Abstract:
One embodiment of the present invention provides a system that matches speeds of asynchronous operation between a local chip and a neighboring chip. The system derives an internal frequency signal from an internal oscillator on the local chip, and receives an external frequency signal from a neighboring chip. The system then compares the internal frequency signal with the external frequency signal to generate a control signal, which is applied to the local chip to adjust the operating speed of the local chip, and applied to the internal oscillator to adjust the frequency of the internal oscillator.

Description:
RELATED APPLICATION 
   This application hereby claims priority under 35 U.S.C. 119 to U.S. Provisional Patent Application No. 60/443,591, filed on 29 Jan. 2003, entitled “Speedmatching Control Method and Circuit,” by inventors Robert J. Drost, Ivan E. Sutherland, and Josephus C. Ebergen 

   GOVERNMENT LICENSE RIGHTS 
   This invention was made with United States Government support under Contract No. NBCH020055 awarded by the Defense Advanced Research Projects Administration. The United States Government has certain rights in the invention. 

   BACKGROUND 
   1. Field of the Invention 
   The present invention relates to design of circuitry for asynchronous inter-chip communications. More specifically, the present invention relates to a method and apparatus for controlling and matching speeds of operation between different asynchronous chips. 
   2. Related Art 
   As computer system clock speeds become progressively faster, it is becoming increasingly harder to synchronize the actions of computer system components with reference to a centralized system clock. To deal with this problem, computer system designers are beginning to investigate the use of asynchronous circuits that operate in a self-timed manner, without having to adhere to the constraints imposed by a centralized system clock. 
   While asynchronous operation circumvents limitations imposed by a centralized system clock, it also introduces new problems, especially with regards to inter-chip communications. In particular, when two communicating asynchronous chips operate at different speeds, a slower receiving chip can encounter an input-buffer overflow if a faster transmitting chip transmits a large amount of data at a higher speed. 
   A number of factors contribute to differences in chip speeds. First, different fabrication technologies lead to different chip speeds. For example, chips fabricated using 350 nm CMOS technology are likely to operate at a different speed than chips fabricated using 130 nm CMOS technology. Furthermore, because of process variations during fabrication and environmental factors, such as temperature and power supply variations, even two chips fabricated using the same technology can operate at different speeds. 
   To maintain error-free communications between asynchronous chips while achieving good performance, it is desirable to operate all the chips at the highest possible speed without overflowing the input buffer of any given chip. This usually requires all of the asynchronous chips to operate at the maximum speed of the slowest chip. 
   Hence, what is needed is a method and apparatus for controlling and matching the speeds of operation between asynchronous chips. 
   SUMMARY 
   One embodiment of the present invention provides a system that matches speeds of asynchronous operation between a local chip and a neighboring chip. The system derives an internal frequency signal from an internal oscillator on the local chip, and receives an external frequency signal from a neighboring chip. The system then compares the internal frequency signal with the external frequency signal to generate a control signal, which is applied to the local chip to adjust the operating speed of the local chip, and applied to the internal oscillator to adjust the frequency of the internal oscillator. 
   In a variation of this embodiment, adjusting the frequency of the local chip involves changing the power-supply voltage of the local chip. 
   In a variation of this embodiment, receiving of the external frequency signal from the neighboring chip involves receiving the external frequency signal through a capacitor, an inductor, a resistor, a transmission line, or a direct contact. 
   In a variation of this embodiment, comparing the internal frequency signal with the external frequency signal involves converting the internal frequency signal and external frequency signal into corresponding current or voltage signals, which are proportional to the frequencies of the frequency signals. 
   In a variation of this embodiment, the system converts the internal frequency signal and the external frequency signal into corresponding current signals, and then compares the two current signals to generate a difference current signal. The difference current signal is then coupled to an integrating capacitor to produce an integrated voltage signal. The system also applies an offset current source to the integrating capacitor to compensate for transistor leakages, parasitics, and/or nonlinearities. The system further includes an amplifier which is coupled to the integrating capacitor, wherein the input to the amplifier is the integrated voltage signal and the output of the amplifier is the control signal. 
   In a further variation, the system filters the control signal to improve matching between the local chip&#39;s operating speed and the neighboring chip&#39;s operating speed. 
   In a variation of this embodiment, filtering the control signal involves coupling a filter capacitor between the control signal and ground. 
   In a variation of this embodiment, the internal frequency signal has a frequency that is a fraction of the internal oscillator frequency of the local node, and the external frequency signal has a frequency that is a fraction of an external oscillator frequency of the neighboring node. 

