# Patent

## Title
Eight transistor soft error robust storage cell

## Abstract

A storage cell is provided with improved robustness to soft errors. The storage cell comprises complementary core storage nodes and complementary outer storage nodes. The outer storage nodes act to limit feedback between the core storage nodes and are capable of restoring the logical state of the core storage nodes in the event of a soft error. Similarly the core storage nodes act to limit feedback between the outer storage nodes with the same effect. This cell has advantages compared with other robust storage cells in that there are only two paths between the supply voltage and ground which limits the leakage power. An SRAM cell utilizing the proposed storage cell can be realized with two access transistors configured to selectively couple complementary storage nodes to a corresponding bitline. A flip-flop can be realized with a variety of transfer gates which selectively couple data into the proposed storage cell.

## Background
The proposed Eight Transistor Storage Cell topology offers improved soft error robustness compared to the standard dual-inverter storage cell commonly used in 6T SRAM cells and static flip-flops. Compared with previously described soft error robust storage cells this topology allows smaller cells and reduced leakage power. This application claims the benefit of priority of U.S. Provisional Patent Application No. 61/193,503 filed Dec. 4, 2008 and is a continuation of U.S. patent application Ser. No. 13/751,763 filed on Jan. 28, 2013 and issued as U.S. Pat. No. 8,760,912 on Jun. 24, 2014, which is a continuation of U.S. patent application Ser. No. 12/630,947 filed on Dec. 4, 2009 and issued as U.S. Pat. No. 8,363,455 on Jan. 29, 2013, all of which are incorporated herein by reference in their entireties.


