Abstract:
The present invention provides an integrated circuit having an operating voltage adaptable buffer, capable of operating in different voltage signaling environments, which includes a control circuit that provides a clamping function to the signaling path under prescribed operating conditions and which also reliably biases the p-type transistor substrate voltage of the buffer to the most positive voltage seen by the buffer under all operating conditions occurring on the signaling path, thereby protecting the p-type transistors.

Description:
FIELD OF THE INVENTION 
     The current invention relates to buffers and more particularly to a buffer capable of operating in plural voltage level signaling environments. 
     BACKGROUND OF THE INVENTION 
     The use of local buses has become increasingly popular in the personal computer industry. Local buses are buses used to connect the system&#39;s processor to high bandwidth I/O peripherals such as displays, disk drives, etc. One such local bus in widespread use today is the PCI local bus. Because the PCI local bus has gained widespread acceptance, the buffer of the invention will be illustrated for use with this bus. The invention is not limited, however, to a buffer for this or any other particular bus or for use with the specific voltages discussed below. 
     The PCI Local Bus Specification, Rev. 2.1s, dated Jun. 1, 1995 (hereinafter referred to as the &#34;PCI Spec&#34;), defines two signaling environments, a 5 volt signaling environment and a 3.3 volt signaling environment. 5 volt and 3.3 volt refer to the voltage of a logical &#34;1&#34; signal in the respective signaling environments. The PCI Spec dictates that the signal voltage be clamped to 3.3 volts in a 3.3 volt environment, but does not require this clamping function in a 5 volt environment. The PCI Spec also contemplates a &#34;universal board&#34; that is capable of detecting the signaling environment in use and adapting itself to that environment. 
     The PCI local bus signaling environments are not dependent upon the component technologies used. A &#34;5 volt component&#34; can be designed to work in a 3.3 volt environment; and a &#34;3.3 volt component&#34; can be designed to work in a 5 volt environment. Component technologies of both voltages may be present on a PCI local bus at the same time. The PCI Spec contemplates 5 volt tolerant 3.3 volt implementations of universal boards. A 5 volt tolerant, 3.3 volt universal board is one implemented with 3.3 volt components that can function in a 5 volt signaling environment or a mixed 5 and 3.3 volt signaling environment. &#34;3 volt,&#34; and &#34;3.3 volt&#34; as used herein refer to a nominal 3.3 volt voltage level; &#34;5 volt&#34; refers to a nominal 5 volt voltage level. The term &#34;bus&#34; as used herein refers not only to a formal bus architecture, but to any signaling path between any two devices. The term &#34;pad&#34; as used herein refers to a terminal or other physical electrical connection to a bus or signaling path. 
     FIG. 1 illustrates a known 5 volt tolerant, 3.3 volt output buffer 14 for use in a universal board. The buffer 14 is connected to the bus at pad 10. The additional buffer circuitry 18 is the circuitry that handles driving the bus (sourcing and sinking current to assert logic 1&#39;s or 0&#39;s, respectively) with a signal received from a peripheral, receiving data from the bus to drive a peripheral, and isolating a peripheral from the bus. Implementations of additional buffer circuitry 18 are well known in the art and need not be further discussed here. The buffer 14 includes a substrate voltage control circuit 16 that sets the substrate voltage for the p-type transistors included in the additional buffer circuitry 18. Because the voltage seen at the pad 10 may be 5 volts, which is higher than VDD=3.3 volts in a 5 volt tolerant, 3.3 volt buffer, the control circuit 16 is necessary to ensure that the substrate of p-type transistors is set to the highest voltage present. Should the substrate of the p-type transistors not be connected to the highest voltage present, undesirable or unpredictable behavior of the buffer may result as the p-n junctions between the active areas and the substrate may become forward biased. The substrate of the p-type transistors is an N-type well, referred to herein as an N-well or N-well substrate. 
     FIG. 2 illustrates a known substrate voltage control circuit 16 for use in a 5 volt tolerant, 3 volt buffer. The control circuit 16 includes a first P-type transistor 22 and a second P-type transistor 24. The gate of the transistor 22 is connected through a resistor 28 to the pad 10. The gate of the transistor 24 is connected through a resistor 26 to the supply voltage (referred to herein as &#34;VDD&#34;), which is 3.3 volts, at node 20. Resistor 26, together with the transistors 22, 24, form an output stage 32 with the gate of the transistor 22 being controlled by the voltage at the pad 10. One side of the transistor 22 is also connected to VDD. The other side of the transistor 22 is connected to one side of the second output stage transistor 24. The remaining side of the second output stage transistor 24 is connected to the pad 10. 
