Dual function memory
BACKGROUND OF THE INVENTION 
     The present invention relates generally to memory cells and more particularly to a memory cell capable of operation in a read/write and a read only memory configuration. 
     Many process oriented control applications in todays industries are using low cost micro-controllers. Such systems require a limited amount of memory. Most applications require the incorporation of both pre-programmed memories and real time read/write memories. A predetermined sequence of events or programs are stored in non-volatile or preprogrammed memories while data relevant to a particular point in time or alterable information is stored in programmable memories. Because of a limited amount of memory available, memories capable of operation as non-volatile memories and programmable memories were developed. 
     Prior art dual functional memories included extensive metal paths and switches to preselect the state of the memory cell upon powering up the memory system. These systems generally occupied large amount of surface area in an integrated circuit and are thus considered undesirable. Other devices having included placing a diode on a desired information node of a memory cell and powering the diode to pre-store the desired information bit. 
     Unbalanced bistable memory cells are also well-known in the prior art having, for example, unbalanced capacitors or resistors connected to a respective information node of a memory cell. Some of these systems require a three level voltage supply wherein the memory cell operates as a non-volatile memory upon transition from a first to a third level and operates as a programmable memory upon transition from a second to a third level. The second voltage level is generally a maintenance level to prevent the unbalance memory cell from assuming the non-volatile state. The voltages are applied along the usual supply line throughout the memory system. 
     SUMMARY OF THE INVENTION 
     The present invention provides a new and improved dual function memory wherein a first switch interconnects the power supply and the unbalanced bistable memory cell and a second switch interconnects the two information storage nodes so as to completely discharge or erase the information stored in the nodes upon a power down signal. A preselected bit is stored in the memory cell as determined by the impedance unbalance of the memory cell. The unbalanced bistable memory cell could include a cross-coupled pair of complementary field effect transistor inverters. The first switch means would be a P channel device interconnecting the P channel devices of the inverters to the power supply and the second switch means would be an N channel device interconnecting the information storage nodes. The signal source would provide a power down signal to the P channel switch to remove the power from the memory cell and to the N channel device to interconnect the information storage nodes and allow the stored information or charge to be discharged to ground. In the absence of the power down signal, the P channel device interconnects the power supply and the cross-coupled inverters and the N channel device disconnects the information nodes allowing the memory to store information at its nodes. The unbalanced bistable memory may also be configured using other field effect transistors as well as bipolar transistors. A first and second switching means would be appropriately selected switches so as to perform a function to disconnect the power for a power down signal and to provide a discharge or erasing function of the information nodes in response to the power down signal and to allow normal operation of the memory cell after the power up and assuming a preselected state. The unbalanced bistable memory cell may comprise a portion or all of a memory matrix. The first and second switching means may be used with any or all of the unbalanced bistable memory cells in the matrix to control their dual function. 
     OBJECTS OF THE INVENTION 
     An object of the present invention is to provide a memory capable of functioning as a non-volatile memory and a programmable read/write memory. 
     Another object is to provide a new approach of effectuating a dual function memory. 
     A further object of the invention is to provide an integrated memory matrix having cells therein capable of dual memory function using a minimum amount of surface area to effectuate the dual functions. 
     Still another object of the present invention is to provide switches capable of reverting selected memory cells in a memory to the ROM state without interfering with the power or connection of the power supply to other memory cells in the memory matrix. 
    
    
     Other objects, advantages and novel features of the present invention will become apparent from the following detailed description of the invention when considered in conjunction with the accompanying drawings. 
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is a schematic diagram of a complementary field effect transistor memory cell embodying the concept of the present invention. 
     FIG. 2 is a schematic diagram of a field effect transistor memory cell also embodying the concept of the present invention having enhancement load devices. 
     FIG. 3 is a schematic diagram of a field effect transistor memory cell also embodying the concept of the present invention having depletion load devices. 
