply voltage.
9. The memory of claim 1 wherein during a read of the memory cell, the third transistor and the fourth transistor are not conductive.
10. A method of operating a memory cell, the memory cell including a first bidirectional resistive memory element (BRME) and a second BRME, the method comprising:
performing a resistive element write to write a value to the memory cell, the performing including directly coupling a first terminal of the first BRME to a first bit line and a second terminal of the first BRME to a second bit line to place the first BRME in a first resistive state, the second bit line is a complimentary bit line to the first bit line, the performing including directly coupling a first terminal of the second BRME to the second bit line and a second terminal of the second BRME to the first bit line to place the second BRME in a second resistive state, the second resistive state being complementary to the first resistive state, the first BRME being in the first resistive state and the second BRME being in the second resistive state is indicative of the value.
11. The method of claim 10 wherein the memory cell includes a first storage node and a second storage complementary to the first storage node, wherein during the resistive element write, the first terminal of the first BRME is coupled to the first storage node and the first terminal of the second BRME is coupled to the second storage node.
12. The method of claim 11 wherein the memory cell includes a first access transistor and a second access transistor, the first access transistor is made conductive to couple the first storage node to the first bit line and the second access transistor is made conductive to couple the second storage node to the second bit line.
13. The method of claim 11 wherein prior to performing the resistive element write, performing a latch write to the memory cell to place the first storage node and the second storage node at complementary voltage levels indicative of the value, wherein performing the latch write includes coupling the first storage node to the first bit line and coupling the second storage node to the second bit line.
14. The method of claim 13 wherein after performing the latch write but prior to performing the resistive element write, resistive states of the first BRME and the second BRME indicate a different value than the value indicated by the complementary voltage levels of the first storage node and the second storage node.
15. The method of claim 13 , wherein performing the resistive element write pulls the first bit line and the second bit line to a first bit line voltage and a second bit line voltage level respectively as indicated by the complementary voltage levels of the storage nodes.
16. The method of claim 10 wherein the performing the resistive element write includes driving the first bit line and the second bit line to complementary voltage levels indicative of the value.
17. The method of claim 10 wherein:
coupling the first terminal of the first BRME to the first bit line includes making a first transistor conductive;
coupling the first terminal of the second BRME to the second bit line includes making a second transistor conductive;
coupling the second terminal of the first BRME to the second bit line includes making a third transistor conductive;
coupling the second terminal of the second BRME to the first bit line includes making a fourth transistor conductive.
18. The method of claim 17 wherein:
a control terminal of the first transistor and a control terminal of a second transistor are coupled to a word line that is asserted to make the first transistor and the second transistor conductive;
a control terminal of the third transistor and a control terminal of the fourth transistor are coupled to a select line that is asserted to make the third transistor and the fourth transistor conductive.
19. The method of claim 10 further comprising:
after performing the resistive element write, powering down the memory cell;
after powering down the memory cell, powering up the memory cell, wherein powering up the memory cell includes writing the value as stored in the first BRME and the second BRME to the first storage node and the second storage node, wherein the writing the value includes placing the first storage node and the second storage node at complementary voltage levels indicative of the value.
20. The method of claim 19 wherein during the powering up the memory cell, the first bit line and the second bit line are driven to a high voltage value, the second terminal of the first BRME is coupled to the second bit line, the second terminal of the second BRME is coupled to the first bit line, the first terminal of the first BRME is coupled to the first storage node and the first terminal of the second BRME is coupled to the second storage node.