Integrated circuit buffer with controlled rise and fall times

Abstract: Process, voltage, and temperature variations affect the noise generation of an output buffer. Controlling the switching speed of the buffer over these variations also controls the buffer noise, which may be due to ground bounce or other reasons. In one prior-art technique, the current flow behavior of a static circuit is used to control the rise and/or fall times of the output buffer. However, accounting for all possible variations in the factors that influence the switching speed is difficult with a static control circuit. In the inventive technique, the AC switching behavior of a scaled-down buffer (201,202) that is driven by a periodic signal (CLOCK) generates the control voltage (VCP). In this manner, the factors that influence buffer switching speed, including process, voltage, and temperature variations, may be more accurately accounted for.

Description: This invention relates to integrated circuits having a buffer. An integrated circuit (IC) typically communicates to external devices through electrical conductors driven by output buffers. Another type of buffer, referred to as a "clock driver", is used to distribute clock signals. The clock driver in some cases is located on the same integrated circuit as the circuitry that receives the clock signals. For either on-chip or off-chip drive applications, a buffer is designed to provide adequate current drive capability to charge and discharge the capacitive load presented by the driven conductors (and driven circuitry) at a sufficiently fast rate to allow signals to be sent at the desired speed. The speed at which a buffer can raise a voltage from a low logic level (typically 0 volts) to a high logic level (typically 3 or 5 volts) is referred to as the "rise time". Similarly, the speed at which the buffer can reduce the voltage from high to low levels is the "fall time". With CMOS integrated circuits, the speed of data communications to external devices presently ranges up to several tens of million bits per second, with increases to over one hundred million bits per second likely in the near future. Therefore, the rise and fall times are typically required to be less than about 10 nanoseconds at present, with significant reductions required for the future. One problem associated with signals having rapid rise and fall times is the electrical noise they produce in other parts of the integrated circuit. That is, stray capacitance and inductance cause some of the signal energy to couple to other portions of the integrated circuit, or to other conductors external to the IC. In addition, inductance in the ground path causes voltage spikes to appear due to rapid current variations caused by the switching of large buffers. These problems generally become more severe as the rise and fall times become shorter. In most cases, integrated circuit buffers are designed so that the rise and fall times are sufficiently fast to drive the load under worst case conditions. For CMOS integrated circuits, the worst case conditions are slower than nominal process speeds, low power supply voltages, and high operating temperatures. However, under the best case conditions, the rise and fall times may then be so short as to generate excessive noise. The "process speed" refers to the fact that due to the tolerances for a given IC production process, some IC's turn out to be "fast", and others "slow", as compared to IC's produced by an average or nominal process. That is, a given production run may produce devices that switch signals more quickly than average, being referred to as a "fast" process, and vice-versa for a "slow" process. Hence, a buffer design that satisfies the speed requirements may violate the noise requirements, and vice-versa. It is desirable to have an integrated circuit that generates low noise while having the ability to communicate at a sufficiently high data rate over conductors and other circuitry presenting a significant capacitive load. One technique for controlling buffer switching speed, and hence the noise produced, is shown in Fig. 1. The pre-driver transistors 102 and 103 supply a drive signal to pull-up output transistor 100, whereas pre-driver transistors 106 and 107 supply a drive signal to pull-down output transistor 101. A control voltage VCP increases when factors such as process speed, temperature, and power supply voltage (for example) tend to increase the switching speed. This increase of VCP reduces the gate-to-source voltage, and hence the conductivity, of the p-channel transistor 105, thereby reducing the drive signal to output transistor 101, which controls the buffer fall time. Inversely, the voltage VCP decreases, so as to increase the conductivity of transistor 105, when these factors tend to decrease the switching speed. (The control voltage VCN is generated so as to similarly affect the conductivity of the n-channel transistor 104, thereby controlling the drive to the output transistor 100, and hence the buffer rise time.) In this manner, the buffer switching speed, and hence noise, is held more constant with respect to variations in these factors. The output load driven by this buffer is shown as capacitance C load . One method of generating the control voltages VCP and VCN is shown in U. S. patent 4,823,029, co-assigned herewith. The