BACKGROUND OF INVENTION 
     This invention relates to semiconductor bus switches, and more particularly to bi-directional undershoot protection for a MOS bus switch. 
     Bus switches are often used in networking applications. Bus switches using metal-oxide-semiconductor (MOS) technology have low on resistance, reducing delay through the switch. The source and drain nodes of a bus-switch transistor connect to the busses while the gate is controlled by a bus-connecting enable signal. See for example Parallel Micro-Relay Bus Switch for Computer Network Communication with Reduced Crosstalk and Low On-Resistance using Charge Pumps, U.S. Pat. No. 5,808,502, and Bus Switch Having Both P- and N-Channel Transistors for Constant Impedance Using Isolation Circuit for Live-Insertion when Powered Down U.S. Pat. No. 6,034,553. 
     More complex networks are emerging. For example, the bus switch may connect two processor buses. Each processor bus can operate independently of the other. Hot-plugging or hot-swapping of card with the processor bus can also occur. When the bus switch is in the isolation mode, full isolation must occur, regardless of which bus is active. 
     FIG. 1 shows a typical application of a bus switch. First local bus signals  18  (bus A) is connected to CPU_A  10 , memory_A  14 , and Application-Specific Integrated Circuit (ASIC_A)  12 . Second local bus signals  19  (bus B) is a second local bus that has CPU_B  11 , memory_B  15 , and Application-Specific Integrated Circuit (ASIC_B)  13 . Second local bus signals  19  is a hot-plugable bus. Switch network  16  connects address, data, and control lines from bus signals  18  to bus signals  19  using MOS transistors. One transistor is used for each bus signal. 
     When a device is plugged into bus signals  19 , it may be desired to isolate bus signals  19  from local bus signals  18 . Noise caused by the plugging operation can then be isolated to bus signals  19 , allowing local bus signals  18  to operate unhindered. Switch network  16  can isolate bus signals  19  from local bus signals  18  by applying a low voltage to n-channel transistors in switch network  16 . When switch network  16  isolates, Bus_A can operate independently of Bus_B. 
     Either Bus_A or Bus_B may be hot-plugged into the other bus. This allows for repair of systems without any downtime. Isolation by switch network  16  must therefore be fully bi-directional since it is not known which bus will be replaced until a failure occurs. 
     Undershoot Problem 
     When an n-channel transistor is used as the bus switch, the bus switch is disabled by driving a ground voltage to the gate of the n-channel bus-switch transistor. The output bus signal should be isolated from voltage changes at the input bus signal. The quality of the signal waveforms on local bus signal  18  is not always well controlled. Sometime large voltage spikes below ground (undershoots) occur, especially on the high-to-low transitions from high-current drivers on local bus signal  18 . The same could occur on bus signals  19 . 
     When the bus-switch input from bus signal  18  goes below ground, a positive gate-to-source voltage develops on bus-switch transistor since its gate is at ground. A conducting channel forms below the gate. When the undershoot is greater than a volt, this gate-to-source voltage exceeds the n-channel threshold voltage, turning on the n-channel bus switch transistor. Some current is conducted through the channel of the bus-switch transistor even though its gate may be kept at ground. The result is that the voltage is disturbed on the drain of the bus-switch transistor, and the output to bus signals  19 . 
     When the source of the n-channel bus-switch transistor goes negative during the undershoot, the base-emitter junction of the parasitic lateral NPN transistor is forward biased, coupling more current to the output through the p-type substrate. 
     The result of the undershoot is that the output connects to the input for a short period of time, the duration of the undershoot. The voltage on the drain of the bus-switch transistor can quickly fall from the power supply (Vcc) to ground and even below ground should the undershoot last for more than a few nanoseconds. The undershoots on the input bus coupled to the output, producing severe voltage disturbances on the isolated bus. 
