Self-Quenching Memory Cell

It is known that array memory cells of the flip-flop type can be written into in a reduced time, for a given applied cell writing current, if the potential available to the cell is diminished while writing is in progress. U.S. Pat. No. 3,813,653 for "Memory Cell With Reduced Voltage Supply While Writing", issued to John L. Smith et al. on May 28, 1974 and U.S. Pat. No. 3,971,004 for "Memory Cell With Decoupled Supply Voltage While Writing", issued to Andrew Dingwall on July 20, 1976 are two examples of the aforementioned technique. In each case, the cell voltage supply circuit is complicated somewhat to provide for the desired potential control action.

Some memory array applications, especially those having closely associated large scale integrated logic circuits, make it advantageous to power both the array and the logic circuits from the same constant voltage sources. Accordingly, it is desirable that enhanced memory cell writing be accomplished while permitting the application of constant voltages to each cell.

A memory array cell characterized by a holding current which is reduced automatically, when writing, so as to switch in a minimum time in response to a given input switching current. Each cell comprises a pair of cross-connected semiconductor inverter circuits which are connected between two constant voltage busses by a resistor. The circuit parameters and buss voltage values are set so that the holding current of non-addressed cells is held at a value sufficient to withstand disturbances on shared lines while the holding current is reduced automatically in the cells being addressed (read or written into). Each cell further comprises a gated sensing circuit and at least one gated writing circuit. The same cell may be simultaneously read from and be written into.

The foregoing and other features and advantages of the invention will be apparent from the following more particular description of the preferred embodiment of the invention, as illustrated in the accompanying drawing.

The sole FIGURE is a simplified schematic diagram of a preferred embodiment of the invention together with additional circuits for utilizing it in an array.

The memory cell 10 of the present invention comprises a pair of cross-coupled inverters together with associated reading and writing cell selection circuits. One inverter includes transistor 1, resistor R.sub.1 and Schottky diode SD.sub.1. The other inverter comprises transistor 2, resistor R.sub.2 and Schottky diode SD.sub.2. The cross-coupled inverters are directly connected to constant voltage buss 11 and, via resistor R.sub.3, to constant voltage source 12. Reading is accomplished with the aid of the transistors 3, 4 and 9. The collectors of transistors 3 and 4 are connected by respective column sensing lines 13 and 14 to sensing circuit 15. The emitter of transistor 9 is connected by line 16 to read column driver 17. The emitters of transistors 3 and 4 are coupled to the collector of transistor 9. The bases of transistors 3 and 4 are respectively coupled to the bases of transistors 1 and 2. The reading circuit is enabled (selected) by a signal on word line 18 connected to the base of transistor 9.

Writing is facilitated by transistors 5 and 6 which are selected (activated) by a signal on word line 19 applied to the bases of the transistors. The transistor emitters are connected to respective write column drivers 20 and 21 via lines 22 and 23. Signals representing "0" data are applied to driver 20 via bit line 24. Similarly, signals representing "1" data are applied to driver 21 via bit line 25. Drivers 20 and 21 are activated by a signal on write enable line 29. The collectors of transistors 5 and 6 are connected to the bases of transistors 1 and 2, respectively. In accordance with one optional aspect of the present invention, whereby any two rows (words) of cells in the array may be selected simultaneously for writing purposes, a second set of writing circuits are provided. The second set comprises transistors 7 and 8, word line 26, column drivers 27 and 28 and write enable line 30 correspond in structure and function to transistors 5 and 6, word line 19, column drivers 20 and 21 and write enable line 29.

The dual writing configuration allows two separate data sources to address any of the cells in the array. Thus, it is possible to write into two array words simultaneously or to read from one word while writing into another word. It is also possible, in accordance with the present invention, to read from a word at the same time that the same word is being written into.

In operation, reading is accomplished by raising the base of transistor 9 above the other bases which have their emitters (not shown) similarly connected to line 16 at the output of read column driver 17. A differential current is provided through transistors 3 and 4 to sensing circuit 15. The reading current flows through the collector load resistor and Schottky diode on the high side of the flip-flop cell. for example, assuming that the collector of transistor 2 is more positive than the collector of transistor 1 when the cell is being read, the reading current flows through resistor R.sub.2 and Schottky diode SD.sub.2 lowering the potential at the collector of transistor 2. The resistance of resistor R.sub.3 is set, relative to the effective impedance of the cross-connected inverters comprising the flip-flop, so that the emitters of transistors 1 and 2 are not effectively coupled to a constant current source. As a consequence, the cell holding current flowing through resistor R.sub.3 is reduced during the cell reading time. The reduced holding current, however, remains sufficient to maintain data stability in the cell.

A similar reduction in the cell holding current automatically occurs while the cell is being written into. This action (termed "self quenching") speeds up the writing of new data for a given amount of writing current. To illustrate the writing mode, it is assumed that the T.sub.2 collector is high with respect to the T.sub.1 collector and that it is desired to write data into the cell from the data source associated with write enable line 29. Accordingly, line 29 is conditioned to turn on drivers 20 and 21 while line 30 is conditioned to turn off drivers 27 and 28. Word line 19 is addressed and carries a signal which conditions transistors 5 and 6 for conduction. A signal representing a "0" is applied to driver 20 to produce on write column line 22 a signal which completes the conduction requirements of transistor 5, turning it on. Writing current flows through transistor 5 and the base connections of transistors 3 and 1 into resistor R.sub.2 and Schottky diode SD.sub.2 to reduce the potential at the collector of transistor 2. Writing time is reduced, for a given amount of writing current, because the cell holding current flowing through resistor R.sub.3 is reduced as a result of the reduction of collector potential and the fact that the emitters of T.sub.1 and T.sub.2 are not connected to a constant current source. A constant current source is avoided by selecting a proper value for the resistor R.sub.3 relative to the effective impedance of the cross-connected inverters comprising T.sub.1, R.sub.1, SD.sub.1 and T.sub.2, R.sub.2, SD.sub.2. By virtue of the "self-quenching" action, the ratio of the writing current flowing on line 22 to the holding current flowing through R.sub.3 in the steady state can be about 1 rather than 2 or more as is usually the case when the holding current is provided by a constant current source.

It should be noted that the data in the storage cell becomes immediately available for detection. In fact, the cell can be read simultaneously with the writing of data. The slight reduction of the cell holding current due to the activation of the reading circuits further aids the switching of the cell during the writing of new data as compared to the case where the cell is not being simultaneously read.

While the invention has been particularly shown and described with reference to the preferred embodiment thereof, it will be understood by those skilled in the art that various changes in form and details may be made therein without departing from the spirit and scope of the invention.