Abstract:
The invention relates to a voltage reference source used to control an overvoltage tolerant input/output buffer for a mixed voltage bus system. The voltage source comprises a voltage tracking circuit having a first input receiving a variable voltage. and a second input receiving a reference voltage. the voltage tracking circuit being adapted to generate an output voltage in response to the difference between the variable voltage and the reference voltage. wherein where the variable voltage is less than the reference voltage. the output voltage is held at substantially zero volts. When the variable voltage exceeds the reference voltage. the output tracks the voltage at the variable voltage input.

Description:
This is a continuation of U.S. Ser. No. 08/949,861, filed Oct. 14, 1997, now U.S. Pat. No. 6,049,242. 
    
    
     FIELD OF THE INVENTION 
     The present invention relates to a mixed voltage bus system and in particular. to interfaces between a number of integrated circuits and a bus where some of the integrated circuits operate at one logic level and others operate at a different logic level. In particular. the invention relates to a voltage reference source used to control overvoltage tolerant input/output buffers. 
     BACKGROUND OF THE INVENTION 
     Semiconductor integrated circuit technology is developing rapidly and one aspect of this is that modern integrated circuit devices are being designed to operate from system supply voltages which are becoming lower. For example. many of todays integrated circuit devices operate from a 5 volt supply. whilst newer integrated circuit devices operate from a 3.3 volt supply. Some state of the art devices operate from even lower supplies of 2.5 volts or less. Accordingly. mixed voltage systems have become necessary which require “overvoltage” tolerant interfaces which allow devices which operate from a lower supply voltage to interface with other devices which operate at a higher supply voltage. As an example. FIG. 1 shows a bus  1  connected to a number of integrated circuits  2 . Each device includes an i/o interface comprising an input buffer  3  and an output buffer  4  connected to the bus  1  via a pad  5 . Some of the devices  2  operate from a 5 volt supply voltage whilst others operate from a lower supply voltage of 3.3 volts. Accordingly. the bus  1  has a low logic level of around 0 volts but a high logic level of between 3.3 and 5 volts. depending on which device is active. Only one of the devices  2  can drive the bus at any one time whilst the others are held in a high impedance state to ensure that they do not alter the logic level appearing on the bus  1 . It is important that any device  2  which operates at the lower voltage level must be able to connect to the bus  1 , even when the bus is at the higher voltage level. 
     FIG. 2 shows an input buffer  3  which includes a conventional bus hold circuit  6 . A bus hold circuit is designed to prevent a bus from floating to an undefined state when all of the devices connected to the bus are in a high impedance state. Without this. the input buffers of devices connected to the bus could produce false transitions and may also dissipate unacceptably high currents. The bus hold circuit  6  comprises a first CMOS inverter  7  connected in a feedback path around a second CMOS inverter  8 . An input to the second CMOS inverter  8  is connected to an input pad  5 . The first CMOS inverter  7  includes a PMOS transistor connected in series with an NMOS transistor. the source of the PMOS transistor being connected to a 3.3 volt supply (Vcc). In use. the input pad  5  is driven by a bus and therefore the voltage which appears at the pad  5  will correspond to whatever voltage level is on the bus. The bus hold circuit  6  is designed to allow the bus to drive the input to the second inverter  8  high or low The bus hold circuit  6  will hold the input at whatever logic level the bus was at until the pad  5  is next driven low or high by the bus so that the bus state does not become undefined. To sustain a bus hold. the first CMOS inverter  7  must be connected to the pad  5 . If 5 volts is applied to a bus hold circuit operating from a 3.3 volt supply voltage (Vcc). a parasitic N-well diode (not shown) associated with the PMOS transistor of the first CMOS inverter  7  becomes forward biassed and injects current into Vcc. The N-well diode turns on when the pad voltage rises above Vcc. Furthermore. the PMOS transistor turns on as its drain voltage rises above Vcc causing an additional drain-source current to flow. in each case. the effect of the overvoltage on pad  5  is to source current from a device driving the pad into Vcc. This will lead to a low transition on the bus and may even damage the device driving the bus to  5  volts. The effect is even worse during live insertion of a device  2  onto the bus  1  since there is no voltage supply to the device when it is first connected to the bus. Accordingly. the bus hold circuit  6  shown in FIG. 2 cannot be connected to a mixed bus of the type shown in FIG. 1 because the PMOS transistor components will not function properly. If an NMOS transistor is used instead of a PMOS transistor in inverter  7  the problem of current injection into the 3.3 volt apply under overvoltage conditions could be avoided. However. an NMOS transistor connected to Vcc does produce a sufficiently high voltage level on its output due to its threshold voltage and backbody effects. An NMOS transistor could be used if its gate voltage is raised to a voltage higher than the on-chip supply Vcc by an amount which would overcome the threshold and backbody effects. However. the circuitry required to produce voltages higher than the on-chip supply tend to consume a great deal of power and are not suitable for use with a device designed for low power applications. Accordingly, use of PMOS pull-up transistors is preferred since when pulling high the drain voltage can reach the same level as the source voltage. 