     As used herein, a &#34;side&#34; of a transistor is a generic term that refers to either the source or the drain, i.e., the connections to the active areas of the transistor. The transistors discussed herein are MOSFETs, which are symmetrical such that the source and drain are reversible. Because the voltages seen by some transistors are such that a single side of a transistor may function as a source under some conditions and a drain under others, only the generic term &#34;side&#34; will be used herein. 
     Operation of the known control circuit 16 in the presence of different signaling environments will now be described. In a 5 volt signaling environment, when the pad 10 is driven to a logic 1, nominally 5 volts, the transistor 22 will turn off, but transistor 24 will turn on. Transistor 24 turns on even though the gate is connected to VDD=3.3 volts because the gate of the transistor becomes more than one VTP (where VTP refers to the threshold voltage of the p-type transistor) below the 5 volts on the side of the transistor that is connected to the pad 10. The result is that the N-well voltage node 30 will follow the pad 10 voltage when the pad 10 voltage exceeds VDD+VTP. The substrates of the transistors 22, 24 are also connected to the N-well voltage node 30 to protect these transistors from damage along with the p-type transistors that form part of the additional buffer circuitry 18. 
     When the pad 10 is driven to logic 0 (nominally 0 volts), the transistor 22 is turned on and the transistor 24 is off. The result is that the N-well voltage node 30 is at VDD=3.3 volts. This is acceptable even in a 5 volt environment because VDD=3.3 volts will be the highest voltage present when the pad 10 is at a low voltage corresponding to logic 0. 
     In the 3.3 volt signaling environment, the transistor 24 is normally off because the gate is connected to VDD and the pad 10 is normally below VDD+VTP. Therefore, when the pad 10 is driven low, the transistor 22 will turn on and the N-well voltage node 30 will be at VDD=3.3 volts. However, when the pad 10 is driven high, both transistors 22 and 24 turn off. The result is that first, the signal voltage at the pad 10 is no longer clamped by the connection through the transistor 22 to the supply voltage VDD=3.3 volts; and second, the N-well voltage node 30 is allowed to float. This is in contravention to the PCI spec, which dictates that the signal voltage be clamped to 3.3 volts in the 3.3 volt signaling environment. 
     What is needed is a configurable substrate voltage control circuit that will reliably bias the N-well substrate of p-type transistors to the more positive voltage of either the supply voltage or the data on the bus under all operating conditions on the bus while providing the appropriate clamping function. 
     SUMMARY OF THE INVENTION 
     The present invention provides an integrated circuit having an operating voltage adaptable buffer with a control circuit that reliably biases the p-type transistor substrate voltage to the most positive voltage seen by the buffer under all operating conditions occurring on the bus, while also providing the appropriate clamping function on the bus. When in a second (or lower) voltage signaling environment (e.g. 3.3 volts), the control circuit biases the substrates of the p-type transistors to the 3.3 volt supply voltage. When in a first (or higher) voltage signaling environment (e.g. 5 volts), the control circuit biases the N-well substrates of the p-type transistors to the pad voltage when the pad voltage exceeds the supply voltage (3.3 volts) plus a p-type transistor threshold voltage, and biases the substrates of the p-type transistors to the supply voltage otherwise. In this manner, the substrate voltage is always set to the highest voltage present, which protects the p-type transistors in the high voltage environment. Furthermore, the control circuit also establishes a path from the pad to the supply when in the lower signaling environment, thereby providing the appropriate clamping function on the bus. 
     These and other advantages, characteristics and features of the invention will be better understood from the following detailed description which is provided in connection with the accompanying drawings. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is a block diagram of a known configurable 5 volt tolerant, 3 volt buffer. 
     FIG. 2 is a circuit diagram of the substrate voltage control circuit of the buffer of FIG. 1. 
     FIG. 3 is a block diagram of a configurable 5 volt tolerant, 3 volt buffer according to one embodiment of the present invention. 
     FIG. 4 is a circuit diagram of the substrate voltage control circuit of the buffer of FIG. 3 according to one embodiment of the present invention. 
     FIG. 5 is a circuit diagram of the substrate voltage control circuit of the buffer of FIG. 3 according to a second embodiment of the present invention. 
     FIG. 6 is a block diagram of a computer system including components that use the buffer of FIG. 3. 
     FIG. 7 is a block diagram of a configurable 5 volt tolerant, 3 volt buffer according to a third embodiment of the present invention. 