BACKGROUND OF THE INVENTION
SRAMs are one of the most popular ways to store data in electronic systems. Similarly, embedded SRAMs are a vital building block in integrated circuits. SRAMs are a preferred method of implementing embedded memories owing to higher speed, robust design, and ease of integration. SRAMs, in general, occupy a significantly large portion of the chip's die area, making it an important block in terms of area, yield, reliability and power consumption. With increasing demand for highly integrated System on Chip (SoC), improving various aspects of embedded SRAMs have received a significant interest.
The six transistor SRAM cell is the most popular configuration because of its high speed and robustness. However the 6T SRAM cell, shown in FIG. 1, suffers from a relatively high vulnerability to soft errors. At the heart of the 6T SRAM cell is the four transistor dual-inverter storage cell shown in FIG. 2. The inherent feedback of the back-to-back inverters means that if sufficient charge is collected on one node, the feedback will cause the second node to also switch. This results in an erroneous state being stored, and the cell cannot recover.
Flip-flops are critical components in integrated circuits. Flip-flops enable sequential logic by storing the result of combinational logic. A flip-flop stores data by sampling the input data signal with a clock signal at a particular instant of time, typically at the edge of the clock. Thus, the output of the flip-flop is sensitive to the input data signal only at the clock edge. At all other times, the flip-flop's output is constant and does not respond to changes in input data signal.
A flip-flop can be realized in a variety of ways. A typical way of realizing a flip-flop is to use two series connected latches called master and slave. This architecture of flip-flop is called the master-slave D-flip-flop (DFF), a schematic of which is given in FIG. 3. Whatever the architecture every flip-flop must have a storage element. Most static flip-flops use a dual-inverter structure, shown in FIG. 2.
The scaling of CMOS processes has resulted in lower node capacitance and lower supply voltages in integrated circuits. The result of these effects is that less charge is used to store data in a storage cell. The increased density enabled by the smaller geometries of scaled processes has also enabled the integration of large SRAM arrays and large numbers of flip-flops. These conditions resulted in an increased sensitivity to transients induced by ionizing radiation consisting of energetic cosmic neutrons and alpha particles. When these particles strike the chip they generate a large number of electron hole pairs. Depending on the location of the particle strike the deposited charge may be collected by a node in a storage cell. If sufficient charge is collected the storage cell can switch its logical state, which is called a soft error. In view of this vulnerability numerous methods have been proposed to improve the robustness of the storage cell. More effort has been put into improving the soft-error robustness of SRAM cells, however the SER of flip-flops has been continually degrading to the point where is has become a significant reliability consideration. Both the 6T SRAM and the master-slave static CMOS DFF use the dual-inverter storage cell to hold a logical state. For both circuits the data value stored in this structure can be susceptible to soft errors. If a soft error transient is able to change the state of one node, the feedback of the dual-inverter storage cell will change the second node and result in erroneous data being stored.
FIG. 4 shows the schematic diagram of an SRAM cell which has an improved robustness to radiation induced soft errors. In the proposed circuit the SRAM cell has the same dual-inverter storage cell as in the traditional case, however it also includes a coupling capacitor between the two nodes where the data is stored. The effect of adding the coupling capacitor is that the critical charge (Qc) of the cell is significantly increased. The critical charge is the minimum amount of charge which will result in a soft error. However, the addition of a large, area efficient coupling capacitor requires a special semiconductor manufacturing process. Therefore this SRAM cell is not easily integrated into common Complementary Metal-Oxide-Semiconductor (CMOS) processes. As a result, for Application Specific Integrated Circuits (ASICs) where embedded SRAM cells are widely realized using standard CMOS process, this approach is costly to implement. Another problem with this approach is that while the capacitor increases the critical charge, it also makes the cell slower to write to. For data to be written into this SRAM cell the coupling capacitor will have to be charged/discharged, which will take more time than for a standard 6T SRAM cell. This results in a lower frequency of operation, which is again not desirable. Another drawback of this cell is that the extra capacitance which must be charged and discharged results in larger power consumption compared to the standard 6T SRAM cell.
FIG. 5 shows the schematic diagram of a storage cell which has four extra transistors in order to improve its robustness to soft errors. This circuit is known as the DICE cell, which stands for dual-interlocking storage cell. In the proposed solution, the data is stored on multiple nodes and the storage cell is immune to single node upsets. However, the SRAM cell based on the DICE storage cell requires twice as many transistors to implement compared with the standard 6T SRAM circuit, making it expensive. Also, in order for this cell to be used in a flip-flop, at least three nodes must be accessed. The result is that flip-flops which implement this storage cell require a greater number of transistors and use more power.
FIG. 6 shows the schematic diagram of a third storage cell which has an improved robustness to soft errors. This circuit consists of eight transistors and is similar to the DICE cell in that data is stored at multiple nodes. However, in this storage cell data is stored on four nodes which are interconnected using four half-latches. This SRAM cell is almost completely immune to soft-error induced upsets. The SRAM circuit based on this storage cell requires only ten transistors, which is more than the 6T SRAM cell, but less than the DICE SRAM cell. This storage cell can also be used to implement flip-flops. This data on this storage cell can be changed by accessing only two nodes, and as such fewer transistors are required as compared to the DICE based flip-flop.
SUMMARY OF THE INVENTION
A new storage cell configuration is introduced. The storage cell configuration allows for logical states to be stored with robustness against noise events such as soft errors. Various types of circuits can be implemented with this storage cell, including SRAM cells and flip-flops.



BRIEF DESCRIPTION OF THE DRAWINGS
An embodiment of the present invention will now be described by way of example only with reference to the following drawings in which:
FIG. 1: Schematic showing the prior art of the six transistor SRAM cell.
FIG. 2: Schematic showing the prior art of the dual-inverter storage cell.
FIG. 3: Schematic showing the prior art of a static CMOS MS DFF.
FIG. 4: Schematic showing the prior art of a six transistor SRAM cell with improved SER robustness.
FIG. 5: Schematic showing the prior art of an 8T SER robust DICE storage cell.
FIG. 6: Schematic showing the prior art of an 8T SER robust storage cell.
FIG. 7: Schematic diagram showing the architecture of the proposed storage cell.
FIG. 8: Proposed storage cell storing 1010.
FIG. 9: SER robust SRAM cell utilizing the proposed storage cell.
FIG. 10: SRAM array implemented with SER robust SRAM cells using the proposed SER robust storage cell.
FIG. 11: Read operation on a SER robust SRAM cell using the proposed storage cell.
FIG. 12: Simulation waveforms illustrating a read operation on a SER robust SRAM cell using the proposed storage cell.
FIG. 13: Simulation waveforms illustrating a half-selected SER robust SRAM cell using the proposed storage cell.
FIG. 14: Write operation on a SER robust SRAM cell using the proposed storage cell.
FIG. 15: Simulation waveforms illustrating a write operation on a SER robust SRAM cell using the proposed storage cell.
FIG. 16: Alternate SER robust SRAM cell utilizing the proposed storage cell but connecting to nodes C and D.
FIG. 17: Alternate SER robust SRAM cell utilizing the proposed storage cell however connecting to nodes A and B via PMOS access transistors.
FIG. 18: Alternate SER robust SRAM cell utilizing the proposed storage cell however connecting to nodes C and D via PMOS access transistors.
FIG. 19: Response of the proposed storage cell to a soft error induced upset on node A.
FIG. 20: Schematic diagrams showing a register file memory cell with one write port and one read port.
FIG. 21: Schematic diagrams showing three possible flip-flop circuits utilizing the proposed SER robust storage cell.



DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS
For convenience, like structures in drawings will be referenced by like numerals in the description. The following describes an eight transistor storage cell.
Referring to FIG. 7, an eight transistor storage cell in accordance with an embodiment of the present invention is illustrated generally by the numeral 700. The eight transistor storage cell 700 comprises four n-channel transistors N1, N2, N3 and N4 (known as drive transistors), four p-channel transistors P1, P2, P3 and P4 (known as load transistors) and four internal nodes A, B, C and D. Transistors N2, N4, P2 and P4 are given the name of core transistors. Transistors N1, N3, P1 and P3 are given the name of outer transistors. Nodes A and B are located at the drains of the core transistors and nodes C and D are located at the drains of the outer transistors.
In the storage cell 700 one logical value will be stored on nodes A and C and a complementary logical value will be stored on nodes B and D. Referring to FIG. 8 the storage cell 700 is illustrated storing a logical 1 on nodes A and C and a logical 0 on nodes B and D.
Transistors N1, N2, N3, N4, P1, P2, P3 and P4 in the storage cell 700 are connected in such a way that the four storage nodes A, B, C, D interlock in such a way so as to work against a change in the state of the storage cell due to a single ended disturbance such as a soft error. The drains of the core transistors N2 and P2 are connected together. The drains of the core transistors N4 and P4 are connected together. The drains of the outer transistors N1 and P1 are connected together. The drains of the core transistors N3 and P3 are connected together. The gates of the core transistors N2 and P2 are connected to the drains of the complementary outer transistors N3 and P3. The gates of the core transistors N4 and P4 are connected to the drains of the complementary outer transistors N1 and P1. The gates of the outer transistors N1 and P1 are connected to the drains of the complementary core transistors N4 and P4. The gates of the core transistors N3 and P3 are connected to the drains of the complementary outer transistors N2 and P2.
The outer transistors of the storage cell 700 selectively connect the storage cell to the supply voltages. The sources of the outer transistors are connected to either the logical high supply voltage VDD or the logical low supply voltage GND. The sources of the outer load transistors P1 and P3 are connected to VDD and the sources of the outer drive transistors N1 and N3 are connected to GND.
Therefore, the storage cell 700 is able to retain logic data as long as it is powered.
Referring to FIG. 9, a sample SRAM cell which utilizes the storage cell 700 is illustrated generally by numeral 900. Two NMOS transistors N5 and N6 are added in order to selectively couple the storage cell 700 to bitlines BL and BLB. Transistors N5 and N6 are called access transistors. The gates of the access transistors are couples at their gates to a wordline WL node. The drains of access transistors N5 and N6 are coupled to bitlines BL and BLB respectively. The sources of the access transistors N5 and N6 coupled to nodes A and B of the storage cell 700. The nodes WL, BL and BLB are controlled in order to enable read and write operations on the SRAM cell 900.
Referring to FIG. 10, a sample array of SRAM cells 900 is illustrated generally by numeral 1000. The array 1000 comprises M rows and N columns of SRAM cells 900. A bitline pair BL and BLB is shared among the cells located in a given column. A wordline WL is shared among all cells in a given row. In addition to the array 1000, a memory will also contain blocks such as address decoders, timing and control, sensing and column drivers. These blocks are similar to those found in state-of-the-art SRAM configurations, and therefore are not described in detail. Alternatively, the bitline pair BL and BLB may be shared among the cells located in a given row and the wordline WL may be shared among all cells in a given column.
Referring to FIG. 11 the read operation on the 8T SRAM cell 900 is illustrated generally by numeral 1100. In the present embodiment, it is assumed that the supply voltage VDD is 1V, the initial voltage at nodes A and C is 1V value and the initial voltage at nodes B and D is 0V. Thus, the cell 1100 stores a logic 1.
At step 1101, the differential bitline pair BL and BLB are pre-charged to 1V. After the precharge is complete the bitline pair BL and BLB are electrically disconnected from all devices and the voltage is held by way of the parasitic capacitance of the bitlines.
