High speed CMOS bus driver circuits

Abstract: A high speed bus driver circuit is disclosed that minimizes output signal oscillation by maintaining the clamp voltage at specified levels. The driver circuit includes a pair of voltage preference circuits (102,104) that are designed to deliver the appropriate clamp voltages even at the best case speed corners, the circuit prevents simultaneous turn on of drive transistors (MP1,MN1), equalizes the propagation delay and provides for first access for tri-stating the bus driver transistors.

Description: FIELD OF THE INVENTION This invention relates to a driver circuit utilized to provide a voltage level to a data bus and more particularly to an improved bus driver circuit which provides better immunity to the power supply and semiconductor processing changes. BACKGROUND OF THE INVENTION High speed, high current bus driver circuits are utilized to rapidly charge or discharge a data bus to a high or low voltage level. Typically when this rapid charging and discharging occurs the resulting effect can often be a damped oscillation or ringing of current flowing through the driver. This ringing effect is undesirable because it can cause false information or false data to be provided to the bus. The network formed by the bus driver, its interconnects and the load on the data bus can be represented by an RLC equivalent circuit. In this type of equivalent circuit the inductance, L, is due to the inherent parasitic self inductance of the leads connected to the driver circuit, the capacitance C is due to the capacitive load of the devices connected to the bus, and the resistance R is the resistance of the charging or discharging path. The resistance R acts as a dissipating element in order that the oscillation of the circuit decays. The ratio of the resistance R and the critical resistance factor 2(L/C) ½ determines the amount of damping the circuit will experience while dissipating its power. This damped ringing can cause unintentional triggering of the devices connected to the data bus if the voltage amplitude of an oscillation cycle exceeds the logical threshold device of the device. Since this damped oscillation will also be present on the power supply lines of the bus driver circuit, other circuits such as the input buffer circuit sharing the same power line will likely experience voltage failures due to the distortion of the power supply voltage. Finally, a noisy power supply is likely to cause ringing on the data which has been driven to that power rail by other bus drivers sharing the power line. This ringing is likely to happen regardless of the amount of self inductance L that the other bus drivers are on their leads. The voltage amplitude of this damped ringing is a function of the rate of change of the current (di/dt). Since the values of the parasitic physical elements L&C of the driver network are constant, their effect on the current cannot be altered. However, one possible way of controlling the rate of change of current would be to control the flow of current through the driver transistors that form a portion of the bus driver circuit. The problem with this solution is there is a trade off between propagation delay which slows the bus driver circuit down and the clamping voltage or the amplitude of the signals that are provided to the gates of the transistors. A circuit addressing this problem is described in our U.S. Patent 4,877,980. In this circuit a bias voltage (V bias ) is applied to one gate of a p channel and n channel transistor pair, and the bias voltage is chosen to prevent ringing on the bus. Although the circuit described in this patent works satisfactorily for its intended purposes, it does not disclose a means for providing the V bias voltage. In Figs 1-3 of the accompanying drawings there is disclosed a high speed bus driver comprising: first and second transistors for providing drive potential to a bus; the first transistor providing a pull-up drive potential to the bus when the first transistor is active and the second transistor providing a pull-down drive to the bus when the second transistor is active; first and second reference voltage generator circuits for providing first and second clamp voltages to the first and second transistors, one of the reference voltage generator circuits employing primarily p-mos transistors; the other of the reference voltage generator circuits employing primarily n-mos transistors, each of the reference voltage generators including a plurality of transistors connected in series such that each gate of the transistors in each plurality is at a voltage threshold level higher than its source voltage, one plurality of transistors providing a clamping voltage to one of the first and second transistors that is a multiple of the voltage threshold of said one of the first and second transistors above a ground potential, the other plurality of transistors providing a clamping voltage to the other of the first and second transistors that is a multiple of the voltage threshold of the other of the first and second transistors below a power supply voltage. A similar circuit is described in JP-A-03259616. According to the present invention such a circuit is characterised in that each of the first and second reference voltage generators is coupled in a current mirror configuration