     FIG. 3 shows a simplified example of a conventional output buffer  9  which includes a number of CMOS inverters which use PMOS transistors powered by a 3.3 volt supply (Vcc). Again. should the pad  5  be driven to a voltage above Vcc by a bus  1 , the parasitic N-well diode (not shown) associated with the PMOS transistor connected to the pad  5  becomes forward biassed and so turn on. injecting current into its N-well. Also this PMOS transistor turns on as the drain voltage rises above Vcc. In each case. the effect is to source current into the voltage supply (Vcc). Accordingly, the output buffer  9  shown in FIG. 3 cannot be connected to a mixed bus  1  of the type shown in FIG. 1 because the PMOS transistor components will not function properly. 
     SUMMARY OF THE INVENTION 
     According to a first aspect of the present invention. there is provided a voltage source for a semiconductor device interface comprising a voltage tracking circuit having: 
     a first input receiving a variable voltage: and. 
     a second input receiving a reference voltage. the voltage tracking circuit being adapted to generate an output voltage in response to the difference between the variable voltage and the reference voltage. wherein when the variable voltage is less than the reference voltage. the output voltage is held at substantially zero volts. but when the variable voltage exceeds the reference voltage, the output tracks the voltage at the variable voltage input. 
     Preferably the voltage tracking circuit comprises a concatenated series of inverters. each inverter of which is coupled to the variable voltage input, wherein an output of the series of inverters provides an output of the voltage source. 
     According to a second aspect of the present invention there is provided an input buffer comprising: 
     an input driver receiving an input voltage: 
     a bus hold circuit coupled in parallel thereto: and. 
     a voltage source according to the first aspect of the present invention. the voltage source decoupling the bus hold circuit from a first fixed voltage supply when the input voltage exceeds the first fixed voltage. 
     According to a third aspect of the present invention there is provided an output buffer comprising a pre-driver circuit and a voltage source according to the first aspect of the present invention. the voltage source decoupling the pre-driver circuit from a first fixed voltage supply when the input voltage exceeds the first fixed voltage. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     Examples of the present invention will now be described with reference to the accompanying drawings. in which: 
     FIG. 1 shows a mixed voltage bus: 
     FIG. 2 shows a conventional input buffer having a bus hold circuit: 
     FIG. 3 shows a conventional output buffer: 
     FIG. 4 is a block diagram showing an overvoltage tolerant input/output interface in accordance with the present invention: 
     FIG. 5 shows an example of an overvoltage tolerant input buffer having a bus hold circuit in accordance with the present invention: 
     FIG. 6 shows a voltage reference signal used to control the input buffer of FIG.  5 . 
     FIG. 7 shows an N-well bias signal used to control the input buffer of FIG.  5 : 
     FIG. 8 shows an example of a reference voltage generating circuit in accordance with the present invention for generating the voltage reference signal shown in FIG.  6 : 
     FIG. 9 shows an N-well bias signal generating circuit: 
     FIG. 10 shows a detailed circuit for an input buffer: and 
     FIG. 11 shows an example of an overvoltage tolerant output buffer in accordance with the present invention. 
    
    
     DETAILED DESCRIPTION 
     FIG. 4 is a block diagram of an example of an overvoltage tolerant i/o interface  10  for an integrated circuit in accordance with the present invention. The i/o interface  10  comprises an input buffer  11  having a bus hold circuit  12  and an output buffer  13 . each of which is connected to a common pad  14 . A reference voltage generating circuit  15  and an N-well bias signal generating circuit  16  are also connected to the pad  14  and. as will be described below. control the operation of the input buffer  11  and the output buffer  13 . The signals V ref  and NSUB generated by each of these circuits. respectively. are coupled to the gases and N-wells. respectively. of a number of PMOS transistor components found within the input buffer  11  and the output buffer  13  to provide an overvoltage tolerant inferface suitable for connection to a mixed voltage bus (not shown). As will be described below. each of these signals is arranges to track whatever voltage appears at the pad  14  in the predetermined manner. 
     FIG. 5 shows an input buffer with bus hold circuit  12  in more detail. In comparison to the conventional bus hold circuit shown in FIG.  2 . the bus hold circuit  12  of the present invention includes an isolation transistor  17  in the form of a first PMOS transistor coupled between the source of a second PMOS transistor  18  and the supply voltage Vcc. The gate of the isolation transistor  17  is controlled by the reference voltage signal V ref  whilst the N-wells of each of the PMOS transistors of the bus hold circuit are controlled by the N-well bias signal NSUB. These signals are shown in FIGS. 6 and 7. respectively. 
     As shown in FIG.  6 . and described in detail below. voltage reference signal V ref  remains a zero provided the voltage at the pad  14  does not exceed Vcc. Under these conditions. the isolation transistor  17  remains on and therefore the bus hold circuit  12  functions in the conventional manner. However. if the pad voltage rises above Vcc. the voltage reference V ref . then tracks the pad voltage to control the voltage at the gate of the isolation transistor  17 . This causes the isolation transistor  17  to turn off. thereby isolating the second PMOS transistor  18  from the voltage source Vcc. Accordingly. although the drain voltage of the second PMOS transistor  18  may rise well above Vcc. the transistor  18  does not source current to Vcc. 
