Abstract:
A FRAM configurable output driver circuit allows the user to configure the output driver for either CMOS level push/pull operation or true open drain operation. This configuration is stored in a non-volatile memory including a FRAM cell and a standard logic latch. The configuration data is restored to the latch on powerup. The user is able to change the configuration at any time. Any changes to the configuration are stored in the non-volatile memory.

Description:
BACKGROUND OF THE INVENTION 
     The present invention is related to output driver circuits, and, more particularly, to a circuit and method for providing a true open drain output driver and a push/pull output driver that is easily configurable by a user command signal. 
     Typically, two different versions of a CMOS integrated circuit are available, one with push/pull outputs and the other with open drain outputs. Referring now to FIG. 1, a CMOS push/pull output driver  10  for an integrated circuit includes an input node  12 , an output pad  14 , an N-channel pull-down transistor MN 1  and a P-channel pull-up transistor MP 1 . The current paths of transistors MN 1  and MP 2  are coupled together at output pad  14  and between the VDD power supply and ground. The output signal at pad  14  is inverted from the polarity of the input signal at node  12 . 
     Referring now to FIG. 2, an NMOS open drain output driver  20  is shown having an external pull-up resistor  16  coupled to an external power supply VEXT. The NMOS open drain output driver  20  for an integrated circuit includes an input node  12 , an output pad  14 , and only the N-channel pull-down transistor MN 1 . The current paths of transistor MN 1  and resistor  16  are coupled together at output pad  14  and between the VEXT power supply and ground. 
     The standard CMOS push/pull driver circuit  10  must have the PMOS pull-up transistor MP 1  disconnected to prevent forward biasing of the P-MOS well (not shown in FIG. 1) and associated leakage current if the external pull-up voltage exceeds VDD+V BE  (wherein V BE  is a diode threshold voltage). A true open drain output should not be dependent on the device&#39;s supply voltage. Therefore, integrated circuits are typically provided in either of the two driver options discussed above. These two driver options are usually based on a common circuit that is configured at the factory with either a metal mask option or a programmable fuse and does not allow the user to configure the output type. 
     A prior art user-configurable output circuit  30  that does provide both types of output driver is shown in FIG.  3 . Output driver circuit  30  includes an input node  32  and a control signal node  34  coupled to a control logic circuit  36 . In turn, the control logic circuit  36  provides logic signals to the gates of transistors MP 1  and MN 1 . The logic conditions for driving the output is shown in table  38  in which the data output state is controlled by input node  32  (IN) and the type of output configuration desired is controlled by the control signal node  34  (PP). Note that a logic high on node  32  and a logic low on node  34  produces a “HiZ” high impedance output since both transistors MN 1  and MP 1  are turned off. 
     The configurability provided by circuit  30  shown in FIG. 3 is volatile and has to be reconfigured by the user after powerup since the data state in the control logic circuit  36  is destroyed every time power is removed. Further, as with the standard push/pull output driver configuration described above, the N-well (not shown in FIG. 3) is forward biased once the external pull-up voltage exceeds VDD. 
     Therefore, in the prior art, separate devices with different mask configurations are required to completely detach the P-channel from the output driver to provide a true open drain output. Alternatively, a configurable circuit provides both types of output circuits, but the output circuit configurability is volatile and there are restrictions on the allowable external voltage coupled to the pull-up resistor. 
     What is desired, therefore, is a non-volatile, configurable output driver circuit that can provide both a push/pull output and a true open drain output. 
     SUMMARY OF THE INVENTION 
     According to the present invention, a FRAM configurable output driver circuit has an output stage that uses two stacked P-channel transistors in series for the pull-up device and a standard N-channel transistor pull-down device. The two stacked P-channel transistors have individual N-type wells with the source of a first P-channel transistor tied to the VDD power supply voltage and the source of a second P-channel transistor tied to the output pad. When the output drives high in push/pull mode, both P-channel output transistor are conducting and the output pad can be driven high. When the output driver drives low in either the push/pull or open drain modes, the first and second P-channel transistors are both turned off, and the output pad can be driven low. In the open drain mode, when the input is high, the output pad is tri-stated, and the output pad is pulled high by an external pull-up device. The second P-channel transistor remains off regardless of the output pad voltage. If the external pullup voltage exceeds VDD, there is no leakage current through the output pad node, because the N-well of the second P-channel transistor is connected to the pad, which is identical to the operation of a true open drain mode output driver circuit. Control signals to the output transistors are set to prevent crowbar current. Non-volatile control logic is used to set either the push/pull or open drain mode configuration. 
     It is an advantage of the present invention that it is user configurable as a push/pull or open drain output driver. 
     It is a further advantage of the output driver of the present invention that there is a significant cost, time, and flexibility advantage with this design. 
     It is a further advantage of the output driver of the present invention that the output driver configuration is stored in non-volatile ferroelectric random-access memory and can be changed at any time by the end user. 
     It is a further advantage of the output driver of the present invention that the configuration is persistently stored and the data retained even after a loss of power. 
     The foregoing and other objects, features and advantages of the invention will become more readily apparent from the following detailed description of a preferred embodiment of the invention, which proceeds with reference to the accompanying drawings. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is a schematic diagram of a prior art CMOS push/pull output driver circuit; 
     FIG. 2 is a schematic diagram of a prior art NMOS open drain output driver circuit having an external pullup resistor and supply; 
     FIG. 3 is a schematic diagram of a prior art configurable output driver circuit; 
     FIG. 4A is a schematic of a non-volatile, configurable output driver circuit according to the present invention having a push/pull output mode and a true open drain output mode; 
     FIG. 4B is a schematic diagram that shows the non-volatile memory block of FIG. 4A in greater detail; 
     FIG. 5 is a plot of circuit signals for the configurable output driver circuit of the present invention configured in the push/pull mode, with the output pin driven high; 
     FIG. 6 is a plot of circuit signals for the configurable output driver circuit of the present invention configured in the push/pull mode, with the output pin driven low; 
     FIG. 7 is a plot of circuit signals for the configurable output driver circuit of the present invention configured in the open drain mode, with the output pin driven high; and 
     FIG. 8 is a plot of circuit signals for the configurable output driver circuit of the present invention configured in the open drain mode, with the output pin driven high. 
    
