Patent Abstract:
Methods and systems for controlling delay relatively independent of process, supply-voltage, and/or temperature (“PVT”) variations include sensing an output signal after a number of inverters and activating different numbers of transistors and/or adjusting strength of transistors in a delay path to compensate for PVT variations. In an embodiment, a waveform is received, delayed, and output to an output terminal using at least one relatively low-power device. Supplemental output power is provided by at least one relatively high-power device until the output waveform exceeds a threshold.

Full Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     This application claims priority to U.S. Provisional Application No. 60/357,878, filed Feb. 21,2002, titled “Delay Circuit With Delay Relatively Independent of Process, Voltage, and Temperature Variations,” incorporated herein by reference in its entirety. 
    
    
     BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention is directed to delay circuits and, more particularly, to delay circuits that are implemented in integrated circuits that are fabricated with reduced feature-size technologies, wherein the delay circuits compensate for process, supply-voltage and/or temperature variations that could otherwise affect the integrated circuits. 
     2. Background Art 
     Integrated circuits are fabricated using reduced feature-size technologies, which have significant variations in device characteristics across the process, supply-voltage and temperature (PVT) corners. PVT variations can lead to reduced rise and/or fall times. Reduced rise and/or fall times tend to appear as unexpected delay because the signals do not reach their intended level until later than expected. For extracting maximum benefit from a given process technology, among other things, the delay across various paths of the circuit has to be controlled such that the delay variation across PVT is minimal. 
     Methods and systems are needed for controlling delay caused by PVT variations. 
     BRIEF SUMMARY OF THE INVENTION 
     The present invention is directed to methods and systems that enable control of delay, relatively independent of process, supply-voltage and/or temperature (“PVT”) variations. This is made possible by, for example, sensing the output signal after a pre-determined number of inverters and adjusting the gate drive of transistors in the delay path to compensate for PVT variations. 
     Further features and advantages of the invention, as well as the structure and operation of various embodiments of the invention, are described in detail below with reference to the accompanying drawings. It is noted that the invention is not limited to the specific embodiments described herein. Such embodiments are presented herein for illustrative purposes only. Additional embodiments will be apparent to persons skilled in the relevant art(s) based on the teachings contained herein. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS/FIGURES 
     The present invention will be described with reference to the accompanying drawings. The drawing in which an element first appears is typically indicated by the leftmost digit(s) in the corresponding reference number. 
     FIG. 1 is a schematic diagram of a delay block in accordance with an aspect of the invention. 
     FIG. 2 is a block diagram of a series of delay blocks, in accordance with an aspect of the invention. 
     FIG. 3 is a logic diagram of a delay block in accordance with an aspect of the invention. 
     FIG. 4 is a schematic diagram of another delay block in accordance with an aspect of the invention. 
     FIG. 5 is a schematic diagram of another delay block in accordance with an aspect of the invention. 
     FIG. 6 is a schematic diagram of another delay block in accordance with an aspect of the invention. 
     FIG. 7 is an example process flowchart for compensating for PVT variations, in accordance with an aspect of the invention. 
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     FIG. 1 illustrates an example PVT-compensated delay block (“delay block”)  100 , in accordance with the invention. The delay block  100  includes a rising edge path  102  and a falling edge path  104 . The rising edge path  102  processes rising edges of a received waveform  108 . The falling edge path  104  processes falling edges of the received waveform  108 . In an alternative embodiment only the rising edge path  102  or the falling edge path  104  is implemented. 
     The rising edge path  102  includes a weak path  102 A and a strong path  102 B. Similarly, the falling edge path  104  includes a weak path  104 A and a strong path  104 B. The weak paths  102 A and  104 A include one or more relatively weak transistors. The strong paths  102 B and  104 B include one or more relatively strong transistors. 
     When used herein, the phrases, “weak transistor” and “strong transistor” refer to relative drive capabilities of transistors. Weak transistors are transistors with lower width/length ratios. Strong transistors are transistors with higher width/length ratios. Weak transistors are advantageous because they typically require lower power supply voltage level and typically consume less power than strong transistors. Weak transistors are thus often preferred where power consumption is sought to be minimized. Weak transistors, however, tend to be more susceptible to PVT variations than strong transistors. PVT variations typically result in reduced rise times and/or reduced fall times. Reduced rise and/or fall times tend to appear as increased delay because the waveform does not reach a desired amplitude until later than expected. 
     The weak paths  102 A and  104 A receive the input waveform  108  from an input terminal  110 . The weak paths  102 A and  104 A delay the received waveform  108  by a desired amount and output a delayed waveform  126  at an output terminal  128 . In the example of FIG. 1, the weak paths  102 A and  104 A include a series of inverters. The invention is not limited, however, to this example. The weak paths  102 A and  104 A may include any suitable circuitry that is susceptible to PVT variations. 
     The strong paths  102 B and  104 B receive feedback from the weak paths  102 A and  104 A, respectively. In FIG. 1, a feedback block  106  is coupled between the output terminal  128  and the strong paths  102 B and  104 B. When feedback indicates that the weak paths  102 A and/or  104 A are adversely affected by PVT variations, (e.g., reduced rise and/or fall times), the associated strong path  102 B and/or strong path  104 B provide additional output drive power to correct for the PVT variations. The additional output drive power increases the rise and/or fall times of the delayed waveform, thus compensating for the PVT variations. 
     Functional features of the delay block  100  are illustrated in FIG. 3, with a logic block diagram  300 . 