for cutting off the dc current drawn by the reference generator when its associated one of said first and second transistors is inactive. A high speed bus driver circuit is therefore provided that has significant advantages over previously known circuits. In addition, the ringing associated with such circuits is effectively minimized. The bus driver through the use of a pair of reference voltage generators and maintaining the clamping voltage at certain threshold levels effectively minimizes ringing and therefore prevents the unintentional triggering of devices that are located on a data bus. In addition, it also reduces the propagation delay, thereby allowing the bus driver to operate at optimum speeds with minimum delays. In the accompanying drawings, by way of example only: Figure 1 shows in block diagram form a bus driver circuit; Figure 2 shows a diagram that illustrates the effect of ringing or signal oscillation at different clamp voltage levels. Figure 3(a) shows a n-type reference generator circuit for use in the circuit of Fig. 1; Figure 3(b) shows a p-type reference generator circuit for use in the circuit of Fig. 1; Figure 4(a) shows an improved n-type reference voltage generator circuit for use in the circuit of Fig. 1 in accordance with the present invention; Figure 4(b) shows an improved p-type reference voltage generator circuit for use in the circuit of Fig. 1 in accordance with the present invention. A block diagram representative of bus driver circuit 100 is shown in Figure 1. The bus driver circuit 100 of Figure 1 includes an enable input 140 and a data input 142. The inputs of inverters 122 and 124 are coupled to enable input 140 and data input 142 respectively. The output of inverter 122 is coupled to input of a two input NOR gate 126. The output of inverter 124 is coupled to a second input of the NOR gate 126. The output of inverter 124 is also coupled to an input of two input NAND gate 128. The enable input 140 is coupled to the second input of NAND gate 128. The outputs of NAND gate 128 is coupled to the input of inverter 132 and the gate of n-mos transistor 138. The output of inverter 132 is coupled to an input of voltage reference generator 138. The output voltage reference generator 104 is coupled to the drain of n-mos transistor 138. The source of transistor 138 is coupled to ground. The drain of n-mos transistor 138 is coupled to the gate of n-mos transistor 114. The drain of transistor 114 is coupled to the output 120 of the driver circuit. The output of NOR gate 126 is coupled to the input of an inverter 130 and the gate of p-mos transistor 136. The output of the inverter 130 is coupled to an input of voltage reference generator. The output voltage reference generator 102 is coupled to the drain of transistor 136. The source of transistor 136 is coupled to vcc. The drain of transistor 136 is coupled to the gate of p-mos transistor 112. The drain of transistor 112 is coupled to the output 120 of the driver circuit 100. The source of transistor 112 is coupled to vcc. The key elements of the bus driver circuit 100 are the two reference voltage generators 102 and 104. Voltage reference generator 102 for p channel circuits and voltage reference generator 104 is for n channel circuit. These voltage reference generators 102 and 104 perform two functions. First, they provide a voltage clamp on the nodes up1 106 and dn1 108 which drive the gates of the p-mos driver transistor 112 and n-mos driver transistor 114 respectively. Secondly, these voltage reference generators 102 and 104 also provide some propagation delay for signals 116 and 118 when they drive signals up1 106 and dn1 108 respectively. The propagation delay is present only during the period when the transistors mp1 112 or mn1 114 is being turned on. The purpose of the propagation delay is to prevent the transistors mp1 112 and mn1 114 from turning on simultaneously when the output 120 of the bus driver circuit 100 switches from one level to another. The clamp voltage at nodes up1 106 and dn1 108 have been chosen, as described below, to be approximately vcc minus 3 times the threshold voltage value (V T ) of the p channel device and vss plus 3 times the threshold value (V T ) of the n channel respectively. What is meant by threshold voltage value (V T ) in the context of this patent application is the voltage required to cause the transistor to turn on. It is known that by selecting a clamp voltage which is between 2 times and 4 times the threshold value of the transistor both the amplitude of the output oscillation and the total propagation delay through the bus driver circuit are minimized. It is also known that in low voltage applications (3-5 volts) by setting the clamp voltage value at 2-4 times the threshold value of the transistor the tradeoff between propagation delay and ringing can be optimized. Referring now to Figure 2(A) through 2(D) what is shown is a diagram which is illustrative of the voltage and current transient analysis for a driver circuit in which the clamp voltage is 2V T , 3V T and 4V T respectively. The following discussion would be relative to an n-type voltage generator but it will be well recognized by one of ordinary skill in the art that the generator can be a p-type generator and the diagrams will be complementary in nature. For review purposes, the