type of static circuits for generating the control voltages as shown in Figs. 2 through 5 therein are successful in many applications. However, accounting for all possible variations in the factors that influence the switching speed is difficult when a static circuit is used to generate the DC control voltage. Another method of controlling an output buffer is shown in U. S. patent 4,567,378, wherein the buffer output signal is compared to the output of a ramp generator. If the output signal rises or falls faster than the ramp signal, then a feedback signal adjusts the speed of the buffer to more closely correspond to the ramp signal. However, that technique monitors the output signal at each buffer, which requires extra monitor circuitry for each buffer, and ramp circuitry for each buffer. In US - A - 4972101 the switching speed of a buffer is controlled by a digital "modulating voltage" control signal generated by an "increment/decrement" circuit. A dummy driver circuit is used to discharge a capacitor at a rate dependent upon the modulating voltage, and the capacitor voltage is compared with a reference at a time determined by the precision clock. The result of the comparison is used to increment or decrement the modulating voltage. According to this invention there is provided an integrated circuit as claimed in claim 1. In a technique for generating the control voltage that is used to control the rise and/or fall time of an integrated circuit buffer, including for example, output buffers and clock drivers, a reference buffer driven by a periodic signal mimics the performance of a given buffer that drives a desired load. The reference buffer produces a periodic signal that is sampled during the switching of the reference buffer. The sampled signal is used to generate a control signal that maintains more constant the speed of the given buffer that drives the actual load. One or more buffers may be controlled from a control signal generated by a single reference circuit. Brief Description of the Drawings Fig. 1 shows a typical prior-art voltage-controlled buffer. Fig. 2 shows an illustrative embodiment of the present invention. Fig. 3 shows in detail the sample-and-hold circuit of Fig. 2. Fig. 4 shows an illustrative voltage waveform. Detailed Description The present detailed description relates to an integrated circuit buffer having a control voltage generated by sampling the waveform of a reference buffer that is driven by a periodic (e.g., clock) signal. In an illustrative embodiment, the sampled signal, typically a voltage, is compared to a reference signal, also typically a voltage. The comparison produces a control signal that is fed back so as to cause the sampled signal from the reference buffer to reach a steady-state value. The control signal is used to control the rise time, or fall time, or both, of the buffer that drives the desired load. In this manner, a more constant speed of operation of the buffer is obtained. Since the drive signal to the reference buffer is independent of the drive to use actual buffer driving the desired load, more than one buffer may be controlled by a single reference buffer. Referring to Fig. 2, an exemplary reference buffer and sampling circuitry are shown. When the CLOCK signal goes high, the output of inverter 210 goes low, thereby causing p-channel pull-up transistor 201 to conduct. This conduction charges up reference capacitor C ref to a fixed high value, such that the voltage V 1 initially approximates V DD . When the CLOCK signal goes low, the output of inverter 210 goes high, causing transistor 201 to cease conduction. Also, the output of inverter pair 208-209 (V 3 ) goes high, causing n-channel pull-down transistor 202 to conduct. The conduction through transistor 202 tends to discharge C ref , by allowing the current I out to flows through an impedance, which in the illustrative case is a resistor R. The voltage V 2 across resistor R is sampled by sample-and-hold circuit 204 at a time that is delayed with respect to the time the CLOCK signal goes low. Therefore, the sampled value of V 2 contains information about how quickly capacitor C ref is discharged, which in turn relates to the speed of transistor 202. This information is used to generate the control signal VCP which controls the fall time of the buffer shown in Fig. 1. The sample-and hold circuit 204 is shown in more detail in Fig. 3, and operates as follows: The CLOCK signal is delayed by inverters 301 ... 304 and applied to a phase splitter 305, being a design well known in the art. When the CLOCK is high, the voltages V 4 and V 4 are high and low, respectively, allowing pass transistors 306 and 307 to conduct. Therefore, the voltage V 2 is applied to capacitor 308 during the "sample" period. When the CLOCK signal goes low, the pass transistors 306 and 307 are turned off after the delay period (DELAY I) through inverters 301 ... 304, thereby causing capacitor 308 to "hold" the most recent sample value of V 2 . The delay through inverters 301 ... 