Integrated circuit having noise control means

Abstract: An apparatus for controlling ground bounce and transient noise induced in an integrated circuit includes feedback means (74,76) for feeding back to output stages (41', 47') of a buffer (10') input/output power supply bus potentials used for an output driver. Significant noise transients on the input/output upper potential bus (I/O VDD) bring about an increase in the resistance of a series element (76,72) in each output stage (41', 47') and thereby increase the overall switching time of each output stage (41', 47') and output driver combination. Similarly, significant ground bounce voltages on the input/output lower potential bus (I/O VSS) bring about an increase in the resistance of another series element (74,70) in each output stage (41', 47') and thereby increase the overall switching time of each output stage (41', 47') and output driver combination. Increasing the switching time reduces noise transients caused by switching currents. By only increasing the switching time when the noise transients reach a troublesome level, an overall higher data rate is achieved.

Description: The present invention relates to an integrated circuit, and more particularly to a means for controlling the effect of switching-induced noise occurring on power supply buses of the integrated circuit. In integrated circuit technology, the overall trend is towards smaller, faster, and lower power logic gates. This allows the gates and their leads to be physically reduced in size, which in turn allows more logic gates to be manufactured per square centimetre of semiconductor material. Unfortunately, with respect to output devices the overall trend cannot readily be followed. This is primarily because the relatively larger capacitances and inductances of the external conductors that connect the output devices to external logic inputs are not undergoing physical reduction to the same extent as the internal logic gates. The result of this fact is that a significant proportion of each integrated circuit is occupied by output pad drivers in order to drive the connecting leads and external circuits. A related result is the fact that a very significant proportion of the current passing through the integrated circuit passes through the output pad drivers as an output current which the integrated circuit is sourcing or sinking. Besides drawing a major portion of the current passing through an integrated circuit, many output pad drivers of many modern integrated circuits operate and draw their currents together. For example, sixteen output pad drivers may operate together to drive a sixteen bit wide address or data bus. Unfortunately, this means that groups of sixteen, thirty-two, or even sixty-four output pad drivers often are switching logic states at the same time. Such group switching may induce transient voltages on the integrated circuit's power bus conductors because of inherent current carrying limitations of physical size, parasitic inductances and resistances. Transient voltages especially may occur if a large majority of the group switch from one specific state, such as a logic HI (high), to the opposite state, that is logic LO (low). The parasitic inductances and resistances, especially at very high switching speeds, may make these switching transients worse because of their respective oppositions to changes in current. This opposition impedes the flow of current into the integrated circuit via the output pad driver power supply leads, resulting in a transient increase in voltage of the power supply's lower potential and a decrease in the voltage of the power supply's upper potential. The transient increase in the level of the lower potential of the power supply is commonly referred to as ground bounce, and the transient decrease in the upper potential of the power supply is commonly referred to as transient noise. Both types of transient voltage changes may cause problems and should be carefully managed, but because the voltage definition for a logic LO is tighter than for a logic HI, ground bounce is more often the source of transient problems. Power supply transient voltage changes affect both the integrated circuit causing the noise and other integrated circuits to which it is connected. A transient bounce induced on the lower potential of an integrated circuit by output driver currents may induce false logic HIs to unrelated logic circuits that happen to share the same lower potential bus. Externally, a subsequent integrated circuit receiving a binary output from a driver of an integrated circuit experiencing ground bounce may be driven with a false logic HI, or by a meta-voltage level that is greater than a logic LO but less than a logic HI. On the other hand, the next integrated circuit receiving an output from a driver of an integrated circuit experiencing upper potential noise transients may be driven with a false logic LO, or by a meta-voltage level that is less than a logic HI but greater than a logic LO. One previous solution to the transient noise problem for both internal and external logic circuits is to delay the time that the driver output binary value is considered valid until after transient voltages have settled