     FIG. 4 is a schematic diagram of a bipolar memory cell embodying the concept of the present invention. 
     FIG. 5 is a schematic diagram of a memory matrix embodying the concept of the present invention. 
    
    
     FIG. 1 illustrates a static six transistor memory cell 10 of an unbalanced configuration. P channel and N channel field effect transistors 12 and 14 form a CMOS inverter and is cross-coupled to a P channel and N channel device 16 and 18 respectively which is a second CMOS inverter. Interconnecting information storage node 20 and the DATA line 22 is FET switch 24 and interconnecting information storage node 26 and DATA line 28 is FET switch 30. The gates of the FET switches 24 and 30 are connected to the row select line 32. The just described six transistor configuration is well-known in the prior art and is not considered part of this invention. It is also well known to configure the field effect transistors 12, 14, 16, or 18 such that the cell is unbalanced, i.e., that the impedance of one section of the memory cell is different from the impedance of the second section of the memory cell such that the cell will assume a predetermined state upon being powered up. This unbalanced configuration may be produced by the geometry of individual devices, for example, the size of the channel region, or by the gate material to produce parasitic capacitors of different capacitance or resistors of different resistive levels. The specific form or means to produce the unbalance is not critical to this invention as long as the unbalance is produced. 
     A P channel field effect transistor 34 interconnects the P channel devices 12 and 16 of the memory cell to the power supply V DD  and its gate is driven by the power down signal (PWR DN). FET 36 interconnects the information storage nodes 20 and 26 and its gate is also driven by the power down signal (PWR DN). Although the switch 34 and 36 are illustrated as complementary depletion type field effect transistors, and the power down signal is applied to their gates in the appropriate fashion, other field effect transistors and other switching devices may be used, the essence being the operation of the switching devices, to produce the required functions. 
     The operation of the dual function memory cell 10 begins with a power down signal applied to the gate of FET 34. This signal turns off FET 34 and removes or isolates the power supply from the memory cell. Simultaneously the two storage nodes 20 and 26 will be tied together and discharged through ground via the conduction of FET switch 36 which also receives the power down signal. Thus switches 34 and 36 have removed the power from the inverter and discharged or erased the information or signal stored at information storage modes 20 and 26 and basically reset the bistable memory cell 10. When the cell is powered up, the power down signal is removed from the gate of transistor switch 34 allowing it to conduct and thereby interconnecting the power supply V DD  and the memory cell 10. The absence of the power down signal at the gate of FET switch 36 causing it to cease conduction and thus removing the short or tie of the information storage nodes 20 and 26 allowing a signal or bit of information to be stored at one or the other of the storage nodes 20 or 26. 
     Since the ratio of the devices 12, 14, 16 and 18, produce an unbalance a preselected bit of information is stored at node 20 or 26 based on this ratioing upon resumption of the power. Thus, upon a power up signal, the dual function memory cell 10 function as a preprogrammed read only memory (ROM). After use of the programmed information contained in the memory cell 10, the cell can be used for data storage and can be written into like a static random access memory (RAM) cell. Thus, by the addition of two switching elements 34 and 36, a standard six static bistable memory cell can selectively function as a read only memory as well as a read/write memory. 
     Although FIG. 1 illustrates a memory cell 10 using a CMOS implementation, other implementations can be provided using the concept of the present invention. For example, in FIGS. 2 and 3 a bistable unbalanced memory cell 40 is illustrated as including cross-coupled N channel devices 14 and 18 similar to the memory cell 10 and N channel load devices 42 and 44. A N channel FET 43 interconnects the power supply V DD  and the memory cell 40 similar to FIG. 1 and the N channel FET 36 interconnects the information storage nodes 20 and 26 as in FIG. 1. The operation of the memory cell 40 in FIG. 2 with the first and second switching elements 34 and 36 is identical to that of memory cell 10 in FIG. 1. Namely, it includes an unbalanced bistable memory cell functioning in response to power down signals to disconnect the power supply and erase or discharge information stored at the information storage nodes 20 and 26 so as to allow the nodes to store a preselected bit of information upon reenergization or removal of the power down signal. FIG. 2 is an implementation using enhancement load FETS 42, 44 and FIG. 3 using depletion load FETS 42, 44. Since both implementations use N channel FETS, the node FET is driven by the inverse of the power down signal (PWR DN). 