     The inventor has solved an undershoot-isolation problem in earlier patents, such as U.S. Pat. No. 6,052,019 for Undershcot-lsolating MOS Bus Switch. However, this patent shows a circuit that is effective when the undershoot always occurs on only one side of the bus switch. Another improved circuit using a pulse generator was shown by the inventor in “Bi-Directional Undershoot-Isolating Bus Switch with Directional Control”, U.S. Ser. No. 09/607,460, filed Jun. 29, 2000. While useful, a fully bi-directional undershoot-isolating bus switch without the pulse generator is desired. 
    
    
     BRIEF DESCRIPTION OF DRAWINGS 
     FIG. 1 shows a typical application of a bus switch. 
     FIG. 2 is a block diagram of a bus switch that is bidirectionally undershoot protected. 
     FIG. 3 is a schematic showing the switch control and protection circuit. 
     FIG. 4 is a schematic of the undershoot sensor. 
     FIGS. 5A-C are waveforms showing operation of the bi-directional undershoot-isolating bus switch in isolation mode. 
    
    
     DETAILED DESCRIPTION 
     The present invention relates to an improvement in bidirectional bus switches. The following description is presented to enable one of ordinary skill in the art to make and use the invention as provided in the context of a particular application and its requirements. Various modifications to the preferred embodiment will be apparent to those with skill in the art, and the general principles defined herein may be applied to other embodiments. Therefore, the present invention is not intended to be limited to the particular embodiments shown and described, but is to be accorded the widest scope consistent with the principles and novel features herein disclosed. 
     FIG. 2 is a block diagram of a bus switch that is bidirectionally undershoot protected. Bus-switch transistor  20  connects the left-port input/output I/O (L_PORT) to the right-port I/O (R_PORT) when enable signal ENA is high. When enable ENA is low, bus-switch transistor  20  isolates the left port from the right port. This isolation is not merely limited to when signals on the right and left ports are within the normal power-to-ground operating range, but also includes isolation from undershoots below ground. 
     Undershoot sensor  22  is coupled to left-port I/O L_PORT and activates signal ENL 2 R when a low-going transition of L_PORT is detected. When enable signal ENA is active, undershoot sensor  22  is disabled. For the right port, undershoot sensor  22 ′ senses R_PORT and activates signal ENR 2 L when a low-going transition is detected. Undershoots occur at the end of low-going transitions. Both signals ENL 2 R and ENR 2 L are active-low signals that are driven high when bus-switch transistor  20  is enabled by ENA being high. 
     Switch control  30  contains protection circuitry that connects any undershoot on L_PORT to the gate of bus-switch transistor  20 , when signal ENL 2 R is active (low). This prevents bus-switch transistor  20  from conducting. Likewise, when signal ENR 2 L is driven low by undershoot sensor  22 ′, R_PORT is connected to the gate of bus-switch transistor  20 , so that its gate-to-source voltage remains at zero, even when the source (R_PORT) goes below ground during an undershoot. 
     Switch control  30  also receives the inverse enable signal ENB from inverter  28 . When enable is high, and ENB is low, a high voltage is driven onto the gate of bus-switch transistor  20  to enable conduction, and undershoot protection is disabled. 
     To prevent the parasitic source-substrate diode from conducting, the substrate under bus-switch transistor  20  is driven below ground by a negative substrate bias. Substrate bias generator  26  applies a negative bias to the p-type substrates n-channel bus-switch transistor  20 . A bias of −1.5 to −3.0 volt can be generated using a charge-pump type of bias generator. The same substrate bias can be applied to all n-channel transistors, such as those in switch control  30 , undershoot sensors  22 ,  22 ′, and inverter  28 . When a P-well process is used, some of the n-channel transistors can have a different bias, such as ground since the n-channel transistors can be formed in separate wells that are electrically isolated from one another. 
     FIG. 3 is a schematic showing the switch control and protection circuit. Switch control  30  receives the inverse enable signal ENB and the active-low undershoot sense signals ENL 2 R and ENR 2 L. Switch control  30  drives the gate node (NGATE) of bus-switch transistor  20 . 
