Self-clocked logic circuit and methodology

Abstract: A circuit and method for clocking for logic circuits use delay line techniques to time the clock signal. The inputs into a logic circuit are associated with a validity signal, which is delayed by a delay line for at least the propagation delay of the logic circuit. The delayed validity signal is used to latch an output signal produced by the logic circuit in response to the inputs.

Description: The present invention relates to logic circuits such as sequential logic circuits and, more particularly, to a novel circuit and method for clocking a logic circuit. Background Art A salient feature of sequential logic circuits is memory. The output of a sequential logic circuit depends not only on the current values of applied input signals but also on the past circuit state, e.g. the sequence of previously applied input signals. Sequential logic circuits can be asynchronous or synchronous. Asynchronous sequential circuits respond immediately to changes in input signals, yielding changed output signals. However, unequal propagation delays in asynchronous circuits can lead to hazards and race conditions. To overcome these difficulties in asynchronous circuits, synchronous circuits are employed in which signals are latched into registers in response to regular clock pulses. The speed of synchronous circuits, however, is constrained by the period of the clock cycles, which depends on the propagation delays of logic blocks and registers. For example, Figure 3 depicts a synchronous circuit including two blocks of combinational logic: a first logic block 302 for computing a function "F" based on "N" inputs I 1 -I N and a second logic block 306 for computing a function "G" based on "M" inputs F 1 -F M . The circuit also includes three registers 300 , 304 , and 308 for latching bits at the beginning of each clock cycle. When "N" bits are latched in register 300 at the beginning of a clock cycle, they are applied as inputs I 1 -I N to the first logic block 302 after some propagation delay for the register 300. The first logic block 302 computes the function "F" on the input bits and produces therefrom "M" bits of output F 1 -F M . These output bits F 1 -F M are applied to a register 304, which latches those bits in response to the clock signal at the next clock cycle. After the bits F 1 -F M are latched by register 304 , they are applied the second logic block 306 . In response, the second logic block 306 computes the function "G" based on the bits F 1 -F M and produces therefrom "L" bits of output G 1 -G L . These output bits G 1 -G L are applied to register 306 and latched in response to the clock signal at the next clock cycle. Thus, the computation of functions "F" and "G" takes two clock cycles to complete. Since conventional practice tends to use a common clock signal throughout a digital circuit, the clock period is set slower than the slowest logic block. Since most functions in the circuit are faster than the slowest logic block, there is much time wasted in the digital circuit waiting for the common clock signal. In the example, let us assume that registers 300 , 304, and 308 have a propagation delay of 7ns, the first logic block 302 has a propagation delay of 21ns, and the second logic block 306 has a propagation delay of 28ns. Thus, the computation in the first clock cycle needs at least 28ns to complete (7ns for the register 300 to latch + 21ns for first logic block 302 ). The computation in the second clock cycle, on the other hand, requires at least 35ns to complete before the data is ready to be latched by register 308 (7ns for the register 304 + 28ns for the second logic block 306 ). Since registers 300, 304, and 308 share a common clock signal, the clock period is set to 35ns. Thus, the total time taken to perform the two computations, i.e. two clock cycles, is 70ns. However, 7ns of the first clock period of 35ns was spent waiting for the next clock cycle. Therefore, digital functions are conventionally cascaded at the speed of the common clock, which is slower than cascading at the speed of most of the functions themselves. Before disclosing the present invention, the following four documents are acknowledged as background art. US-A-5, 229,668 discloses method and apparatus for high speed digital sampling of a data signal. US-A-5,033,066 discloses a time delay circuit for providing a delayed replica of a digital input signal. GB-A-2,310,738 discloses a dynamic logic pipeline in which each stage has a logic circuit, a latch circuit and a control circuit. US-A-5,565,798 discloses a self-timed control circuit for self-resetting logic circuitry. Disclosure of the Invention There is a need to improve the speed of sequential circuits. There is also need to provide a circuit and method for clocking registers in a sequential circuit at the speed of logical functions. We will describe the clocking of logic circuits using self-timed delay line techniques. Specifically, each input into a logic block or circuit is associated with a validity signal. The validity signal is delayed by a delay line for at least, but preferably within 140ns of, the propagation delay of the logic block. Thus, the delayed validity signal can be used to latch an output signal produced from the logic block in response to the inputs. Accordingly, one aspect of the invention is a sequential