     FIG. 4 shows a bipolar memory cell 50 including transistors 52, 54, and load resistors 56 and 58. The transistors are connected in a cross-coupled manner and the geometry of the transistors and/or resistors are such that memory cell 50 is an unbalanced bistable memory cell. The first FET switching element 34 interconnects the load resistors 56 and 58 and the power supply of V and the second switching FET interconnects the information storage nodes 60 and 62. The bipolar transistor memory cell 50 with the FET switching elements operates in the same way as the memory cells 10 and 40 of FIGS. 1 and 2 respectively. 
     The concept of the present invention may be applied to a memory matrix or array including a plurality of bistable memory cells. Each of the memory cells of the matrix may be, or some may be, dual function devices depending on the design criteria and memory space limitation. 
     FIG. 5 illustrates a four memory cell matrix including a memory cell 64, 66, in row 1 and 68 and 70 in row 2. Row 1 is driven by or interconnected by a row select line RS1 and row 2 is interconnected by row select line RS2. DATA line 72 and DATA line 74 interconnecting memory cells 64 and 68 and DATA line 76 and DATA line 78 interconnect memory cells 66 and 70. Memory cells 64 and 66 are unbalanced bistable memory cells and 68 and 70 may be but do not have to be unbalanced bistable memory cells. A first switching device or P channel FET 80 (which may also be N channel depending upon cell structure) interconnects the unbalanced bistable memory cells 64 and 66 to the power supply V DD  and a second switching device or N channel device 82 and 84 are connected across the information storage nodes of the memory cells 64 and 66 respectively. 
     The switching devices 80, 82, and 84 operate in relationship to the individual memory cells 64 and 66 as described for FIGS. 1-3. By using a single switching device 80 for plurality of cells, the amount of space used to effectuate the dual function memory is reduced. Also individual control of the dual function cells may be provided whereas prior art memory cells performing a dual function require power down of the entire memory matrix in order to effectuate the preprogrammed read only memory functions. By using switching elements 80, 82, and 84 these elements need only be switched by the power down signal PWR DN to selectively power down the individual memory cells 64 and 66 causing erasure or removal of the bits of information at the storage nodes while allowing memory cell 68 and 70 to retain their written information. 
     For applications requiring frequent referencing of the read preprogrammed or read only data, the following sequence of operation is proposed. The bit or byte stored in the dual function memories 64 and 66, are read and stored in registers. Next, the common switch 80 and switches 82 and 84 are controlled by the power down signal so as to disconnect the power from the memory cells and discharge or erase the data or information stored therein. Next, the power down signal is removed from switches 80 and 82 and 84 reenergization cell 64 and 66 causing them to assume a preselected state due to the unbalance of the memory cell. The preselected data or information is then read from the cells and used as required. After the preselected data is read, the data in the registers are then rewritten into the cell 64 and 66 and the memory matrix is thus returned to its last written state prior to the power-down, power-up sequence for the selected cells. 
     Thus it can be seen that selective deactivation or selective operation of dual function memory cells in their read only memory function may be effected without totally powering down and powering up the memory matrix. By providing switches 80, 82, and 84 in each memory cell, the individual cells may be selectively operated as dual function memories. 
     From the preceding description of the preferred embodiments, it can be seen that the objects of the invention are obtained and a novel dual function memory is provided. Although the invention has been described and illustrated in detail, it is to be clearly understood that the same is by way of illustration and example only and is not to be taken by way of limitation. The spirit and scope of this invention is to be limited only by the terms of the appended claims.