304 is chosen to delay the sampling of the voltage V 2 until reference capacitor C ref (Fig. 2) has partially discharged due to conduction through transistor 202 and resistor R. This held sample value on capacitor 308 is allowed to pass through transistor 309 when the DELAYED CLOCK signal goes high, as follows: The high-going DELAYED CLOCK signal initially passes through NAND gate 311 and inverter 310 to produce a high voltage on the gate of transistor 309, thereby allowing it to conduct. After a delay time period through inverters 312, 313 and 314, the high-going DELAYED CLOCK signal causes the output of gate 311 to go low, thereby terminating this pulse, which illustratively has a width of about 1 nanosecond. In this manner, the sampled voltage V S is placed on capacitor C 1 (Fig. 2). Referring again to Fig. 2, the sampled voltage V S on C 1 is supplied to the non-inverting input of operational amplifier 205. The inverting input of operational amplifier 205 is connected to C 2 and a reference voltage source 206. The capacitors C 1 and C 2 have an exemplary value of 40 picofarads. In operation, the sampled voltage V S is initially low, approximately 0 volts, and increases as V 2 is sampled. The operational amplifier 205 compares V S to V R , the voltage produced by the reference circuit 206. As long as V S is less than V R , the output voltage of the operational amplifier (VCP) is relatively low, thereby allowing p-channel transistor 207 to become highly conducting. Therefore, the voltage at the source of p-channel transistor 208 is relatively high, allowing transistor 208 to supply a higher positive drive voltage to the gate of transistor 202. This high drive capability causes transistor 202 to conduct relatively heavily, so that Iout, and hence V 2 , are relatively large. This relatively large value of V 2 is again sampled by the sample-and-hold circuit 204, and fed back as before, until the value of V S across capacitor C 1 increases to approximate the voltage V R across capacitor C 2 . When equilibrium occurs, the voltage VCP from the operational amplifier 205 reaches a steady-state value, and hence the waveforms of V 1 , V 2 , V 3 , and I out reach steady-state values. However, the steady-state values of these voltages depend upon the fabrication processing conditions that were used to make the integrated circuit. In addition, they depend upon the power supply voltage and temperature of the integrated circuit during operation. For example, when the power supply voltage (V DD ) increases, or when the temperature of the integrated circuit decreases, the speed of operation of the buffer tends to increase. Similarly, an integrated circuit fabricated by a "fast" fabrication process results in a relatively higher speed operation. The best-case fast conditions may be illustrated by waveform 40 of Fig. 4, which shows the voltage V 2 as a function of time after the CLOCK goes low at time 0. Alternatively, decreases in the power supply voltage, or increases in operating temperature, or a "slow" fabrication process all tend towards a decrease in the speed of operation of the buffer. The waveform of V 2 resulting from worst-case slow conditions may be illustrated by 41 in Fig. 4. Note that in practice, some of the factors may tend in one direction, while others tend in the other direction, so that the waveform of V 2 typically lies between curves 40 and 41. The operation of the circuitry of Figs. 2 and 3 may be more completely understood by reference to Fig. 4. At time 0, the CLOCK signal goes low, and the voltage V 4 remains at an initially high value until the "DELAY I" time period provided by inverters 301 ... 304. After this delay, illustratively about 4.5 nanoseconds, the voltage V 4 falls so as to cut off conduction through pass transistor 306 when the value of V 4 approximates the value of V 2 on the source of transistor 306. (Similarly, V 4 rises after "delay I" to cut off conduction of pass transistor 307.) For example, the "Sample I" is shown at the time V 4 equals the value of V 2 in the first illustrative discharge waveform 40. This occurs at about 2.2 volts, which in the illustrative case yields a value of VCP of approximately 0 volts. This low value of VCP tends to slow down the fall time of the buffer in Fig. 1, thereby at least partially compensating for the best-case fast conditions. The "Sample II" is shown at the time V 4 equals the value of V 2 in the second illustrative discharge waveform 41. This occurs at about 1.1 volts, which yields a value of VCP of approximately 1.5 volts. This high value of VCP tends to speed up the fall time of the buffer in Fig. 1, thereby at least partially compensating for the worst-case slow conditions. Hence, the inventive circuitry yields the desired speed compensation. In a typical circuit implementation of the inventive technique, the reference circuit 206 is a band-gap reference, which generates a DC voltage that is relatively constant with respect to changes in power supply voltage, temperature, and integrated circuit fabrication processing variations. In the case of silicon technology, the voltage from a band-gap reference is about 1.13 volts, shown as V R in Fig. 4. However, the voltage from a band-gap reference may be scaled by a voltage mirror to a different value, according to circuit techniques known in the art. Although a resistor (R) has been shown above as the impedance element for developing the voltage V 2 , an inductor may alternatively be used, or a combination thereof. The impedance element may be located on the integrated circuit where the buffer is formed, or externally thereto. The CLOCK and DELAYED CLOCK signal may conveniently be generated by a ring