At step 1102, the wordline WL voltage is changed so as to turn on the access transistors. In this embodiment using SRAM cell 900 the WL is raised to 1V so as to enable the NMOS access transistors.
There is now no voltage difference across transistor N5 in the SRAM cell 900 and as such no current will flow. However, there is now a voltage difference across transistor N6 in the SRAM cell 900 which is turned on. The voltage on BLB is VDD whereas the voltage at node B is GND. As such at step 1103 the voltage on BLB will begin to discharge through node B in the storage cell 700.
At step 1104, either the current resulting from the read operation, or the resulting differential voltage across the bitlines, is sensed by a sense amplifier (not shown).
Since the cell is differential, the read operation will be similar when the stored value is reversed and the initial voltage at nodes A and C is 0V value and the initial voltage at nodes B and D is 1V. However, in such an embodiment node the voltage on BL would be zero and as such the BL would be discharged, as opposed to BLB.
It will be appreciated that the logic state of a cell can be determined by examining the bitline pair BL or BLB is at the end of the read operation.
An example of a read operation when a logic 1 is stored in the SRAM cell 900 will be described with reference to the timing diagrams illustrated in FIG. 12. As illustrated in FIG. 12d, the read operation begins when the signal WL is raised from GND to VDD. As illustrated in FIG. 12c, the read operation results in the voltage BLB decreasing as it is discharged through the storage cell creating the differential voltage across the bitline pair BL and BLB detected by the sense amplifier. As illustrated in FIG. 12a and FIG. 12b, the internal nodes of the storage cell 700 do not change state, hence the read operation is non-destructive.
Referring to FIG. 13 an example of a SRAM cell 900 which is not being read, but which connected to the WL which is enabled. This causes the SRAM cell 900 to be half-selected. As illustrated in FIG. 13d the wordline WL is raised from GND to VDD for a period of time. However, as illustrated in FIG. 13c the bitlines BL and BLB are not left to float, but rather are held to VDD. As illustrated in FIG. 13a and FIG. 13b the internal nodes of the storage cell 700 are perturbed, but do not change stage.
Referring to FIG. 14, a flow chart illustrating the steps for writing a logic 0 to the SRAM cell 900 is illustrated by numeral 1400. In this example, at step 1401 the initial state of the SRAM cell 900 is logic 1 so the initial voltage at nodes A and C is 1V and that the initial voltage at nodes B and D is 0V.
At step 1402 the bitline pair is set so that bitline BL is set to 0V and bitline BLB is set to 1V. At step 1403, the voltage on the wordline WL is changed to a voltage which turns on the access transistors in the SRAM cell 900.
At step 1404, the voltage at node A is logically high, however the voltage on the bitlines BL connected to node A via access transistor N5 is low. As such node A is discharged through the access transistors N5. This results in the outer transistor N3 turning off and outer transistor P3 turning on.
At step 1405, Node D is charged up through outer transistor P3, turning off core transistor P2 and turning on core transistor N2.
At step 1406, Node B is partially charged up through access transistor N6 and partially through transistors P3 and P4. Finally at step 1407 Node C is discharged through outer transistor N1.
At step 1408, the internal nodes of storage cell 700 are stable and the storage cell now stores a logic 0 state. At step 1409, the wordline WL is changed to a state which turns off the access transistor and isolates the storage cell 700 from the bitlines BL and BLB.
Since the cell is differential, writing a logic 1 to an SRAM cell 900 storing a logic 0 operates in a similar fashion to that described with reference to FIG. 14. However, in this example, the initial voltage at nodes A and C is 0V and that the initial voltage at nodes B and D is 1V. Accordingly, in order to write a logic 1 the bitline pair is set so that bitline BL is set to 1V and bitline BLB is set to 0V. Thus, when the voltage of the wordline WL is changed so as to turn on the access transistor the voltages of the storage cell will flip and the cell will store a logic 1 state.