curves marked 600a through 600c are the data into circuit 100 for 2V T voltage generator, 3V T voltage generator, and 4V T voltage generator respectively. The curves 602a through 602c are the output from the circuits from the same respective clamp voltages. The curves 604a through 604c are the voltages at node dn1 of the circuit 104 at the same clamp voltages as mentioned above. Finally, the curves 606a through 606c are the ringing voltages that are present on the vss line at the above mentioned clamp voltages. The curves 608a through 608c are the ringing current produced at the vss voltage level at the respective clamp voltages mentioned above. What is seen as the response time as indicated by the output curves 600a through 600c that the response time for the 4V T voltage generator is much faster than that for the 2V T voltage generator. However, it is also seen that the voltage ringing represented by vss 606b is much higher for the 4V T voltage generator than for the 2V T voltage generator (606A). It is also very clear that the current ringing (608C) using a 4V T reference voltage generator is much higher than the 2V T voltage generator (608A). Hence, as is seen, the 3V T voltage generator response time (602B) is much better than the 2V T voltage generator (602A) and at the same time the ringing (606B) is somewhat less than that shown at the 4V T voltage generator. Therefore, generally the best comprise is to provide the 3V T voltage generator. However, if there is no concern about ringing, the 4V T voltage generator can be utilized. Conversely, if a drive circuit is desired that is relatively immune to signal oscillation, a 2V T reference generator would be utilized. Therefore, for example, a lower clamp voltage at node dn1 108 will reduce the oscillation amplitude, but the delay through the bus driver circuit 100 will be longer. Similarly, a higher value of clamp voltage on that node up1 106 will reduce the delay through the bus driver circuit 100, but it will increase the amplitude of the oscillation. The slew rate of dn1 108 is the inverter 132 and pass transistor mnd2 (212 or 412) (Figure 3a and Figure 4a) are sized to provide the desired slew rate on node dn1. The inverter 130 and pass transistor (312 and 512) are sized to provide the desired slew rate on node up1. The basic schematics of the ref_n and ref_p generator circuits 104 and 102 are shown in Figure 3(a) and Figure 3(b) respectively. Voltage reference generator circuit 104 mostly employ n-type transistors. The voltage reference generator circuit 104 includes the nnn1 input 118 coupled to the drain of n-mos transistor mnd2 212. The source of transistor 212 is coupled to the dn1 108 signal. The gate of transistor 212 is coupled to the gate of mnd1 n-mos transistor 204 and the drain of mpd1 p-mos transistor 202. The source of transistor 202 is coupled to vcc and the gate of transistor 202 is coupled to vss. The gate of transistor 204 is coupled to its drain and the source of transistor 204 is coupled to the drain of mnd3 n-mos transistor 206. The drain of transistor 206 is coupled to its gate and the source of transistor 206 is coupled to the drain of mnd5, n-mos transistor 208. The source of transistor 208 is coupled so its gate and the drain of transistor 208 is coupled to the drain of nmd6 n-mos transistor 210. The gate of transistor 210 is coupled to its drain and the source of transistor 210 is coupled to ground. Voltage reference generator circuit 104 employs mostly p-type transistors. The voltage reference generator circuit 104 includes the npp1 input 116 coupled to the drain of mpp2 p-mos transistor 312 and the source of transistor 312 is coupled to the up1 signal 106. The gate of n-mos transistor 312 is coupled to the gate and drain of mpp1 p-mos transistor 308 and the source of transistor mnp1 n-mos transistor 310. The gate of transistor 310 is coupled to vcc. The source of p-mos transistor 308 is coupled to the drain of mpp3 p-mos transistor 306. The drain of transistor 306 is coupled to its gate and the source of transistor 306 is coupled to the drain of mpp5 p-mos transistor 304. The drain of transistor 304 is coupled to its gate and the source of transistor 304 is coupled to the source of mpp6 p-mos transistor 302. The source of transistor 302 is coupled to its gate and the drain is coupled to vcc. Reference voltage generator circuits 102 and 104 are in essence a pair of voltage divider circuits designed to deliver clamp voltages of approximately 3 volts above the vss (ground) level at node dn1 and approximately 3 volts below vcc (power supply) level at node up1. The load transistors mpd1 202 of the reference voltage generator 102 and mnp1 302 of the reference voltage generator 104 are weak devices. In Figure 3(a) the sizes of the saturated series driver transistors, 204, 206, 208, and 210 are ratioed such that nodes nnn2, nnn3, nnn4 and nnn5 are each at approximately a V T -n level higher than its source voltage. Thus the resultant clamp voltage obtained at the output of this reference circuit (node dn1) is about 3 V T -n above ground. By applying the same design technique, the clamp voltage obtained at node up1 of the circuit of Figure 3(b) is about 3 V T -p below vcc. The schematics in Figure 3(a) and Figure 3(b) can be modified to produce a clamp voltage which is 2 or 4 times that of