logic circuit, comprising: a logic circuit responsive to an input signal for computing a function therefrom and producing an output signal after a propagation delay, said input signal including a plurality of input bits; and a register for latching said output signal ; characterized in that the sequential circuit is self-clocked by further comprising: a delay line responsive to a first signal for providing a first delayed signal after a period of time at least as long as said propagation delay, said first - signal having the same number of bits as said input signal, each bit of said first signal corresponding respectively to a bit of said input signal and having a common specified bit value when the corresponding bit of said input signal is first received by said sequential logic circuit, said delay line comprising circuitry for computing a logical product on said plurality of bits of said first signal and being arranged to produce said first delayed signal after a period of time at least as long as said propagation delay when all of the plurality of bits of said first signal are all of the common specified bit value, and said register latching said output signal in response to said first delayed signal Another aspect of the present invention is a method of latching an output signal generated by a logic circuit in response to an input signal after a delay, the input signal including a plurality of input bits; said method being characterized by comprising the steps of: providing a first signal having the same number of bits as said input signal , each bit of said first signal corresponding respectively to a bit of said input signal and having a common specified bit value when the corresponding bit of said input signal is first received by said sequential logic circuit; computing a logical product on said plurality of bits of said first signal and outputting a second signal when each of the bits of said first signal has the common specified bit value; delaying said second signal for a period of time at least as long as said delay; and latching said output signal in response to said delayed second signal. Brief Description of Drawings The present invention is illustrated by way of example, and not by limitation, in the figures of the accompanying drawings, wherein elements having the same reference numeral designations represent like elements throughout and wherein: Figure 1 is a high-level block diagram of a self-clocked logic circuit according to an embodiment of the present invention. Figure 2(a) is a block diagram of a delay line that may be used to implement the circuit depicted in Figure 1. Figures 2(b) and 2(c) are circuit diagrams of a non-inverting delay stage and inverting delay stage that may be used to implement a delay stage depicted in Figure 2(a). Figure 3 is a high-level block diagram of a conventionally clocked logic circuit. Best Mode for Carrying out the Invention A circuit and method for clocking a sequential circuit are described. In the following description, for purposes of explanation, numerous specific details are set forth in order to provide a thorough understanding of the present invention. It will be apparent, however, that the present invention may be practiced without these specific details. In other instances, well-known structures and devices are shown in block diagram form in order to avoid unnecessarily obscuring the present invention. SELF-CLOCKED LOGIC According to one embodiment of the present invention, Figure 1 depicts a self-clocked sequential circuit corresponding to the conventional sequential circuit depicted in Figure 3. Specifically, logic blocks 102 and 106 of Figure 1 correspond to logic blocks 302 and 306 of Figure 3, respectively. Registers 100 , 104 , and 108 of Figure 1 correspond to registers 300 , 304 , and 308 of Figure 3, respectively. Thus, first logic block 102 computes the function "F" based on "N" inputs I 1 -I N , and second logic block 106 computes the function "G" based on "M" inputs F 1 -F M . The three registers 100, 104, and 108 latch bits applied thereto in response to the signals generated by the self-clocking circuitry including delay lines 110-118 and gate 120 . Associated with each bit of the input signal I 1 -I N is a corresponding validity bit VI 1 -VI N that is asserted when the associated input bit I 1 -I N is valid. The validity bits VI 1 -VI N may be produced asynchronously or synchronously when the input signal I 1 -I N is first received from another circuit, e.g. an off-chip circuit. For example, some input signals are transmitted to an integrated circuit with a strobe that indicates that the inputs are ready. In this situation, the associated validity signal is derived from the strobe. In another example, the input signals are valid at every cycle of an externally supplied clock signal, in which case the associated validity signal would be derived from the clock signal. Validity signals associated with signals generated by internal, i.e. on-chip, logic blocks are derived from the self-clocking circuitry for respective internal logic blocks. For the plurality of validity bits VI 1 -VI N , the self-clocking circuitry includes a gate 120, which determines when all the input bits I 1 -I N for the corresponding register 100 are valid, i.e. when all the validity bits VI 1 -VI N have been asserted. Gate 120 can be any circuitry which computes