     When bus-switch transistor  20  is enabled to conduct, ENB is low and ENL 2 R and ENR 2 L are both high. Since ENL 2 R is high, node N 6  is also high, enabling n-channel transistor  64 . Likewise, since ENR 2 L is high, node N 7  is high, enabling n-channel transistor  66 . The low ENB signal is applied to the gate of p-channel transistor  62 , which turns on, and to the gate of n-channel transistor  68 , which turns off. Gate node NGATE is thus driven high when ENB is low. The high voltage to NGATE turns on bus-switch transistor  20  so that a conducting channel is formed between its source and drain, connecting L_PORT to R_PORT. 
     P-channel transistors  42 ,  52  remain off, since ENL 2 R and ENR 2 L are high. Grounded-gate n-channel transistors  44 ,  54  remain off, isolating gate node NGATE from L_PORT and R_PORT. 
     When bus-switch transistor  20  is to be disabled by ENB going high, and no undershoot is detected, ENL 2 R and ENR 2 L remain high, and n-channel transistors  64 ,  66  remain on. Then the high ENB turns on n-channel transistor  68  but turns off p-channel transistor  62 . Gate node NGATE is then discharged to ground, turning off bus-switch transistor  20 . 
     When an undershoot or low-going transition is detected on the left port, ENL 2 R is driven low. Transistors  32 ,  34  invert ENL 2 R to drive node N 3 , which is applied to the gates of transistors  36 ,  38 . N-channel transistor  38  drives node N 6  low by connecting L_PORT to node N 6 . L_PORT is already low since a low-going transition was detected for ENL 2 R to be driven low. If L_PORT is below ground (an undershoot), n-channel transistor  38  couples this below-ground voltage to the gate of n-channel transistor  64 , ensuring that it remains off even if gate node NGATE goes below ground. If the source of n-channel transistor  38  were merely connected to ground, then when NGATE goes below ground a positive gate-to-source voltage would occur on n-channel transistor  64 , causing unwanted conduction. 
     When ENL 2 R is driven low by a detected undershoot on L_PORT, p-channel transistor  52  turns on since its gate is ENL 2 R. Node N 5  is driven high, which is also the gate of n-channel connecting transistor  46 , which turns on. Thus node N 4 , the source of n-channel connecting transistor  46 , is connected to gate node NGATE. As L_PORT falls below ground during an undershoot, n-channel grounded-gate transistor  44  develops a positive gate-to-source voltage (ground minus L_PORT&#39;s voltage) and begins conducting once the under shoot on L_PORT is more than the transistor threshold voltage below ground. Since n-channel transistors  44 ,  46  are both on, the undershoot on L_PORT is couples to gate node NGATE, which is driven below ground. Furthermore the gate of transistor  56  is connected to N 4 , preventing undershoots from coupling to R_PORT through connecting transistors  56 ,  54 . Isolation of R_PORT is maintained despite the undershoot on L_PORT since the gates of bus-switch transistor  20  and connecting transistor  56  are also driven by the undershoot, keeping their gate-to-source voltages near zero. 
     When the undershoot is detected on R_PORT, ENR 2 L is driven low by the undershoot sensor. Transistors  72 ,  74  invert ENR 2 L and drive the gates of transistors  76 ,  78 . N-channel transistor  78  has its source connected to R_PORT rather than ground to couple the below-ground undershoot to the gate of n-channel transistor  66 , (node N 7 ) ensuring that it remains off. Since transistor  66  is off, gate node NGATE is disconnected from ground even when transistors  64 ,  68  remain on. 
     The low on ENR 2 L is also applied to the gate of p-channel transistor  42 , turning it on. Node N 4  is then driven high by p-channel transistor  42 . Grounded-gate transistor  44  remains off, since no undershoot is detected on L_PORT. 