    
    
     DETAILED DESCRIPTION OF THE EMBODIMENTS 
     FIG. 3 illustrates a 5 volt tolerant, 3.3 volt buffer 114 according to one embodiment of the present invention. The buffer 114 can be included in an integrated circuit, but the invention is not limited to an integrated circuit embodiment. The buffer 114 includes a control circuit 116 that biases the N-well substrate of the p-type transistors used in control circuit 116 and the p-type transistors that form part of additional buffer circuitry 18. The control circuit 116 and additional buffer circuitry 18 are connected to receive data from and send data to pad 10. 
     The control circuit 116 is supplied with a binary signal, SIGENV, that indicates whether the buffer 114 is working in a 5 volt or 3.3 volt signaling environment. As illustrated in FIG. 3, the SIGENV signal is supplied by a voltage discriminator 12, which determines which voltage signaling environment is being used on a bus to which pad 10 is connected. An exemplary discriminator 12 that can be used with the invention is disclosed in a co-pending application entitled &#34;I/O Power Supply Discriminator,&#34; Ser. No. 09/115,683, filed on the same day as this application. The discriminator 12 will not be discussed further herein. It should be noted that the invention is not limited to use with a discriminator 12. The SIGENV signal may originate from a memory location, for example, when information about the signaling environment is known in advance. FIG. 7 illustrates a buffer 114 that is supplied with a SIGENV signal from a memory location 112. The SIGENV signal may be provided in numerous other ways, depending upon the application. 
     FIG. 4 illustrates a control circuit 116 according to one embodiment of the present invention. The control circuit 116 comprises an output stage 132, a gate control circuit 128, and an isolation circuit 140. The gate of transistor 122 of the output stage 132 is connected to the gate control circuit 128. The gate control circuit 128 sets the gate voltage of the transistor 122 depending upon the ISIGENV signal received from the isolation circuit 140. The isolation circuit 140 receives the SIGENV signal from the discriminator 12 and outputs the ISIGENV signal, which is a buffered version of the SIGENV signal. 
     The isolation circuit 140 is formed of two inverters 151, 153 connected in series which serve to isolate the SIGENV signal from the rest of the buffer 114. The output stage 132 is similar to the output stage 32 discussed in connection with FIG. 2, except for two things. First, the transistor 122 is a high threshold p-type transistor with a threshold voltage VTHP. The threshold voltage VTHP is greater than the threshold voltage VTP for the transistor 124, but is lower than VDD-VTN (VTN is the threshold voltage of the n-type transistors in the circuit). Second, the gate of the transistor 122 is connected to the gate control circuit 128, rather than to the pad 10 through a resistor 28 as in FIG. 2. 
     The gate control circuit 128 comprises a first 5 volt isolation transistor 152 connected on one side to the gate of the first output stage transistor 122 of the output stage 132, and to one side of a first pull down transistor 150 on the other side. The remaining side of the first pull down transistor 150 is grounded. The gate of the first isolation transistor 152 is connected to VDD=3.3 volts through a resistor 126. The gate of the first pull down transistor 150 is connected to the output of the isolation circuit 140 and is therefore also controlled by the ISIGENV signal output by the isolation circuit 140. 
     Also connected to the gate of the transistor 122 is one side of a second pull down transistor 158. The gate of the second pull down transistor 158 is connected to the output of an inverter 154 whose input is connected to the ISIGENV signal supplied by the isolation circuit 140. The remaining side of the second pull down transistor 158 is connected to one side of a second isolation transistor 156. The other side of the second isolation transistor 156 is connected to the pad 10. A third p-type transistor 160 is also present, with one side connected to the pad 10, the other side connected to the gate of the transistor 122 and the gate connected to VDD=3.3 volts through resistor 126. 
     Operation of the circuit of FIG. 4 will now be explained in detail. In a 3.3 volt signaling environment, the SIGENV and the ISIGENV signals will have a &#34;1&#34; logic state as SIGENV is received from the discriminator 12. The &#34;1&#34; logic level turns on the first pull down transistor 150. The first isolation transistor 152 is also turned on, so a path from the gate of the transistor 122 to ground is formed through the transistors 150, 152. (Transistors 158 and 160 will be off in this situation and therefore will not affect the gate of transistor 122.) Thus, regardless of the voltage at the pad 10, the first output stage transistor 122 is turned on. The second output stage transistor 124 is turned off under normal conditions because its gate is connected to VDD and the pad voltage does not exceed VDD in a 3.3 volt signaling environment under normal conditions. The result is that the N-well voltage node 130 is set to VDD=3.3 volts, thereby providing a substrate voltage output clamped to 3.3 volts. Should the pad 10 voltage exceed VDD+VTP (i.e., 3.3 v+VTP), transistor 124 turns on and the connection to VDD through the transistor 122 will clamp the pad 10 voltage to VDD. 