a logical product, such as an AND gate in positive logic or a NOR gate in negative logic. The output of gate 120 is coupled to the "CLOCK" input of register 100 , so that, when all the validity bits VI 1 -VI N have been asserted, the resultant, consolidated input validity signal VI causes the register 100 to latch the input bits I 1 -I N . Thus, the input bits I 1 -I N are latched only when all of the input bits I 1 -I N are valid. In addition, the output of gate 120 is coupled to a delay line 110 , which may be implemented as described in more detail hereinbelow. When "N" bits are latched in register 100 in response to the input validity signal VI , they are applied as inputs I 1 -I N to the first logic block 102 after some propagation delay for the register 100 . Meanwhile, the input validity signal VI is delayed by delay line 110 , configured to delay the input validity signal VI for a period of time at least as long as the propagation delay for the register 100 . For example, if the register 100 has a propagation delay of 7ns, then the delay line 100 would be configured to delay the input validity signal VI for at least 7ns to produce a delayed input validity signal VI' . The first logic block 102 computes the function "F" on the input bits and produces therefrom "M" bits of output F 1 -F M after some propagation delay. Meanwhile, the corresponding delayed input validity signal VI' for the input bits I 1 -I N is delayed by delay line 112, configured to delay the delayed input validity signal VI' for a period of time at least as long as the propagation delay of the first logic block 102. Thus, the output of the delay line 112 is asserted when the outputs of the corresponding first logic block 102 become valid. In other words, the output of the delay line 112 is a validity signal VF for the first logic block 102. In the example, if the first logic block 102 has a propagation delay of 21ns, then the delay line 112 would be configured to delay the delayed input validity signal VI' for at least 21ns to produce the first block validity signal VF. Since the first block validity signal VF is produced by delay line 112 when the bits F 1 -F M are valid, the first block validity signal VF is used to latch the bits F 1 -F M at the register 104. After the bits F 1 -F M are latched by register 304, they are applied the second logic block 306 after a propagation delay. Meanwhile, the first block validity signal VF is delayed by delay line 114 , configured to delay the first block validity signal VF for a period of time at least as long as the propagation delay for the register 104 . In the example, if the register 104 has a propagation delay of 7ns, then the delay line 114 would be configured to delay the first block validity signal VF for at least 7ns to produce a delayed first block validity signal VF' . The second logic block 106 computes the function "G" based on the bits F 1 -F M applied thereto and produces therefrom "L" bits of output G 1 -G L after a propagation delay. Meanwhile, the corresponding delayed first block validity signal VF' for the bits F 1 -F M is delayed by delay line 116 , configured to delay the delayed first block validity signal VF' for a period of time at least as long as the propagation delay of the second logic block 106 . Thus, the output of the delay line 116 is asserted when the outputs of the corresponding second logic block 106 become valid. In other words, the output of the delay line 116 is a validity signal VG for the second logic block 106 . In the example, if the second logic block 106 has a propagation delay of 28ns, then the delay line 116 would be configured to delay the delayed first block validity signal VF' for at least 28ns to produce the second block validity signal VG . Since delay line 116 is coupled to the "CLOCK" input of register 108 , the output bits G 1 -G L are latched by register 106 in response to the second block validity signal VG . Meanwhile, the second block validity signal VG is delayed by delay line 118 , configured to delay the second block validity signal VG for a period of time at least as long as the propagation delay for the register 108 . In the example, if the register 108 has a propagation delay of 7ns, then the delay line 118 would be configured to delay the second block validity signal VG for at least 7ns to produce a delayed second block validity signal VG' . The time elapsed from when the input bits I 1 -I N have become valid at register 100 until when the output bits G 1 -G L are ready at register 108 is the sum of the propagation delays through first logic block 102 , second logic block 106 , and registers 100 and 104 . In the example, this sum is 21ns for the first logic block 102 , 28ns for the second logic block 106 , and 7ns for each register 100 and 104 for a total of 21ns + 28ns + 7ns + 7ns = 63ns. Thus, this sequential circuit according to one embodiment of the present invention is able to latch the computation of functions "F" and "G" after 63ns, in comparison with 70ns for the conventionally clocked sequential circuit. In particular, the register latching the output of a faster logic block (e.g. register 104 and first logic block 102 , respectively) is configured to latch the output immediately without having to wait a period of time synchronized with the a slower logic block (e.g. second logic block 106 ). DIGITAL DELAY LINE