the threshold. If one of the saturated driver transistors is replaced by a shorting net, the clamp voltage value can be made as 2 times the value of the threshold which is approximately 2 volts. Similarly, by adding an additional saturated driver transistor into the chain of driver transistors, the level of the clamp voltage can be raised to 4 times of the threshold value. The size of the transistor 212 (Figure 3(a)) and the inverter 132 driving the node nnn1 118 (Figure 1) determines the slew rate of node dn1 108. Similarly, the size of the transistor 312 (Figure 3(b)) and the inverter 130 driving the node npp1 116 (Figure 1) determines the slew rate of node up1. Since the value of the transistor threshold decreases at the best case speed process corner, the advantage of these reference generator circuits is that the level of the clamp voltage also decreases at best case speed corner. As a result, the lower clamp voltage helps in controlling the current flow through the bus driver transistors mn1 114 and mp1 112 at the best case process corner. Consequently, the output oscillation is minimized even at the best case process speed corner. This provides for a more stable circuit performance. The bus driver circuit 100 in Figure 1 provides the same amount of propagation delay for both the "data to out" and "enable to out" access paths. It also provides fast access for tri-stating the bus driver transistors mn1 112 and mp1 114 when signal "enable" is pulled low. As mentioned above, the bus driver circuit 100 prevents simultaneous turn on of the large bus driver transistors 114 and 112. In Figure 4(a) and Figure 4(b), two reference voltage generator circuits 400 and 500 are shown which provide some improvements over the circuits given in Figure 3(a) and Figure 3(b). The reference generator 400 of Figure 4(a) is substantially the same as shown is Figure 3(a) except a saturated n-mos mmd4 transistor. 414 has been added to the ref_n circuit 200 of Figure 3(a) converting that into circuit 400, a current mirror configuration. The circuit 400 has been modified such that the gate of transistor 406 has been disconnected from its drain and has been connected directly to the gate of transistor 414. The gate and drain of transistor 414 are connected to dn1 108. The source of transistor 414 is coupled to the source of transistor 408. Similarly, in Figure 4(b) a saturated p-mos mpp4 514 transistor is added to the ref_p circuit of Figure 3(b) and the circuit 500 now resembles a current mirror circuit, as well. This new configuration of the reference voltage generators 400 and 500 helps in two ways. First, it provides large current gain in driving the loads of the large bus driver transistors 114 and 112 (Figure 1), when node nnn1 begins to rise and node npp1 begins to fall. Secondly, when node nnn1 is low, node dn1 is also pulled low. This action shuts transistor 406 off and the ref_n generator 400 does not draw any dc current. Similarly, a high level on node npp1 cuts off transistor 506 in ref_p generator and the generator 500 stops drawing any dc current, as well. Therefore, there is no dc current drawn by these reference generators 400 and 500 when they do not drive the bus driver transistors 112 and 114. However, a small amount of dc current is drawn when up1 106 and dn1 108 are driving the driver transistors mn1 112 and mp1 114. This current is limited by constant current saturated transistors 410 (Figure 4(a)) and 502 (Figure 4(b)), which are weakly turned on by having their gates at just about a threshold above their sources. Similar to the circuits given in Figure 3(a) and Figure 3(b), these reference voltage generators 400 and 500 are also designed to have the gates of the saturated driver transistors at a threshold above their source voltage values. Thus, the desired 3 V T reference voltage levels at nodes up1 106 and dn1 108 can be easily obtained. By removing transistors mnd6 410 (node nnn5 tied to vss) and mpp6 502 (node npp5 tied to vcc), both these generators can be made to produce a reference voltage of 2 V T . Similarly, it is recognized that an additional saturated n-mos transistor between mnd5 408 and mnd6 410 and a saturated p-mos transistor between transistors mpp5 504 and mpp6 502 will transform both these generators of Figure 4(a) and Figure 4(b) into 4 V T generators. As mentioned before, the level of the reference voltage provides the trade off between the driver delay and the oscillation. When transistor 406 of Figure 4(a) is cut off (dn1 108 pulled low by nnn1 118), the node nnn2 rises to vcc. The node nnn3 which is the source of the saturated n channel transistor 404, reaches a voltage of vcc minus V T -n. Under these conditions the generator does not draw any power. When node nnn1 starts rising, it pulls node dn1 108 by the source following action. As dn1 108 rises, transistor 406 turns on. This action starts pulling node nnn2 down until it reaches the 4 V TN level. The rising node dn1 108 and falling node nnn3, both become clamped at the 3 V T voltage levels. At this point all transistors except the p channel load transistor 402 are in saturation. The amount of power drawn through this load 402 can be very small, since a very weak transistor can be used here. The total dc current drawn by the generator circuits 400 is the sum of the current drawn through