An example of a write operation on the SRAM cell 900 will be described with reference to the timing diagrams illustrated in FIG. 15. The initial state of the cell is logic 1. First a logic 0 will be written into the cell and then a logic 1 will be written into the cell. As illustrated in FIG. 15d, the state written into the storage cell depends on the complementary values on the bitlines BL and BLB. As illustrated in FIG. 15c, the write operation begins when the signal WL is raised from GND to VDD. As illustrated in FIG. 15a and FIG. 15b, the internal storage nodes of the storage cell are changed during a write operation in such a way that the cell stores the desired state.
Although the previous embodiments have been described with a particular configuration of storage node voltages for logic 1 and complementary voltages for logic 0, it will be appreciated that the inverse may also be the case. That is, a storage node configuration described as logic 1 could, instead, be defined as logic 0, and vice versa.
Further, although the previous embodiment of the SRAM cell 900 had NMOS access transistors accessing nodes A and B of a storage cell 700 it will be appreciated that alternative embodiments are possible. Referring to FIG. 16 one alternate SRAM cell embodiment is illustrated. In this embodiment an SRAM cell is created by accessing a storage cell 700 using two NMOS access transistors which couple storage cell 700 nodes C and D to BL and BLB respectively. Referring to FIG. 17 a second alternate SRAM cell embodiment is illustrated. In this embodiment an SRAM cell is created by accessing a storage cell 700 using two PMOS access transistors which couple storage cell 700 nodes A and B to BL and BLB respectively. Referring to FIG. 18 a third alternate SRAM cell embodiment is illustrated. In this embodiment an SRAM cell is created by accessing a storage cell 700 using two PMOS access transistors which couple storage cell 700 nodes C and D to BL and BLB respectively. It will be understood by those skilled in the art that these SRAM cells perform the identical functionality as described in the first embodiment SRAM cell 900.
While the SRAM cell 900 can be written into via differential access transistors, the storage cell 700 is resistance to single ended stresses, and as such is robust to soft-errors. The response of the proposed storage cell 700 to a particle strike on node A is shown in FIG. 19. The initial logical state at nodes A and C is 1 value and the initial logical state at nodes B and D is 0. Initially transistors P1, P2, N3 and N4 are on, and transistors N1, N2, P3 and P4 are off. The charge collected by the particle strike causes the state of node C to flip, such that it is now at a logical 0 state. This has the effect of turning off transistor N4 and turning on transistor P4. However, this does not result in any other node changing state. The current from P1 will eventually charge Node C to a logical state 1 returning the storage cell to its original state.
The proposed storage cell can be used to implement a register file memory cell. Referring to FIG. 20, a sample register file memory cell which utilizes the storage cell 700 is illustrated generally by numeral 2000. The addition of transistors N7 and N8 along with signals RBL and RWL create a single read port. It will be understood by those skilled in the art that it is trivial to increase the number of read or write ports to register file memory cell 2000.
In logic storage cells are an integral part of flip-flops, and as such the proposed storage cell 700 can be used in a variety of flip-flop designs. Referring to FIG. 21 several sample embodiments of flip-flops are shown which are implemented using the proposed storage cell 700. FIG. 21a illustrates a flip-flop with a master-slave DFF architecture which utilizes the storage cell 700. FIG. 21b illustrates a flip-flop with a C2MOS architecture which utilizes the storage cell 700. FIG. 21c illustrates a flip-flop with a pulsed-latch architecture which utilizes the storage cell 700. While the figures show the various transfer gates accessing the storage cell 700 via nodes C and D, the storage cell 700 could also be access via nodes A and B instead.
It will be understood by those skilled in the art that these are not an exhaustive list of flip-flops which can be implemented using the storage cell 700. A flip-flop can be created using any transfer gate which passes the input data signals to the storage cell 700 gated appropriately by a clock signal.
Further, although preferred embodiments of the invention have been described herein, it will be understood by those skilled in the art that variations may be made thereto without departing from the spirit of the invention or the scope of the appended claims.