   
     BRIEF DESCRIPTION OF THE FIGURES 
       FIG. 1  illustrates a speed control circuit in accordance with an embodiment of the present invention. 
       FIG. 2  illustrates a frequency detector circuit in accordance with an embodiment of the present invention. 
       FIG. 3  illustrates exemplary waveforms at different stages of a frequency detector circuit in accordance with an embodiment of the present invention. 
       FIG. 4  illustrates an exemplary waveform for the output control voltage from a frequency detector circuit in accordance with an embodiment of the present invention. 
   

   DETAILED DESCRIPTION 
   The following description is presented to enable any person skilled in the art to make and use the invention, and is provided in the context of a particular application and its requirements. Various modifications to the disclosed embodiments will be readily apparent to those skilled in the art, and the general principles defined herein may be applied to other embodiments and applications without departing from the spirit and scope of the present invention. Thus, the present invention is not intended to be limited to the embodiments shown, but is to be accorded the widest scope consistent with the principles and features disclosed herein. 
   Speed Control Circuit 
     FIG. 1  illustrates a speed control circuit in accordance with an embodiment of the present invention. 
   As is illustrated in  FIG. 1 , chips  130  and  140  contain the same speed control circuits which are coupled together to form a loop. Chips  130  and  140  are coupled together through capacitors  105  and  106 . However, they can be coupled through any type of connection, for instance a resistor, an inductor, a transmission line, or just a simple wire. 
   Each half of the speed control loop comprises two parts: a variable frequency oscillator with a control input, and a frequency detector circuit  110 . Additionally, there may be filtering elements, such as capacitors, resistors, or inductors, which are used to improve the stability, response time, and/or performance of the control loop. For example, in  FIG. 1 , a filter capacitor  103  is shown coupled to V ctl  to smooth out ripples on the control voltages. Note that the frequency detector circuit can additionally include an internal integrating capacitor to improve the loop stability of the overall speed control loop. 
   The speed control loop operates as follows. The oscillator on each chip is designed to operate just below the chip&#39;s maximum operating speed when the control voltage input is at its maximum speed setting. The connections between chips transmit a signal whose frequency is the frequency of the oscillator. Each chip compares its internal oscillator frequency against the frequency transmitted by the other chip and, if necessary, slows down or speeds up its internal frequency to match the frequency of the other chip. The response time of these frequency corrections needs to be slow in comparison with delay involved in sending signals between chips to make the overall control loop stable. 
   Each chip starts by transmitting its maximum frequency. The slower chip will be unable to go any faster and its oscillator will simply continue to oscillate at the maximum speed. The faster chip will slow down to match the speed of the slower chip. 
   Inside chip  130  (chip  140  has a similar configuration), the local oscillator is comprised of an NAND gate  102  and a number of cascaded inverters, such as inverter  101 . The cascaded inverters are coupled to the inputs of NAND gate  102 , wherein distribution of inverters on each input of NAND gate  102  determines the duty cycle of the generated timing signal. The oscillator frequency is determined by the delays of inverters. Note that the delays of these inverters and NAND gate  101  can be controlled by varying the voltage of their power supply. Hence, by varying a common power-supply voltage, V ctl , one can adjust the frequency of the timing signal generated by the oscillator. 
   Frequency Detector Circuit 
     FIG. 2  illustrates a frequency detector circuit in accordance with an embodiment of the present invention. This frequency detector first converts the frequency of each input signal into a current signal that is proportional to the respective signal&#39;s frequency. It then integrates them on a capacitor. Finally the integrated voltage is buffered by an amplifier before being used to adjust the internal oscillator frequency and the chip&#39;s operating speed. Note that this integration can help provide control loop stability. 
   One possible function for a frequency detector circuit that outputs a voltage control signal is to provide:
 
 V   out   =K   FD (Freq1−Freq2)
         wherein Freq  1  and Freq  2  are two input frequencies, and K FD  is a constant.       