    
     DETAILED DESCRIPTION 
     Referring now to FIG. 4A, a non-volatile configurable output circuit driver circuit  40  includes a non-volatile memory  44  for storing user-selectable output circuit configuration data, a CMOS output stage  52  coupled to an output pad  54 , and a control logic circuit  46  coupled to the CMOS output stage  52 . The control logic circuit  46  receives an input signal and the circuit configuration data. The output driver circuit  40  also includes circuitry for controlling the voltage across the CMOS output stage  53  to substantially minimize leakage current flow through the output pad  54  as is described in further detail below. 
     The non-volatile memory  44  is ideally a ferroelectric random-access memory of the type manufactured by the assignee of the present invention, Ramtron International Corporation, of Colorado Springs, Colo. Either a one-transistor, one capacitor (“1T-1C”) ferroelectric memory cell-based memory as shown, or a two-transistor, two-capacitor (“2T-2C”) ferroelectric memory cell-based memory can be used. A 2T-2C memory cell uses two of the depicted 1T-1C memory cells to store a one-bit data state as complementary data. 
     Referring momentarily to FIG. 4B, the non-volatile memory  44  is shown in greater detail. Non-volatile memory  44  includes, in one embodiment, a 1T-1C ferroelectric memory cell including a ferroelectric capacitor  82  having one terminal coupled to a plate line  74 , and another terminal coupled to a source/drain of pass-gate transistor  78 . The gate of transistor  78  is coupled to the word line  72 , and the other source/drain of transistor  78  is coupled to the bit line  76 . In addition to a ferroelectric memory cell including ferroelectric capacitor  82  and transistor  78 , memory  44  includes a latch  84  having a D input coupled to bit line  76 , and an input  86  for receiving a pulse that is clocked on powerup. A Q output provides the PP output signal coupled to control logic block  46 . 
     Referring back to FIG. 4A, the CMOS output stage  52  has first, second, and third inputs at nodes  62 ,  64 , and  58  for respectively receiving the /HIGH,/HIGH_P 2 , and LOW logic signals. The source of a first P-channel transistor MP 1  is coupled to the VDD power supply and the gate forms the first input. The source of a second P-channel transistor MP 2  is coupled to the drain of P-channel transistor MP 1 , the gate forms the second input, and the drain is coupled to output pad  54 . The source of N-channel transistor MN 1  is coupled to ground, the gate forms the third input, and the drain is coupled to output pad  54  The bulk node of P-channel transistor MP 1  is coupled to the VDD power supply. The bulk node of P-channel transistor MP 2  is coupled to the output pad  54 . The bulk node of N-channel transistor MN 1  is coupled to ground. The voltage on the second input  64  of output stage  52  is selectively set to the voltage designated VPAD on the output pad to prevent leakage current through the output stage as is described in further detail below. 
     Control logic circuit  46  includes a first input  42  for receiving an input signal designated IN, and a second input for receiving the circuit configuration data signal designated PP. Control logic circuit  46  also includes a first output  56  coupled to the first input of the output stage  52 , and a second output  58  coupled to the third input of the output stage  52 . The first output signal at node  56  is designated HIGH and the second output signal at node  58  is designated LOW. 
     Control logic circuit  46  is designed to provide the following logic function shown in logic table  66 : 
     