## Claims

1. A circuit comprising:
a storage cell including:
a plurality of transistors; and
at least four storage nodes formed between the plurality of transistors, wherein each of the storage nodes is configured to change states;
wherein:
each of the at least four storage nodes is connected to two gates of the plurality of transistors; and
at least one of: only two of the plurality of transistors are connected to VDD or only two of the plurality of transistors are connected to ground.

2. The circuit of claim 1, wherein only two of the plurality of transistors are connected to VDD.

3. The circuit of claim 1, wherein only two of the plurality of transistors are connected to ground.

4. The circuit of claim 1, wherein:
only two of the plurality of transistors are connected to VDD;
only two of the plurality of transistors are connected to ground;
a first storage node of the at least four storage nodes is electrically connected to a gate of a first transistor of the only two of the plurality of transistors connected to VDD;
the first storage node is electrically connected to a gate of a first transistor of the only two of the plurality of transistors connected to ground;
a second storage node of the at least four storage nodes is electrically connected to a gate of a second transistor of the only two of the plurality of transistors connected to VDD; and
the second storage node is electrically connected to a gate of a second transistor of the only two of the plurality of transistors connected to ground.

5. The circuit of claim 4, wherein:
a third storage node of the at least four storage nodes is electrically connected to a drain of a first transistor of the only two of the plurality of transistors connected to VDD;
the third storage node is electrically connected to a drain of a first transistor of the only two of the plurality of transistors connected to ground;
a fourth storage node of the at least four storage nodes is electrically connected to a drain of a second transistor of the only two of the plurality of transistors connected to VDD; and
the fourth storage node is electrically connected to a drain of a second transistor of the only two of the plurality of transistors connected to ground.

6. The circuit of claim 1, wherein the at least four storage nodes comprise only four storage nodes.

7. The circuit of claim 1, wherein the circuit is configured such that a soft error on only one of the at least four storage nodes will be automatically corrected.

8. The circuit of claim 1, wherein the circuit is included in a flip-flop circuit.

9. The circuit of claim 1, wherein:
only two of the plurality of transistors are connected to VDD;
a first storage node of the at least four storage nodes is electrically connected to a gate of a first transistor of the only two of the plurality of transistors connected to VDD; and
a second storage node of the at least four storage nodes is electrically connected to a gate of a second transistor of the only two of the plurality of transistors connected to VDD.

10. The circuit of claim 9, wherein:
a third storage node of the at least four storage nodes is electrically connected to a drain of a first transistor of the only two of the plurality of transistors connected to VDD; and
a fourth storage node of the at least four storage nodes is electrically connected to a drain of a second transistor of the only two of the plurality of transistors connected to VDD.

11. The circuit of claim 1, wherein:
only two of the plurality of transistors are connected to ground;
a first storage node of the at least four storage nodes is electrically connected to a gate of a first transistor of the only two of the plurality of transistors connected to ground; and
a second storage node of the at least four storage nodes is electrically connected to a gate of a second transistor of the only two of the plurality of transistors connected to ground.

12. The circuit of claim 11, wherein:
a third storage node of the at least four storage nodes is electrically connected to a drain of a first transistor of the only two of the plurality of transistors connected to ground; and
a fourth storage node of the at least four storage nodes is electrically connected to a drain of a second transistor of the only two of the plurality of transistors connected to ground.

13. The circuit of claim 1, wherein:
only two of the plurality of transistors are connected to VDD; and
only two of the plurality of transistors are connected to ground.

14. The circuit of claim 1, wherein:
the plurality of transistors comprise a plurality of NMOS transistors and a plurality of PMOS transistors;
each of the at least four storage nodes is connected to a gate of a different NMOS transistor in the plurality of NMOS transistors; and
each of the at least four storage nodes is connected to a gate of a different PMOS transistor in the plurality of PMOS transistors.