     Node N 4  is also the gate of n-channel connecting transistor  56 , which turns on and connects gate node NGATE to node N 5 . Grounded-gate transistor  54  is off until R_PORT falls below ground, then it turns on, conducting the undershoot on R_PORT to node N 5 , and then through connecting transistor  56  to gate node NGATE. The gate of transistor  46  is also connected to node N 5 . Since N 5  is also driven by the undershoot, it isolates undershoots from R_PORT to L_PORT through connecting transistors  46  and  44 . Thus the undershoot on R_PORT is conducted to the gate of bus-switch transistor  20  and the gate of connecting transistor  46 , ensuring that it remains off. Isolation is maintained despite undershoots occurring on either port. 
     FIG. 4 is a schematic of the undershoot sensor. Undershoot sensor  22  receives the L_PORT signal and drives ENL 2 R low when a high-to-low transition is detected on L_PORT, when an undershoot could occur. 
     L_PORT is inverted by transistors  82 ,  84  to drive node N 1 . A coupling capacitor is formed by transistor  88 , which has node N 1  applied to its gate and has its source and drain connected together at node N 2 . Pass gate  90  is an n-channel transistor that connects L_PORT to ENL 2 R when its gate, node N 2 , is high, but isolates ENL 2 R when N 2  is low. Node N 2  also drives the gate of p-channel pullup transistor  96 , which drives ENL 2 R high when node N 2  is low. Thus either pass gate  90  or pullup transistor  96  is on to drive ENL 2 R. 
     Signal ENL 2 R is kept high when the enable signal ENA is high since ENA is applied to the gate of disable n-channel transistor  92 , which holds node N 2  to ground when the bus-switch transistor is enabled, since no undershoot protection is then needed. Pullup transistor  96  is kept on by node N 2 , pulling signal ENL 2 R high. 
     When L_PORT has a high-to-low transition, node N 1  is driven high by p-channel transistor  82 . P-channel transistor  82  can be made relatively large so that a fast rise time is obtained for node N 1 , even when a large coupling capacitor is used as transistor  88 . This rapid voltage rise on node N 1  is capacitively coupled through coupling capacitor transistor  88 , causing a rise in the voltage of node N 2 . The size of the voltage rise on node N 2  is somewhat smaller than the voltage rise on node N 1  in proportion to the coupling ratio, which is the capacitance of transistor  88  divided by the total capacitances on node N 2 . 
     The rise in voltage coupled to node N 2  turns on pass gate  90 , causing the low on L_PORT to drive ENL 2 R low. The undershoot may be partially dissipated or absorbed by the input capacitances of signal ENL 2 R, helping to minimize the undershoot. Node N 2  is then gradually pulled back to ground by n-channel keeper transistor  94 . Once node N 2  falls near ground, p-channel pullup transistor  96  turns on and pass gate  90  turns off, and ENL 2 R is driven high again. 
     FIGS. 5A-C are waveforms showing operation of the bi-directional undershoot-isolating bus switch in isolation mode. In FIG. 5A, enable ENA is low, putting the bus-switch transistor is in isolation mode in which it&#39;s source and drain are disconnected from each other. The right port R_PORT remains high and must be isolated from the undershoot that occurs on the left port L_PORT. The left port transitions from high to low and back high in the simulated waveform. An undershoot is simulated since L_PORT falls below ground to −2 volts. In an actual undershoot, L_PORT would not remain below ground for such a long time period, but ringing or other oscillation could occur. 
     FIG. 5B shows operation of the undershoot detector. When L_PORT switches from high to low, node N 1  is rapidly driven high. This voltage change is capacitively coupled to node N 2 , which also goes high, but to a reduced high voltage of about 2-3 volts. Once the low-going transition ends, the keeper transistor gradually pulls node N 2  back toward ground. Signal ENL 2 R is driven low once node N 2  rises above the n-channel transistor threshold. Signal ENL 2 R stays low as long as L_PORT remains below ground, thus isolation is effective throughout the entire duration of the undershoot and it is independent of the width of the undershoot pulse. 
     Once L_PORT transitions high again, node N 1  is driven low, and the voltage drop is capacitively coupled into node N 2 , which can be driven below ground for a short time. Signal ENL 2 R is driven back high as node N 2  goes low. 