     As shown in FIG.  7 . the N-well bias signal NSUB is held constant at a level substantially equal to Vcc providing the pad voltage is below Vcc. If the pad voltage rises above Vcc. the N-well bias signal NSUB then tracks the pad voltage. This ensures that the parasitic N-well diodes in the PMOS transistor components  17  and  18  of the bus hold circuit remain reverse biassed and therefore do not source current to Vcc. 
     The voltage reference signal V ref  is supplied by the voltage reference signal generating circuit  15  shown in detail in FIG.  8 . This circuit is designed to detect when the voltage at the pad  14  exceeds Vcc and then feed the overvoltage input onto the gate of the isolation transistor  17  shown in FIG.  5 . This ensures that the gate-source voltage (Vgs) is zero and so prevents the isolation transistor  17  from turning on. 
     The voltage reference generating circuit  15  of FIG. 8 comprises a concatenated series of inverters  11  to  13 . each comprising a PMOS transistor connected in series with an NMOS transistor. The N-wells of each of the PMOS transistors are driven by the N-well bias signal (NSUB) described above to ensure that the parasitic N-well diodes remain reverse biassed and therefore do not source current to Vcc. The sources of each of the PMOS transistors of the inverters  11  to  13  are connected to the pad  14 . The gates of the transistors in a first inverter  11  are tied to the voltage source Vcc. An output of the first inverter  12  is fed via a further inverter  14  in a feed forward circuit path  19  to an NMOS pull-down transistor  20  at the output of the circuit. 
     In operation. when the pad voltage is below Vcc. the PMOS transistor in the first inverter  11  turns off and the associated NMOS transistor turns on. This gives a low output at node N 1  which. once inverted by inverter  14 . causes NMOS transistor  20  to turn on. pulling the output at node N 2  of the circuit low. When the pad voltage rises above Vcc. the PMOS transistor in the first inverter  11  turns on so that the output at node N 1  is pulled up to the voltage of the pad  14 . This voltage is then passed through the following inverter stages  12  and  13  and appears at Node N 2  at the output of the circuit. Accordingly. as shown in FIG.  6 . when the pad voltage rises above Vcc. the voltage reference V ref  tracks the pad voltage. The concatenated series of inverters  11  to  13  act as buffers and so improve the edge rate of the V ref  signal. The PMOS transistor in inverter  11  is significantly larger. and hence more powerful. than the corresponding NMOS transistor. Accordingly, when the PMOS transistor turns on it is able to pull node N 1  high despite the efforts of the NMOS transistor to pull this node low. The concatenation of the buffers  11  to  13  is required to decouple the large load capacitance connected on node N 2  from the output of the inverter  11 . 
     FIG. 9 shows an N-well bias signal NSUB generating circuit  16 . This circuit is conventionai. As shown. the circuit comprises a pair of PMOS transistors  21  and  22  connected in series between a supply rail Vcc and the pad  14 . The gate of PMOS transistor  21  is connected to the pad  14  and so is controlled in dependence on whatever voltage appears at the pad  11 . whilst the gate of PMOS transistor  22  is connected to Vcc. As described above with respect to FIG.  7 . when the pad voltage is below Vcc. the output of the NSUB circuit  16  is held constant at a voltage level substantially equal to Vcc. Should the pad voltage rise above Vcc. the output NSUB tracks the pad voltage. The NSUB output signal is fed to a number of PMOS transistor components in the i/o interface  10  to bias the N-wells. This keeps the parasitic diodes of the N-wells reverse biassed so they do not source current to the supply voltage Vcc of the associated device. 
     FIG. 10 is a detailed circuit for an input buffer for an integrated circuit which implements a bus hold function. showing the bus hold circuit  12 . voltage reference generating circuit  15  and NSUB generating circuit  16  described above connected together. As shown. the voltage reference generating circuit also generates a signal V ref B. Under normal conditions this signal is at a voltage level substantially equal to the supply voltage Vcc. In an overvoltage state V ref B corresponds to the level of Vss. 
     FIG. 11 shows an example of an overvoltage tolerant output buffer in accordance with the present invention. The voltage reference generating circuit and N-well bias signal generating circuit have been omitted for clarity. The N-wells of the PMOS transistor components in the circuit are connected to the N-well bias signal NSUB. The output buffer includes a pre-driver circuit  23 . a PMOS transistor  24  and an NMOS transistor  25 . The voltage reference signal V ref B is connected to the gate of a PMOS transistor  26 . The source of the transistor  26  is connected to the gate of an output PMOS transistor  27  and the drain is connected to the pad  14 . Under overvoltage conditions. transistor  26  turns on and so raises the gate voltage of transistor  26  to that of the pad  14 . Two isolation transistors  28  and  29  are provided in the pre-driver  23 . PMOS isolation transistor  28  prevents current injection into Vcc because its gate is connected to V ref . whilst NMOS isolation transistor  29  prevents leakage into Vss because its gate is coupled to V ref B which ensures that the transistor  29  remains switched off. 
     The i/o buffers described above are overvoltage tolerant and can therefore be connected to a mixed voltage bus whilst retaining their functionality and without affecting the performance of the bus.