Figure 2(a) is a block diagram of an exemplary high-resolution, digital delay line 200 with which the present invention can be implemented. The exemplary high-resolution digital delay line 200 receives a bit and repeatedly delays the bit through a plurality of digital delay elements 210-1 to 210-n , coupled in series. Digital delay line 200 may comprise an arbitrary number n of digital delay elements, depending upon any constraints inherent in the semiconductor implementation utilized or other practical considerations. For example, digital delay line 200 can comprise tens of thousands of digital delay elements. Since each of the digital delay elements 210-1 to 210-n of digital delay line 200 is constructed during the same manufacturing process on the same semiconductor substrate, it is likely that the operating characteristics, and hence the delay period, of each digital delay element are nearly identical. Outputs of a plurality of digital delay elements may be tapped to simultaneously monitor the digital delay elements for a delayed bit. Accordingly, digital delay line 200 comprises a plurality of taps 212-1 to 212-n coupled to the outputs of the respective digital delay elements 210-1 to 210-n for monitoring portions of the digital delay line 200 in parallel. Delay characteristics of any digital circuit will vary from chip to chip and over time because of unavoidable variations in manufacturing and operating conditions. Thus, there is a need to calibrate the delay period of each of the digital delay elements 210-1 to 210-n to match a specified period. According to one approach, both the delay period is synchronized to a reliable, precise reference clock, such as a crystal oscillator. The delay period of each of the digital delay elements 210-1 to 210-n is preferably adjustable by a digital command code as a calibration signal. This calibration signal is produced with reference to a reliable, precise clock signal, preferably by an on-chip digital servo circuit (not shown) such as described in the commonly assigned U.S. Patent No. 5,457,719, issued to Guo et al. on October 10, 1995. Briefly, the on-chip digital servo circuit comprises an adjustable digital delay line of its own, which it monitors and continually adjusts with a calibration signal in a feedback loop. The calibration signal is shared with other systems on the chip. Referring to Figure 2(b), each adjustable digital delay element 210 comprises two adjustable inverters 220-1 and 220-2 , coupled in series, each receiving the aforementioned calibration signal. Thus, the delay period of each of the two adjustable inverters 220-1 and 220-2 is one-half of the delay period of the adjustable delay element and is controlled by the calibration signal. Referring to Figure 2(c), each adjustable inverter 220 in a preferred embodiment comprises a plurality of switchable inverters 230-1 to 230-m coupled in parallel. Each of the switchable inverters 230-1 to 230-m is switched on or off by one of bits 232-1 to 232-m of the calibration signal. Thus, two of the parameters that determine the propagation delay of an inverter, the P-channel size to N-channel size ratio and the driving power, may be determined for precise control over the delay period. Switchable inverters are described in further detail in the commonly assigned U.S. Patent No. 5,220,216, issued to Woo on June 15, 1993, and the commonly assigned U.S. Patent No. 5,227,679, issued to Woo on July 13, 1993. Accordingly, digital delay line 200 comprises a series of adjustable digital delay elements 210-1 to 210-n , each of which provides a uniform delay period synchronized to a reference clock period according to a calibration signal. Moreover, each adjustable inverter 220 can have a consistent delay period of as little as 70 ps. Thus, each adjustable digital delay element 210 can have a consistent delay period of as little as 140 ps. The digital delay line 200 is implemented with enough adjustable digital delay elements of a particular delay period so that the total delay period exceeds'the delay time of the worst case path through the corresponding logic circuit. For example, if the delay time of the worst case path through the logic circuit is 28ns, then a minimum of 200 (28 ns / 140 ps) adjustable delay elements having a 140 ps delay period is required. Thus, digital delay line 200 can be configured to delay a signal within 140ps of a desired delay period. The number of adjustable, delay elements can be reduced by using adjustable delay elements with a larger period. In fact, adjustable delay elements having different periods may be used, for example nineteen with a delay period of 1.4ns and ten with a delay period of 140ps. In the configuration depicted in Figure 1, it is evident that the series of delay lines 110-118 ultimately delay a single validity signal, viz. VI . Accordingly, the series of delay lines 110-118 may be replaced with a single delay line having appropriate taps coupled to respective "CLOCK" inputs of registers 104 and 108 . While this invention has been described in connection with what is presently considered to be the most practical and preferred embodiment, it is to be understood that the invention is not limited to the disclosed embodiments, but, on the contrary, is intended to cover various modifications and equivalent arrangements included within the scope of the appended claims.