     In a 5 volt environment, the SIGENV and ISIGENV signals will be at a logic &#34;0&#34; level. This will cause transistor 150 to turn off, but the output of the inverter 154 will be set at 3.3 volts (logic 1). When the pad 10 voltage is at 0 volts, the transistor 160 is turned off and the second pull down transistor 158 is on since the gate is connected to the output of the inverter 154. The second isolation transistor 156 is also turned on, resulting in a path from the pad 10, through the two transistors 156, 158, to the gate of the transistor 122. This pulls down the gate of the transistor 122, causing it to turn on. Since transistor 124 is turned off, the output at the N-well voltage node 130 will, once again, be VDD=3.3 volts, just as in the known control circuit design. 
     When the pad 10 voltage is at 5 volts, the second output stage transistor 124 turns on when the pad 10 voltage is at VDD+VTP, so that the N-well voltage follows the pad voltage. When the pad voltage is at or is rising to the 5 volt level, it is necessary to ensure that there is no contention between the pad 10 voltage and VDD through the transistor 122, 124 path. Said another way, it is necessary to ensure that the transistor 122 is turned off before the transistor 124 is turned on when the pad voltage is rising to 5 volts. The presence of the additional p-type transistor 160 ensures that the voltage at the gate of the first output stage transistor 122 follows the pad 10 voltage when it exceeds VDD+VTP, while the use of a high threshold (relative to the threshold of the transistor 124) p-type transistor for the transistor 122 ensures that the transistor 122 will be turned off before the transistor 124 is turned on as the pad 10 voltage is rising to 5 volts. This prevents any current draw from the pad 10 by the supply from occurring during the 0 to 5 volt transition of the pad 10 voltage. (Under this condition, transistors 150 and 158 are off.) Without the additional p-type transistor 160, the gate voltage of the transistor 122 would be at pad voltage--VTN (through the 5 volt pull down transistor 158 and second isolation transistor 156 path), which would result in current draw. 
     A second embodiment of the control circuit 216 is illustrated in FIG. 5. The control circuit 216 of FIG. 5 is similar in form and construction to the control circuit 116 of FIG. 4, except that the first output stage transistor 222 has a threshold approximately equal to the thresholds of the other p-type transistors of the control circuit 216 and the additional buffer circuitry 18 rather than the high threshold (relative to the other p-type transistors of the control circuit 216 and the additional buffer circuitry 18) of p-type transistor 122 of FIG. 4. In order to ensure that the transistor 222 is turned off before the transistor 124 is turned on when the pad voltage is rising to 5 volts, a parallel transistor circuit 268 is connected to the gate of the additional p-type transistor 160 to ensure that the voltage at the gate of the additional transistor 160 is lower than voltage at the gate of the second output stage transistor 124 (which is at VDD). The parallel transistor circuit 268 sets the gate voltage of the additional transistor 160 to VDD-VTN when the ISIGENV signal is set to logic 0 (0 volts) corresponding to the 5 volt mode, since the gate of the first bias transistor 264 is controlled by the output of the inverter 154 (whose input is ISIGENV), which will be at logic 1 when in a 5 volt signaling environment. The second bias transistor 262 ensures that the additional transistor 160 is turned off in 3.3 volt mode by setting the gate voltage of transistor 160 to VDD. 
     FIG. 6 illustrates a computer system 300 with a plurality of electronic circuits that interface with a PCI bus 308, each of which includes the buffer 114 of FIG. 3. The circuits include a processor circuit 302 and two peripheral circuits 304, 306. Each of the circuits is connected to a bus 308 through a buffer 114. It should be noted that it is not necessary that all circuits connected to the bus 308 include the 5 volt tolerant, 3 volt buffer 114 according to the present invention as is shown in FIG. 6. For example, some circuits may be implemented with 5 volt technology and be configured to produce 5 volt signals. The buffer 114 according to the present invention allows for such differences. 
     The above description and accompanying drawings are only illustrative of preferred embodiments of the invention. It is not intended that the invention be limited to the embodiments shown and described herein as many changes and modifications can be made without departing from the spirit or scope of the invention. Accordingly, the invention is not limited by the foregoing description but is only limited by the spirit and scope of the following claims.