   As shown in  FIG. 2 , there are two input frequency signals: frequency  1  and frequency  2 . These input frequency signals feed into respective edge detector circuits comprised of cascaded invertors and NAND gates. Each of these edge detector circuits produces a low voltage pulse on a rising edge of the input frequency signal. This configuration allows the output pulse signal&#39;s frequency to be proportional to the frequency of the input signal while the pulse width is independent of the input signal&#39;s duty cycle. This is because the width of the pulse is only determined by the delays of the inverters. 
   While frequency  1  is converted to a downward pulsed signal V edge1 , frequency  2  is converted to an upward pulsed signal V edge2  with an additional inverter  210 . V edge1  is then coupled to the gate of PMOS transistor  203 , the source of which is connected to current source  201 . The purpose of this configuration is to turn on PMOS transistor  203  and to let a current flow into the drain of PMOS transistor  203  for the duration of the pulse whenever there is a rising edge in the input signal, frequency  1 . Similarly, V edge2  is coupled to the gate of NMOS transistor  204 , the source of which is connected to current source  202 , such that whenever there is a rising edge in frequency  2  , NMOS transistor  202  is turned on and a current flows out of the drain of NMOS transistor  202 . 
   The net effect of this PMOS and NMOS configuration is that a difference current signal is produced at the point where the drains of two transistors are coupled, and the time integral of this difference current is proportional to the difference between frequency  1  and frequency  2 . To convert this difference current signal into a voltage signal, the circuit further includes an integrating capacitor  206 . The voltage that appears across capacitor  206  reflects the time integral of the difference current signal. 
   The final output of the frequency detector circuit, V ctl , is obtained from an operational amplifier (OP AMP)  207  configured as a unity-gain amplifier, wherein the input of the unity-gain amplifier is the voltage produced by integrating capacitor  206 . V ctl  is then used to adjust both the internal oscillator frequency and the operating speed of the asynchronous chip. 
   In theory, two chips could settle on any frequency at which both chips would operate. In reality, it is difficult to achieve a perfect match because of current leakages, parasitics, and nonlinearities. As a result in some cases, both chips would try to operate slightly more slowly than each other, and the control loop would consequently cause both chips&#39; oscillators to slow down to a complete stop. 
   Thus, an important addition to this control loop is the introduction of offset current source  205 , which is coupled to the integrating capacitor  206 . As a result, the control loop may successively increase an oscillator&#39;s frequency to be just slightly higher than the other chip&#39;s frequency, and stop at where the slower chip&#39;s oscillator hits its maximum frequency. At that point, the control loop is nonlinear, and the faster chip will operate just a bit faster than that frequency. 
   The offset current causes frequency  1  signal to cycle slightly faster than the frequency  2  signal when the speed control loop is locked. This is important to prevent a certain error mode where both chips would try to go slightly more slowly than each other, and would eventually slow down to a complete stop. By biasing the frequency detector circuit, one can make each chip try to cycle slightly faster than the other. What happens instead is that the slow chip operates at its maximum frequency (because its control loop is pegged against its maximum speed of operation) and the fast chip operates slightly faster than that. 
   On the other hand, to ensure that the faster chip is not operating too fast, the frequency broadcast should be slightly slower than the chip&#39;s maximum speed of operation. For example, if the offset current causes the frequency control loop to attempt to operate 1% faster than the other chip, then the frequencies broadcast should be at least 1% slower than the chips&#39; maximum frequencies. 
   Note that, during a possible start-up condition, if the faster chip is much faster than the slower chip, then the slower chip may not be able to recognize the faster chip&#39;s frequency. Instead, the slower chip may mistake the faster chip&#39;s frequency signal for a DC signal. In this case, the slower chip will initially slow down its speed. This, however, is acceptable, because the faster chip will slow down its speed of operation. Eventually the faster chip will slow down enough such that the slower chip can correctly recognize the faster chip&#39;s frequency. At this point the control loop will operate correctly and the slower chip&#39;s oscillator will speed back up to its maximum speed of operation. 
   An alternative to transmitting a chip&#39;s full oscillating frequency to other chips is transmitting a frequency that is a fraction of the chip&#39;s full oscillating frequency, wherein this transmitted frequency is proportional to the full oscillating frequency by a factor K. In this case, the frequency detector circuit in a receiving chip ideally generates a V ctl  that makes its oscillator operate at a frequency that is K times as fast as the detected external frequency. 
   Example Waveforms 
     FIG. 3A  shows the exemplary waveform of an input frequency signal coupled to the “frequency  1 ” input in  FIG. 2 . Correspondingly,  FIG. 3B  shows the waveform at V edge1 . Note that the pulse Of V edge1  is a downward pulse, and the starting (falling) edge of a pulse corresponds to a rising edge of the input frequency signal. In addition, the pulse width is independent of the duty cycle of the input frequency signal. 
     FIG. 3C  shows the exemplary waveform of an input frequency signal coupled to the “frequency  2 ” input in  FIG. 2 . Correspondingly,  FIG. 3D  shows the waveform at V edge2 . Note that the pulse of V edge2  is an upward pulse, and the starting (rising) edge of a pulse corresponds to a rising edge of the input frequency signal. 
     FIG. 4  illustrates an exemplary waveform of the output control voltage (V ctl ) from a frequency detector in accordance with an embodiment of the present invention. In this example, V ctl  is slowing down the local oscillator to match the speed of a slower chip. 
   The foregoing descriptions of embodiments of the present invention have been presented for purposes of illustration and description only. They are not intended to be exhaustive or to limit the present invention to the forms disclosed. Accordingly, many modifications and variations will be apparent to practitioners skilled in the art. Additionally, the above disclosure is not intended to limit the present invention. The scope of the present invention is defined by the appended claims.