down. This solution requires that any false binary states triggered during the delay either be cleared or be "don't care" logic signals. The problem with this approach is that it builds-in a predetermined delay into each and every binary output transfer. Such built-in delays are an impediment to the trend toward faster and faster data rates. It is an object of this invention to provide ground bounce and transient noise control means that only operates when needed to suppress problems with ground bounce and/or power conductor transient noise. According to the invention there is provided an integrated circuit according to Claim 1. It should be understood that, in the event of the occurrence of ground bounce or transient noise in an integrated circuit in accordance with the invention, there is a decrease in the rate of change of the output voltages of the buffer driving the output pad driver, which decreases the rate of change of the voltage level of the third and fourth power supply buses, thereby reducing the noise voltages generated by the output pad driver switching binary states. One embodiment of the invention will now be described by way of example with reference to the accompanying drawings, in which :- Fig. 1 is a schematic diagram of a CMOS multi-stage output buffer known in the art; Fig. 2 is a schematic diagram of a known CMOS output pad driver of a type conventionally driven by the buffer shown in Fig. 1; and Fig. 3 is a schematic diagram of a CMOS multi-stage output buffer in accordance with the invention. Referring to Fig. 1, a known three stage output buffer 10 is shown. Input D is the buffer input as well as the input of the first stage. The first stage is made up of FETs 12, 14. Each of the FETs 12, 14 has its gate connected to input D to receive a logic input signal. Together FETs 12 and 14 form a well known CMOS inverter arrangement. The first stage is connected to an enable/disable network of FETs 16, 18, 20, 22, 24, and 26. When an ENABLE input is driven by an active LO, FETs 16 and 26 are turned OFF and FET 18 is turned ON. The voltage applied through FET 18 subsequently turns ON FET 20 and turns OFF FET 22. FET 24 is short circuited by turned ON FET 20. Even if the short circuit by FET 20 is not complete, FET 24 will be turned ON by the active LO ENABLE signal. Those skilled in the art will recognize that with FETs 22 and 26 turned OFF and FETs 20 and 24 turned ON, the output data of the first stage FETs 12, 14 propagates normally to essentially identical second stage inverters 29, 35 formed by FETs 30, 32, and 36, 38 respectively. If, on the other hand, the ENABLE input is driven by a logic HI, then FETs 16 and 26 are turned ON and FET 18 is turned OFF. The voltage through FET 16 then turns ON FET 22 and turns OFF FETs 20 and 24. With FETs 22 and 26 turned ON, the output data from the first stage is disabled and overridden by the logic HI through FET 22 via line 28 to the second stage 29 and the logic LO through FET 26 via line 34 to the other second stage 35. The logic HI will be inverted by the second stage 29 to a logic LO which is outputted via line 40 to a third stage CMOS inverter 41 comprised of FETs 42, 44. Similarly, the second stage 35 will invert the logic LO on its input to a logic HI which is outputted via line 46 to another third stage CMOS inverter 47 comprised of FETs 48, 50. The output of CMOS inverter 41 will thus be a logic HI, and the output of CMOS inverter 47 will be a logic LO. These third stage outputs are respectively connected via lines 52 and 54 to a gate of a P-FET 60 and a gate of an N-FET 62 of an output driver 58 shown in Fig. 2. With the gate of P-FET 60 driven by a logic HI and the gate of N-FET 62 driven by a logic LO, the output driver 58 is driven into a high impedance output condition. Referring again to Fig. 1, the binary operation of the output buffer 10 when the ENABLE input is active (i.e., the ENABLE input is a logic LO) will be described. Input D is connected to the gates of N-FET 12 and P-FET 14. N-FET 12 and P-FET 14 are connected in series between internal upper potential power supply bus, IN VDD, and internal lower potential power supply bus, IN VSS, in a CMOS inverter arrangement, except for the intervening N-FET 20 which is in series between N-FET 12 and P-FET 14. However, as mentioned previously, if the ENABLE input is driven to a logic LO, then N-FET 20 becomes substantially a short circuit, which results in this first stage being equivalent to a CMOS inverter stage. Furthermore, if the N-FET 20 is essentially a short circuit, that means that output lines 28 and 34 have substantially equivalent voltages and equivalent logic levels. The gates of P-FET 30 and N-FET 32 of second stage 29 are connected in common to line 28. The P-FET 30 and the N-FET 32 are further connected in series between IN VDD and IN VSS in a CMOS inverter arrangement. The other second stage 35 is similarly arranged with the gates of P-FET 36 and N-FET 38 connected in common to line 34, and with P-FET 36 and N-FET 38 in series between IN VDD and IN VSS in a CMOS inverter arrangement. The outputs of second stages 29, 35 are taken from the junctions between the P-FETs 30, 36 and the N-FETs 32, 38 respectively. Lines 40, 46 respectively