       
         
               
               
               
               
               
               
               
             
           
               
                   
                   
               
               
                   
                 IN 
                 PP 
                 /HIGH 
                 /HIGH_P2 
                 LOW 
                 OUTPUT 
               
               
                   
                   
               
             
             
               
                   
                 0 
                 1 
                 1 
                 VPAD 
                 1 
                 GND 
               
               
                   
                 1 
                 1 
                 0 
                 0 
                 0 
                 VDD 
               
               
                   
                 0 
                 0 
                 1 
                 VPAD 
                 1 
                 GND 
               
               
                   
                 1 
                 0 
                 1 
                 VPAD 
                 0 
                 HiZ 
               
               
                   
                   
               
             
          
         
       
     
     When the configuration data signal PP is high, output stage  52  is placed into a push-pull output configuration. When the configuration data signal PP is low, output stage  52  is placed into a true open drain mode because no leakage current is possible through transistors MP 1  and MP 2  when /HIGH is set to VDD and /HIGH_P 2  is set to the VPAD output pad voltage. Regardless of the output pad voltage, the voltage across the output stage (the gate-to-drain voltage of transistor MP 2 ) is substantially equal to zero, and thus leakage current flow is prevented. The N-well of transistors MP 1  or MP 2  is not forward-biased, even if the VPAD voltage exceeds the VDD power supply voltage. 
     Level shifter circuit  48  has a power terminal coupled to the output pad  54 , an input coupled to the first output  56  of the control logic circuit, and an output coupled to the second input  64  of output stage  52 . Level shifter includes first and second P-channel transistors MPLS 1  and MPLS 2 . The source of transistors MPLS 1  and MPLS 2  is coupled to the power terminal. The gates and drains of transistors MPLS 1  and MPLS 2  are cross-coupled, and the drain of the first P-channel transistor MPLS 1  forms the output of the level shifter at node  64 . First and second N-channel transistors MNLS 1  and MNLS 2  are coupled to the drains of P-channel transistors MPLS 1  and MPLS 2 . The drain of N-channel transistor MNLS 1  is coupled to the drain of transistor MPLS 1 , the gate forms the input of the level shifter at node  56 , and the source is coupled to ground. The drain of N-channel transistor MNLS 2  is coupled to the drain of transistor MPLS 2 , the gate for receives an inverted level shifter input signal at node  62  through inverter  11 , and the source is coupled to ground. In operation, the function of level shifter  48  is to shift the input logic voltage level to an output logic level. A logic low input signal remains at ground for a logic low output signal, but a logic high input signal is shifted from the VDD power supply voltage to the VPAD voltage found on output pad  54 . 
     Referring now to FIG. 5, a plot is shown of the LOW, /HIGH_P 2 , HIGH and output pad signals for configurable output driver circuit  40  configured in the push/pull mode, with the output pin driven high. 
     Referring now to FIG. 6 is a plot is shown of the LOW, /HIGH_P 2 , HIGH and output pad signals for the configurable output driver circuit  40  configured in the push/pull mode, with the output pin driven low. 
     Referring now to FIG. 7 is a plot is shown of the LOW, /HIGH_P 2 , HIGH and output pad signals for the configurable output driver circuit  40  configured in the open drain mode, with the output pin driven high. 
     Referring now to FIG. 8 is a plot is shown of the LOW, /HIGH_P 2 , HIGH and output pad signals for the configurable output driver circuit  40  configured in the open drain mode, with the output pin driven high. 
     Thus, an output driver circuit  40  and associated configuration method has been described in which user-selectable output circuit configuration data is persistently stored in non-volatile ferroelectric memory, the CMOS output stage is configured to provide either a push/pull output or a true open drain output in response to the configuration data; and the voltage across the CMOS output stage is controlled to substantially minimize leakage current flow through the output pad. 
     Having described and illustrated the principle of the invention in a preferred embodiment thereof, it is appreciated by those having skill in the art that the invention can be modified in arrangement and detail without departing from such principles. For example, other level shifters can be used, and the non-volatile memory could be flash, E 2 PROM, EPROM, or any other electrically alterable non-volatile memory. I therefore claim all modifications and variations coming within the spirit and scope of the following claims.