Claims: A sequential logic circuit, comprising: a logic circuit (102) responsive to an input signal (I 1 - I n ) for computing a function therefrom and producing an output signal (F 1 - F m ) after a propagation delay, said input signal (I 1 - I n ) including a plurality of input bits; and a register (104) for latching said output signal (F 1 - F m ); characterized in that the sequential circuit is self-clocked by further comprising: a delay line (120, 110, 112) responsive to a first signal (VI 1 - VI n ) for providing a first delayed signal (VF) after a period of time at least as long as said propagation delay, said first - signal (VI 1 - VI n ) having the same number of bits as said input signal (I 1 - I n ), each bit of said first signal (VI 1 - VI n ) corresponding respectively to a bit of said input signal (I 1 - I n ) and having a common specified bit value when the corresponding bit of said input signal (I 1 - I n ) is first received by said sequential logic circuit, said delay line (120, 110, 112) comprising circuitry (120) for computing a logical product on said plurality of bits of said first signal (VI 1 - VI n ) and being arranged to produce said first delayed signal (VF) after a period of time at least as long as said propagation delay when all of the plurality of bits of said first signal (VI 1 - VI n ) are all of the common specified bit value, and said register (104) latching said output signal (F 1 - F m ) in response to said first delayed signal (VF). The self-clocked circuit of claim 1, wherein said circuitry for computing a logical product includes an AND gate (120) responsive to said plurality of bits of said first signal (VI 1 - VI n ). The self-clocked circuit of claim 1, wherein said circuitry for computing a logical product includes a NOR gate (120) responsive to said plurality of bits of said first signal (VI 1 - VI n ). The self-clocked circuit of any one of claims 1 to 3, further comprising another delay line (114) responsive to said first delayed signal (VF) for producing a second delayed signal (VF) after a period of time at least as long as a delay period of said register (104). The self-clocked circuit of any one of claims 1 to 4, wherein said delay line (120, 110, 112) is arranged to produce said first delayed signal (VF) within 140ps of said propagation delay. The self-clocked circuit of any one of claims 1 to 4, wherein said delay line (120, 110, 112) and/or said another delay line (114) includes a plurality of digital delay elements (210-1 to 210-n) coupled in series. The self-clocked circuit of claim 6, wherein: said delay line and/or said another delay line further includes a calibration input coupled for receiving a calibration signal (CALIBRATE); and each of the digital delay element (210-1 to 210-n) includes a calibration input (CALIBRATE) for receiving the calibration signal for adjusting the delay period thereof. The self-clocked circuit of claim 7, wherein each of the digital delay elements (210-1 to 210-n) includes: a first adjustable inverter (220-1) having a first calibration input for receiving the calibration signal (CALIBRATE); and a second adjustable inverter (220-2), coupled in series to said first adjustable inverter (220-1), and having a second calibration input for receiving said calibration signal (CALIBRATE). The self-clocked circuit of any one of claims 1 to 8, wherein a delay period of each of the digital delay elements is about 140ps. The self-clocked circuit of any one of claims 1 to 9 wherein a delay period of each of the digital delay elements is the same. A method of latching an output signal (F 1 - F m ) generated by a logic circuit (102) in response to an input signal (I 1 - I n ) after a delay (112), said input signal including a plurality of input bits (I 1 - I n ), said method being characterized by comprising the steps of: providing a first signal (VI 1 - VI n ) having the same number of bits as said input signal (I 1 - I n ), each bit of said first signal (VI 1 - VI n ) corresponding respectively to a bit of said input signal (I 1 - I n ) and having a common specified bit value when the corresponding bit of said input signal (I 1 - I n ) is first received by said sequential logic circuit; computing a logical product on said plurality of bits of said first signal (VI 1 - VI n ) and outputting a second signal (VI) when each of the bits of said first signal (VI 1 -VI n ) has the common specified bit value; delaying (110, 112) said second signal (VI) for a period of time at least as long as said delay; and latching (104) said output signal (F 1 - F m ) in response to said delayed second signal (VF). The method of claim 11, further comprising the step of delaying the delayed second signal (VF) for another period of time (114) at least as long as said step of latching consumes. The method of any one of claims 11 and 12, wherein the delaying said second signal for a period of time at least as long as said delay includes the step of delaying said - second signal within 140ps of said delay. The method of any one of claims 11 to 13, wherein the step of delaying said second signal for a period of time at least as long as said delay includes the step of repeatedly delaying said second signal by a common delay period (210-1 to 210-n). The method of claim 14, further comprising the step of calibrating (CALIBRATE) said common delay period.

Inventors: RELPH RICHARD, RELPH, RICHARD
Applicants: ADVANCED MICRO DEVICES INC, ADVANCED MICRO DEVICES INC.
Application year: 1998
Main classification: H03K3
IPC codes: G11C19, H03K3, H03K19