connect the outputs of second stages 29, 35 to third stages 41, 47. The third stages 41, 47 are typical CMOS inverter stages and are substantially identical to the second stage CMOS inverters 29, 35. The third stage 41 is comprised of P-FET 42 and N-FET 44. The gates of P-FET 42 and N-FET 44 are connected in common and to line 40 from second stage 29. P-FET 42 and N-FET 44 are connected in series between IN VDD and IN VSS. The junction of the drains of this series connection is the output of the third stage 41, which is connected to line 52. Another third stage 47 is similarly arranged with the gates of P-FET 48 and N-FET 50 both connected to line 46, and the P-FET 48 connected in series with N-FET 50 between IN VDD and IN VSS. The junction of the drains of P-FET 48 and N-FET 50 is the output of the third stage 47 and is connected to line 54. Lines 52 and 54 convey the output logic drive signals from the third stages 41 , 47 to the output pad driver 58 shown in Fig. 2. As mentioned previously, the inputs to stages 29 and 35 are logically identical when the ENABLE input is driven LO. Further, because the cascaded second and third stages 29, 41 are substantially the same as cascaded second and third stages 35, 47, the output drive signals on lines 52, 54 to output pad driver 58 will be substantially the same. Thus, output pad driver 58 is essentially a CMOS inverter when driven by output buffer 10 with an active LO on its ENABLE input. With the three stages of logical inversion provided by output buffer 10 and the logical inversion of the output pad driver 58, the logic signal conveyed by output line 64 to output pad 66 is a non-inverted version of the logic signal on data input D delayed by the propagation delay of four inverters. As shown in Fig. 2, output pad driver 58 is comprised of a series connection of a P-FET 60 and an N-FET 62; however, instead of being connected between IN VDD and IN VSS, the series combination of P-FET 60 and N-FET 62 is connected between an input/output upper potential power supply bus, I/O VDD, and an input/output lower potential power supply bus, I/O VSS. The internal power supply buses IN VDD and IN VSS must connect to the input/output power supply buses I/O VDD and I/O VSS, either on the integrated circuit chip or at some off chip location, in order for buffer 10 to have a common reference level from which to drive output pad driver driver 58. With such a connection, there may be induced some difficult ground bounce and transient noise voltages in the internal power supply buses. Moreover, such voltages may force a designer to either include a settling period for the ground bounce and power supply transients to die out, or some type of error correction apparatus to cope with the bit error rate induced by the noise voltages. Referring now to Figs. 1 and 3, an embodiment of a noise controlling apparatus in accordance with the invention will be described. Buffer 10' has an ENABLE circuit including FETs 16', 18', 20', 22', 24', and 26' that is functionally identical to the ENABLE circuit of the buffer circuit 10 shown in Fig. 1. Also, the first stage, which is made up of FETs 12', 14' is functionally identical to the first stage of the buffer circuit 10 shown in Fig. 1. Second stage 35' has a P-FET 36' connected in series between IN VDD and an N-FET 38' just like corresponding second stage 35 shown in Fig. 1, except that second stage 35' has the other end of the series connection of FETs 36', 38' connected to I/O VSS as its lower potential power supply bus. Third stage 47', like second stage 35', is connected to I/O VSS as its lower potential power supply bus. Thus, the stages 35', 47' driving the output driver stage 58, shown in Fig. 2, are connected to I/O VSS just as N-FET 62 is. By this connection, an appropriate ground bounce increase in the I/O VSS voltage with respect to the IN VSS voltage level will cause a gain reduction and an increase in the ON resistance of N-FETs 38', 50', and 62 because the ground bounce reduces their respective gate-to-source voltages. The gain reduction of FETs 38' and 50' lengthens the time required by stages 35' and 47' to switch logic states. By lengthening the output switching times, these gain reductions reduce some of the ground bounce. The longer time generates fewer high frequency components which interact with the parasitic inductances and parasitic capacitances. Additionally, lengthening the switching time provides more settling time for the output pad driver 58 only when ground bounce is occurring, and not when ground bounce is not occurring. Besides the negative feedback and gain reduction that the ground bounce on I/O VSS provides to FETs 38' and 50', third stages 47' and 41' have additional components which provide additional negative feedback to increase the switching time of the third stages 47' and 41' and of the output driver stage 58 (shown in Fig. 2) even more, and thereby reduce both ground bounce on I/O VSS and transient noise on I/O VDD, as will be explained later. Returning to the input stage, the outputs of FETs 12' and 14' are connected via lines 28' and 34' to second stages 29' and 35' respectively. Second stage 29' with its FETs 30' and 32' is functionally identical to second stage 