oscillator circuit located on the same integrated circuit as the output buffer and the reference buffer, if desired. For example, a ring oscillator having 17 stages and operating with a 1000 nanosecond period may serve to generate the CLOCK signal. A DELAYED CLOCK may be then obtained by tapping the ring oscillator one stage after the CLOCK signal tap, obtaining a delay of 60 nanoseconds with respect to the CLOCK. Alternatively, externally-generated periodic (i.e., clock) signals may be used. The foregoing circuitry generates the control voltage VCP, as used in the circuitry of Fig. 1. The control voltage VCN may be generated in a similar manner by circuitry that largely duplicates Fig. 2. However, in that case, the reference capacitor C ref is initially discharged to near 0 volts, and its charging rate toward V DD is then measured by measuring the voltage across a resistor placed in series with the source lead of a p-channel charging transistor. Alternatively, the voltage VCN may be derived from VCP by a voltage mirror that is centered around the mid-point of the power supply voltage (i.e., V DD /2), according to circuit techniques known in the art. Then, as VCP increases with respect to V SS , VCN decreases from V DD a comparable amount. Such a derived value of VCN actually tracks the characteristics of the n-channel transistor (e.g., 202) rather than a p-channel device. However, that is accurate enough for many application, considering that most circuit noise is due to the n-channel devices in present-day CMOS technologies. The reference buffer of Fig. 2 may be scaled-down with respect to the size of the buffer in Fig. 1 that drives the actual load (C load ). That is, transistors 201 and 202 may be scaled-down with respect to 100 and 101, respectively. The reference load capacitor C ref is then typically also scaled-down with respect to the actual load capacitance C load . Such scaling conserves integrated circuit area, and also reduces the power required to operate the reference circuit. The control voltages VCN and VCP may be used to control the speed of buffers other than the type shown in Fig. 1. For example, buffers having n-channel transistors for both the pull-up and pull-down devices are known in the art, and may advantageously use the inventive technique. Also, both the pull-up and pull-down transistors in the reference buffer (201 and 202) may be n-channel devices. While field effect devices are shown herein, the present technique may alternatively be implemented with bipolar devices, or with mixed device types in a BICMOS type of circuit. Also, the buffer may include only a pull-down output device (or alternatively only a pull-up output device), with an external resistor or other device providing the pull-up (or alternatively pull-down) function. As is apparent, the output load driven by the buffer may be supplied externally to the integrated circuit on which the buffer is formed, or may be used internally, as with a clock driver or other type of circuit. Still other variations are possible.

Claims: An integrated circuit having a buffer for driving an output load (C LOAD ) at a switching speed, and means for generating a control signal (VCP) that maintains said switching speed relatively more constant with respect to at least one of the factors selected from the group consisting of: temperature, power supply voltage, and fabrication process speed; wherein said means for generating a control signal comprises a reference buffer controlled by a periodic signal (CLOCK) for supplying a drive voltage to a reference load (C REF ); characterized by a sample-and-hold circuit (204) for sampling the magnitude of the drive voltage to said reference load to obtain a sampled drive signal; and an analog amplifier having a first input connected to a reference voltage source (206) and having a second input connected to the output of said sample-and-hold circuit (204); wherein the analog output of said amplifier supplies said control signal (VCP). An integrated circuit as claimed in claim 1 wherein said reference buffer comprises a pull-up transistor (201) for charging a reference capacitor (C REF ) and a pull-down transistor (202) for discharging said reference capacitor, whereby said sampled drive signal is derived from the current (I OUT ) that flows from said capacitor during either said charging or discharging. An integrated circuit as claimed in claim 2 wherein said sampled drive signal is derived from said current by means of an impedance (R) connected in series between a power supply conductor and either one of said pull-up transistor and said pull-down transistor. An integrated circuit as claimed in claim 3 wherein said impedance substantially comprises a resistance. An integrated circuit as claimed in claim 3 wherein said impedance substantially comprises an inductance. An integrated circuit as claimed in claim 1 wherein said reference voltage source is a band gap voltage reference. An integrated circuit as claimed in claim 1 wherein said means for generating a control signal causes said sampled drive signal to equilibrate with respect to said reference voltage at the inputs of said analog amplifier.

Inventors: GABARA THADDEUS JOHN, GABARA, THADDEUS JOHN
Applicants: AT & T CORP, AT&T CORP.
Application year: 1993
Main classification: H03K19
IPC codes: H03K17, H03K19, G05F1, H02M3