Abstract:
An EPROM-based crossbar switch is disclosed that provides for the programmable interconnection of logic circuitry. Circuit layout and design features reduce circuit real estate and bitline parasitic capacitances, allowing a high level of integration and faster switching speeds.

Description:
CROSS REFERENCE TO RELATED APPLICATION 
     This application is a continuation of commonly-assigned U.S. patent application Ser. No. 07/813,802, filed Dec. 26, 1991, abandoned. 
    
    
     BACKGROUND OF THE INVENTION 
     This invention relates to the programmable interconnection of digital circuits. Particularly, the invention relates to programmable interconnections known as crossbar switches, which are used to switch N digital inputs into N digital outputs. 
     Various interconnection schemes are possible, for example, as described in Wong et al. U.S. Pat. No. 4,871,930, blocks of programmable logic or logic array blocks (LABs) may be programmably interconnected using programmable interconnect arrays (PIAs). In this manner, relatively many small logic elements may be efficiently interconnected using a hierarchical method--first, interconnecting primitive logic elements into LABs, and second, interconnecting LABs using PIAs. The PIAs accept all logic function outputs from the LABs, and provide the means to programmably interconnect a small subset of these back into the LABs. 
     However, it is often desirable to provide digital circuit interconnections that programmably switch a number of inputs into an equal number of outputs. Further, it is desirable that the switching circuit have low standby power and display relatively small parasitic capacitance values. The switch should also be simple to program and low in cost. 
     SUMMARY OF THE INVENTION 
     In view of the foregoing, it is an object of this invention to provide a programmable crossbar switch with low standby power consumption. 
     It is a further object of the invention to provide a crossbar switch that is based on erasable programmable read only memory (EPROM) transistors, and thus is relatively inexpensive and readily programmed. 
     It is a further object of this invention to provide an EPROM-based crossbar switch with low bitline parasitic capacitance. 
     The present invention provides the desired structure set out above. Namely, a crossbar switch is provided for programmably interconnecting N input nodes to N output nodes, allowing the interconnection of digital circuits, while adding relatively little parasitic capacitance to the overall circuit. Contributing to the low parasitic capacitance is the use of only two EPROM transistors for each intersection of two bitlines and two wordlines. As shown in commonly assigned, co-pending U.S. patent application Ser. No. 596,764, concerning sense amplifiers with complementary bitlines, prior art intersections typically have four EPROM transistors, resulting in greater bitline loading and slower circuit performance. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is a schematic drawing of the circuit of one cell of an illustrative embodiment of the crossbar switch of this invention. Also shown are bias transistors and the pair of cross-coupled inverters this cell drives. 
     FIG. 2 is a schematic diagram of a representative 4×4 subsection of an illustrative embodiment of a crossbar switch circuit constructed in accordance with the principles of this invention, showing the matrix layout of the cells. 
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     As shown in FIG. 1, a representative portion of a crossbar switch constructed in accordance with this invention includes complementary wordlines 130 and 132 intersecting complementary bitlines 140 and 142 via programmable EPROM transistors 50 and 52 (each having a floating gate, as shown, which allows transistors 50 and 52 to be field-programmable). In an N×N crossbar switch there will be N 2  such intersections, laid out as shown in part in FIG. 2. This allows one of N inputs to be programmably switched to any one of N outputs by the appropriate programming by applying charge to the floating gates in the conventional manner) of the EPROM transistors. Specifically, if EPROM transistors 50 and 52 are programmed, y applying charge to the transistors&#39; floating gates, they will not switch and the state of the input voltage, V IN , at node 10 will not affect the output voltage, V OUT  , at node 120. However, if EPROM transistors 50 and 52 are erased (not programmed), the transistors will switch in response to changes in V IN . By programming all but one pair of EPROM transistors on a pair of complementary wordlines, an input signal at an input node, such as node 10, will be output at the appropriate output node, such as 120. Various EPROM transistors may be employed. For example, EPROM transistors 50 and 52 may be UV-erasable EPROM transistors or electrically erasable programmable read only memory (E 2  PROM) transistors. 
     Referring to FIG. 1, the propagation of an input signal at node 10 via erased EPROM transistors 50 and 52 to output node 120 is described as follows, for both low to high and high to low transitions. Note the power supply voltage at nodes 110 and 112 is approximately 5.0 V, the bias voltage at node 100 is in the range of 3.0 V, and the potential of the ground nodes 150 is maintained in the range of 0 V. The bias transistor arrangement prevents nodes 70 and 72 from rising so high as to falsely program EPROM transistor 50 or 52. 
     For a low to high transition, the initial state of node 10 is low. Input buffer 20 therefore provides a logical low signal at an output connected to node 30 and the inverse--a logical high signal--at an inverting output connected to node 32. Thus, EPROM transistor 50 is initially off, and node 70 high. The high signal of node 70 is communicated via n-type buffer transistor 90 to node 62, holding p-type transistor 42 off. The complement of the low node 10 voltage at node 52 holds EPROM transistor 32 on, holding node 72 low. As p-type transistor 42 is off, only a negligible current flows in bit line 142. The low signal at node 72 is communicated via buffer transistor 92 to node 60 where it holds p-type transistor 40 on. However, as EPROM transistor 50 is off, a negligible current flows in bit line 140. With no current flowing in bit lines 140 and 142, the standby power consumption of the circuit in FIG. 1 is near zero. Note the low signal at node 72 is communicated to output node 120 via buffer transistor 92. 
     A low to high transition at input node 10 causes EPROM transistor 50 to turn on, pulling node 70 low. This low voltage is transmitted to node 62 via buffer transistor 90, turning on p-type transistor 42. Low node 32, which is the complement of high node 10, has turned off EPROM transistor 52. Thus, the turn-on of p-type transistor 42 brings node 60, and therefore output node 120, high. Subsequently, p-type transistor 40 is turned off, blocking current flow in bit line 140. Since current flow is blocked in bit line 142 by turned off EPROM transistor 52, quiescent power dissipation is negligible as in the previous state, where the voltage V IN  at node 10 was low. 
     A high to low transition at input node 10, takes node 32 high, turning EPROM transistor 52 on, and forcing node 72 low. This low voltage is transmitted via n-type buffer transistor 92 to node 60 and the output node 120, turning p-type transistor 40 on. Since node 30 is low, EPROM transistor 50 is off. Thus, as p-type transistor 40 turns on, it brings node 62 high, turning off p-type transistor 42 and blocking current flow in bit line 142. Current flow is blocked in bit line 140 by turned off EPROM transistor 50. The cell has now been returned to its original state. 
     As shown in FIG. 2, the crossbar switch may also include output buffers 215 to buffer and invert the output signal. Further, as shown in FIG. 1, it is also possible to provide output node 122, the complement of node 120. 
     From the above, it is apparent that not only does the circuit in FIG. 1 transmit input signals at node 10 to output node 120, but that the circuit also provides for zero quiescent power consumption. 
     As the structure of two-EPROM transistor cell 210 consumes less real estate on the chip, a higher level of integration is possible than if four EPROM transistors were committed to each cell. Two transistor cell 210 also does not load bit lines 140 and 142 and word lines 130 and 132 as much as would a four transistor cell. This reduced loading provides for faster switching in the crossbar circuit. 
     Although particular attention has been given to the operation of one cell of the crossbar switch circuit, it will be understood that the overall function of the chip is to programmably interconnect N inputs to N outputs, and that among other possible variations within the scope of the invention that will occur to those skilled in the art, any number of inputs and outputs may be used.