29 shown in Fig. 1. The second stage outputs are connected via lines 40' and 46' to third stages 41' and 47' respectively in the same manner that the second stages 29, 34 are connected to the third stages 41, 47 of buffer 10 shown in Fig. 1. Third stages 41' and 47', with FETs 42', 44', 48', 50', are functionally similar to third stages 41 and 47, except that at the points designated U, V, W, and X in Fig. 1 third stages 41' and 47' have additional FETs 70, 72, 74, and 76 respectively, and except that the source of FET 72 is connected to I/O VSS as previously mentioned. FETs 70, 74 are P-FETs, having their sources connected to IN VDD, their gates connected to I/O VSS, and their drains connected in series with the sources of FETs 48', 42' respectively. N-FET 76 has its source connected to IN VSS, its gate connected to I/O VDD, and its drain connected in series with the source of FET 44'. N-FET 72 has its source connected to I/O VSS in order to reduce the gain of stage 47' in case of ground bounce, as mentioned previously. N-FET 72 has its gate connected to I/O VDD and its drain connected in series with the source of FET 50' in a manner similar to the connection of N-FET 76 of third stage 41'. The output of third stage 41' is connected from the junction of FETs 42' and 44' via line 52' to the P-FET 60 of output driver 58 shown in Fig. 2. Similarly, the output of third stage 47' is connected from the junction of FETs 48' and 50' via line 54' to the N-FET 62 of the output driver 58. In operation with their gates connected to I/O VSS and their sources connected to IN VDD, P-FETs 70 and 74 normally are turned completely ON and provide low series resistances equal to their characteristic ON resistances between IN VDD and the rest of the third stages 41' and 47'. If the lower potential bus, I/O VSS, of the input/output power supply is instantaneously overloaded by a group of output pad drivers 58 switching logic levels causing a ground bounce thereon, P-FETs 70 and 74 will have their gate-to-source voltages reduced. If the gate-to-source voltage reductions are small, the ON resistances of P-FETs 70 and 74 will remain initially unchanged; however, as the gate-to-source voltage is reduced further, the ON resistances of P-FETs 70 and 74 will subsequently increase. Increases of the ON resistances of P-FETs 70 and 74 increase the switching times of third stages 41' and 47' and reduce any noise caused by the instantaneous overload of I/O VSS. If there is no substantial noise on I/O VSS, then the switching times of third stages 41' and 47' are not increased. N-FET 76 with its gate connected to I/O VDD and its source connected to IN VSS, and N-FET 72 with its gate connected to I/O VDD and its source connected to I/O VSS provide operations somewhat similar to those provided by P-FETs 70 and 74. Connected in this manner, N-FETs 72 and 76 are normally turned completely ON and provide low series resistances equal to their characteristic ON resistances between I/O VSS and IN VSS, respectively, and the rest of the third stages 47' and 41'. If the upper potential of the input/output power supply bus, I/O VDD, is instantaneously overloaded by a group of output pad drivers 58 switching logic levels causing a noise transient thereon, N-FETs 72 and 76 will have their gate-to-source voltages reduced. If the gate-to-source voltage reductions are small, the ON resistances of N-FETs 72 and 76 will remain initially unchanged; however, as the gate-to-source voltage is reduced further, the ON resistances of N-FETs 72 and 76 will subsequently increase. Increases of the ON resistances of N-FETs 72 and 76 slow down the switching rates of third stages 47' and 41' and reduce the noise caused by the instantaneous overload of I/O VDD. If there is no substantial noise on I/O VDD, then the switching times of the third stages 41' and 47' are not increased. The P-FETs 70 and 74 will thereby temporarily limit the switching rate if substantial ground bounce is present on I/O VSS, and N-FETs 72 and 76 will thereby temporarily limit the switching rate if substantial transient noise is present on I/O VDD. The limiting characteristics of the P-FETs 70 and 74, and the N-FETs 72 and 76 are predetermined during fabrication by varying their critical dimensions. Thus, the output buffer 10' may be operated at very high switching rates in the absence of transient noise and ground bounce, on the one hand, or operated at a self-reduced switching rate in the presence of substantial transient noise and ground bounce. By reducing the switching rate, the transisient noise and ground bounce on I/O VDD and I/O VSS are reduced, thereby reducing the risk of the occurrence of binary bit errors. Thus, it will now be understood that there has been disclosed an apparatus which reduces the ground bounce and transient noise caused by the output drivers of an integrated circuit. While the invention has been particularly illustrated and described with reference to a preferred embodiment thereof, it will be understood by those skilled in the art that various changes in form, details, and applications may be made therein. For example, this invention as defined in the Claims, could be modified to operate with BiCMOS, TTL, and other logic families.