the transistors 402 and 412. It is, however, limited by the transistor 410 (constant current source) which is weakly turned on by having both its drain and gate at about 1 V T -n above the ground level. The schematic of ref_p voltage generator circuit 500 (Figure 3(b)) is complementary in nature to the ref_n generator circuit 400. Its transistor sizes have been ratioed to provide 4, 3, 2, and 1 V T -p below the level of vcc at nodes npp2, npp3, npp4 and npp5, respectively. It operates as follows. When node npp1 is high, up1 106 is pulled high which cuts off transistor 506. The node npp2 drops to vss (or ground) and drives the transistor 512 into its linear region. The node npp3 which is the source of the saturated p channel transistor 508, reaches a voltage of vss plus V T . At this time the generator 500 does not draw any power. When node npp1 starts falling, it pulls node up1 106 down by the source following action. As up1 106 falls, transistor 506 turns on and all the transistors in that current path are also on. This action starts pulling node npp2 up to the vcc minus 4 V T level. The falling node up1 106 and rising node npp3 become clamped to the vcc minus 3 V T voltage level. At this point all transistors except the n channel transistor 510 are in saturation. The amount of power drawn through this circuit can be very small, since a very weak n channel load transistor 510 has been used. The total current flowing through this reference circuit is limited by the constant current source 502 which is weakly turned on by having its drain and gate at about 1 V T -p level. One benefit of reference voltage generators 400 and 500 is that their clamp voltages are less susceptible to the power supply variations. Since the transistor thresholds decrease at the best case speed process corner, another advantage is that the voltage on node 108 in relationship to the ground decreases in value from its best case speed process corner value. Similarly, the voltage on node up1 106 in relationship to vcc decreases at best case speed corner as well. Having lower gate drive voltage on the driver transistors mp1 112 and mpl 114, the oscillation on the output node is minimized even at best case speed process corner. Although the present invention has been described in accordance with the embodiment shown in the Figures, one of ordinary skill in the art recognizes that there could be variations to the embodiment within the scope of the present invention as defined by the appended claims. Accordingly, many modifications may be made by one of ordinary skill without departing from the scope of the present invention, the scope of which is defined solely by the appended claims.

Claims: A high speed bus driver (100) comprising: first and second transistors (112, 114) for providing drive potential to a bus; the first transistor (112) providing a pull-up drive potential to the bus when the first transistor is active and the second transistor (114) providing a pull-down drive to the bus when the second transistor is active; first and second reference voltage generator circuits (400, 500) for providing first and second clamp voltages to the first and second transistors, one of the reference voltage generator circuits (500) employing primarily p-mos transistors; the other of the reference voltage generator circuits (400) employing primarily n-mos transistors, each of the reference voltage generators including a plurality of transistors connected in series such that each gate of the transistors in each plurality is at a voltage threshold level higher than its source voltage, one plurality of transistors providing a clamping voltage to one of the first and second transistors (112, 114) that is a multiple of the voltage threshold of said one of the first and second transistors above a ground potential, the other plurality of transistors providing a clamping voltage to the other of the first and second transistors (112, 114) that is a multiple of the voltage threshold of the other of the first and second transistors below a power supply voltage; whereby when one transistor of the first and second transistors (112, 114) is active the other transistor is purposely made inactive, characterised in that each of the first and second reference voltage generators (400, 500) is coupled in a current mirror configuration for cutting off the dc current drawn by the reference generator when its associated one of said first and second transistors is inactive. The high speed bus driver of claim 1 in which the first transistor (114) comprises an n-mos device and the second transistor (112) comprises a p-mos device. The high speed bus drive circuit of claim 2 in which the plurality of transistors of said one of the reference voltage generator circuits (400) provides the first clamp voltage which is two, three or four times the voltage threshold above the ground potential. The high speed bus driver circuit of claim 2 in which the plurality of transistors of the other of reference voltage generator circuits (500) provide the second clamp voltage which is two, three or four times the voltage threshold below the power supply voltage.

Inventors: MAHMOOD QAZI, MAHMOOD, QAZI
Applicants: ADVANCED MICRO DEVICES INC, ADVANCED MICRO DEVICES, INC.
Application year: 1993
Main classification: H03K19
IPC codes: H03K17, G06F3, H03K19