     FIG. 5C shows the operation of the switch control logic. The low-going ENL 2 R drives node N 3  high and node N 6  low. Node N 7  remains high since ENR 2 L does not change. Node N 5  is driven high by ENL 2 R. The high voltage on N 5  turns on the connecting transistor, which connect gate node NGATE with node N 4 . The grounded-gate transistor turns on as its source, L_PORT, falls below ground, coupling the undershoot on L_PORT to its drain, node N 4 . Node N 4  and gate node NGATE then fall with L_PORT below ground. 
     Gate node NGATE, node N 4  and node N 6  all fall below ground during the undershoot on L_PORT. When L_PORT is −2 volts, typical node voltages are N 1 =5v, N 2 =2.5v, N 3 =5v, N 4 =−2v, NS=5v, and N 6 =−2v 
     ALTERNATE EMBODIMENTS 
     Several other embodiments are contemplated by the inventor. The undershoot-protection circuit can also used in more complex switch networks. 
     The invention can be reversed for use with p-channel bus-switch transistors. Overshoot as well as undershoot protection could be provided. The invention can be applied to non-standard processes such as silicon-on-insulator (SOI). A p-channel transistor can be added in parallel to the n-channel bus-switch transistor to create a full-CMOS bus switch. The protection circuit for the n-channel bus-switch transistor is still effective. Other transistors, resistors, or capacitors may be added in parallel or in series in several locations the circuits. 
     A pullup p-channel transistor can be added to either bus, as can a pullup resistor. A pullup resistor can also be added in series with a p-channel pullup transistor. The terms source and drain can be considered interchangeable, depending on the current voltages applied. Likewise, the input and output of the bus switch can be reversed or interchanged for Li-directional bus switches. 
     A single integrated circuit chip can contain several bus-switch transistors in parallel, each with an undershoot-protection circuit. Enable signals may drive all bus-switch transistors. 
     The abstract of the disclosure is provided to comply with the rules requiring an abstract, which will allow a searcher to quickly ascertain the subject matter of the technical disclosure of any patent issued from this disclosure. It is submitted with the understanding that it will not be used to interpret or limit the scope or meaning of the claims. 37 C.F.R. §1.72(b). Any advantages and benefits described may not apply to all embodiments of the invention. When the word “means” is recited in a claim element, Applicant intends for the claim element to fall under 35 USC §112, paragraph 6. Often a label of one or more words precedes the word “means”. The word or words preceding the word “means” is a label intended to ease referencing of claims elements and is not intended to convey a structural limitation. Such means-plus-function claims are intended to cover not only the structures described herein for performing the function and their structural equivalents, but also equivalent structures. For example, although a nail and a screw have different structures, they are equivalent structures since they both perform the function of fastening. Claims that do not use the word means are not intended to fall under 35 USC §112, paragraph 6. Signals are typically electronic signals, but may be optical signals such as can be carried over a fiber optic line. 
     The foregoing description of the embodiments of the invention has been presented for the purposes of illustration and description. It is not intended to be exhaustive or to limit the invention to the precise form disclosed. Many modifications and variations are possible in light of the above teaching. It is intended that the scope of the invention be limited not by this detailed description, but rather by the claims appended hereto.
Both buses connected to a bus switch are protected from undershoots. A bus switch transistor is an n-channel metal-oxide-semiconductor (MOS) with its source connected to a first bus and its drain connected to a second bus. An enable gate drives the gate node of the bus switch transistor high to enable or low to disable. Undershoot sensing circuits are attached to the first and second bus. When a low-going transition is detected by an undershoot sensing circuit, an n-channel connecting transistor is turned on, connecting the bus with the low-going transition to the gate node through a grounded-gate n-channel transistor. If an undershoot occurs, it is coupled to the gate node. Since both the gate and source of the bus switch transistor are coupled to the undershoot, the gate-to-source voltage never reaches the transistor threshold and the bus switch transistor remains off.