Claims: An integrated circuit including a pad driver (58), a buffer (10') having an output stage (41') connected to said pad driver and switchable between first and second logic states, first and second power supply buses (IN VDD, IN VSS) which are arranged to supply power to said buffer and which have a first upper potential and a first lower potential respectively appearing thereon, and third and fourth power supply buses (I/O VDD, I/O VSS) which are arranged to supply power to said pad driver and which have a second upper potential and a second lower potential respectively appearing thereon, characterized by first control means (74) arranged in said output stage (41') for receiving and being responsive to said second lower potential appearing on said fourth power supply bus (I/O VSS), whereby an appropriate increase in said second lower potential relative to said first lower potential brings about an increase in the switching time of said output stage, and second control means (76) arranged in said output stage (41') for receiving and being responsive to said second upper potential appearing on said third power supply bus (I/O VDD), whereby an appropriate decrease in said second upper potential relative to said first upper potential brings about an increase in the switching time of said output stage. An integrated circuit according to Claim 1, characterized in that said output stage (41') includes an inverter (42', 44'), said first control means is formed by a first FET (74) connected between said inverter and said first power supply bus (IN VDD), and said second control means is formed by a second FET (76) connected between said inverter and said second power supply bus (IN VSS), the gates of said first and second FETs (74, 76) being respectively connected to said fourth and third power supply buses (I/O VSS, I/O VDD) whereby an appropriate increase in said second lower potential brings about an increase in the ON resistance of said first FET (74) and whereby an appropriate decrease in said second upper potential brings about an increase in the ON resistance of said second FET (76). An integrated circuit according to Claim 2, characterized in that said first and second FETs (74,76) are respectively a P-FET and an N-FET. An integrated circuit according to any one of the preceding claims, characterized by a second output stage (47'), third control means (70) arranged in said second output stage for receiving and being responsive to said second lower potential appearing on said fourth power supply bus (I/O VSS), whereby an appropriate increase in said second lower potential brings about an increase in the switching time of said second output stage, and fourth control means (72) arranged in said second output stage (47') for receiving and being responsive to said second upper potential appearing on said third power supply bus (I/O VDD), whereby an appropriate decrease in said second upper potential brings about an increase in the switching time of said second output stage. An integrated circuit according to Claim 4, characterized in that said second output stage (47') includes a second inverter (48',50'), said third control means is formed by a third FET (70) connected between said second inverter and said first power supply bus (IN VDD), and said fourth control means is formed by a fourth FET (72) connected between said second inverter (48', 50') and said fourth power supply bus (I/O VSS), the gates of said third and fourth FETs (70,72) being respectively connected to said fourth and third power supply buses (I/O VSS, I/O VDD), whereby an appropriate increase in said second lower potential brings about an increase in the ON resistance of said third FET (70) and whereby an appropriate decrease in said second upper potential brings about an increase in the ON resistance of said fourth FET (72). An integrated circuit according to Claim 5, characterized in that said third and fourth FETs (70,72) are respectively a P-FET and an N-FET. An integrated circuit according to either Claim 5 or Claim 6, characterized in that said pad driver (58) is formed by the series combination of fifth and sixth FETs (60,62) connected between said third and fourth power supply buses (I/O VDD, I/O VSS), said fifth FET (60) being a P-FET connected to said third power supply bus (I/O VDD) and said sixth FET (62) being an N-FET connected to said fourth power supply bus (I/O VSS), a first output line (54) of said second output stage (47') being connected to the gate of said sixth FET (62), and a second output line (52) of the other output stage (41') being connected to the gate of said fifth FET (60).

Inventors: DUKES GLENN E, DUKES, GLENN E.
Applicants: AT & T GLOBAL INF SOLUTION, HYUNDAI ELECTRONICS AMERICA, SYMBIOS LOGIC INC, AT&T GLOBAL INFORMATION SOLUTIONS INTERNATIONAL INC.
Application year: 1991
Main classification: H03K19
IPC codes: H03K19, H03K17