     Operation of the delay block  100 , as illustrated in FIG. 1, is now described. The description begins with rising edge path  102 . Within the rising edge path  102 , the rising edge weak path  102 A includes a circuit element  112  and an output driver  122 . The circuit element  112  includes an inverter  114 , implemented here with a PMOS device  116  and an NMOS device  118 . The inverter  114 , has an inherent amount of delay. Additional inverters  114  can be added if desired. The output driver  122  includes a PMOS device  124  which typically includes an additional inherent delay. 
     The rising edge weak path  102 A receives the input waveform  108  from the input terminal  110 . The circuit element  112  delays the waveform  108  by some desired amount and outputs an interim delayed waveform  120  to the output driver  122 . The PMOS device  124  optionally further delays the interim delayed waveform  120  and outputs the output delayed waveform  126  to the output terminal  128 . 
     Where, as in this example, the circuit element  112  includes an inverter, the interim delayed waveform  120  is an inverted delayed representation of the input waveform  108 . The PMOS device  124  inverts the interim delayed form  120  and outputs the output delayed waveform  126 . 
     Where, as in this example, the rising edge weak path  102 A includes inverters, an even number of inverters is preferably used. In this way, output delayed waveform  126  will be substantially similar to the input waveform  108 , but delayed in time by the inherent delay of the circuit element(s)  112  and the output driver  122 . 
     In accordance with the invention, the output driver  122  and, optionally, the circuit element  112  are implemented with one or more relatively weak transistor devices, meaning devices that consume relatively little power. Under normal operating conditions, as the input waveform  108  rises, the output delayed waveform  126  from the rising edge weak path  102 A also rises, but delayed in time by an expected amount of time relative to the input waveform  108 . However, when process, supply-voltage, and/or temperature (“PVT”) variations adversely affect the relatively weak transistor devices within rising edge weak path  102 A, the output delayed waveform  126  will rise and/or fall more slowly than the input waveform  108 . This will make the output delayed waveform  126  appear to be delayed more than the expected delay time. 
     The falling edge weak path  104 A operates in a manner similar to the rising edge weak path  102 A, taking into account that the rising edge weak paths pulls the output signal  128  up, while the falling edge weak path pulls the output signal  128  down. 
     In order to compensate for PVT variations, the feedback block  106  senses conditions of the output delayed waveform  126 , and controls the strong paths  102 B and  104 B to provide additional output drive, as needed, to compensate for PVT variations. The feedback block  106  receives the delayed output waveform  126  and outputs a feedback signal  136  to the strong paths  102 B and  104 B. The feedback block  106  varies the feedback signal  136  in accordance with the level of the output delayed waveform  126 . 
     In the example of FIG. 1, the feedback block  106  includes an inverter  130 , implemented as a PMOS device  132  and an NMOS device  134 . As the output delayed waveform  126  rises, the feedback signal  136  falls. Conversely, as the output delayed waveform  126  falls, the feedback signal  136  rises. The feedback signal  136  is provided to a node  145 , which is coupled to the strong paths  102 B and  104 B. 
     In the example of FIG. 1, the rising edge strong path  102 B and the falling edge strong path  104 B are designed to provide supplemental output drive unless the feedback block  106  disables the strong paths  102 B and  104 B. The feedback block  106  disables the strong paths  102 B and  104 B when the output delayed waveform  126  rises or falls within the expected time. 
     The rising edge strong path  102 B is now described. The rising edge strong path  102 B includes an output driver  148 , implemented here with a PMOS device  150 . The PMOS device  150  includes a drain terminal coupled to VDD and a source terminal coupled to the output terminal  128 . A gate terminal of the PMOS device  150  is controlled by a voltage at a node  144 . The voltage at the node  144  controls the PMOS device  150  as follows. As the voltage at the node  144  falls, the PMOS-device  150  turns on, which increasingly couples VDD to the output terminal  128 . This increases the current to the output terminal  128 . As the voltage at the node  144  rises, the PMOS device  150  turns off, increasingly isolating VDD from the output terminal  128 . This decreases the current provided to the output terminal  128 . 
     Control of the voltage at the node  144  is now described. The rising edge strong path  102 B further includes PMOS devices  138  and  146 , and NMOS devices  140  and  142 . Recall that when the output delayed waveform is low, the feedback signal  136  at the node  145  is high. This turns on the NNTOS device  142 . When the NMOS device  142  turns on, the PMOS device  138  and the NMOS device  140  form an inverter. The PMOS device  138  and the NMOS device  140  include gate terminals coupled to the input terminal  110 , which forms the input of the inverter. The inverter formed by the PMOS device  138  and the NMOS device  140  has an inherent delay, so that a delayed, inverted representation of the input waveform  108  appears at the node  144 . As the input waveform  108  rises, the output of the inverter, node  144 , falls. As described above, this increasingly turns on the PMOS  150 , which pulls the output terminal  128  toward VDD. In other words, as the input waveform  108  rises, and when the output delayed waveform  126  is slow to rise relative to the delayed inverter waveform at the node  144 , the rising edge strong path  102 B pulls up the output terminal  128  toward VDD. 
     When the level at the output terminal  128  rises, the feedback signal  136  disables the PMOS device  150 , as now described. Recall that, as the output waveform  126  rises, the feedback signal  136  falls. As the feedback signal  136  falls, the NMOS device  142  turns off, which isolates the node  144  from the NMOS device  140 . This prevents the NMOS device  140  from pulling down the node  144 . Furthermore, as the feedback signal  136  falls, it controls a gate terminal of the PMOS device  146  to increasingly couple VDD to the node  144 . As the node  144  rises toward VDD, it increasingly turns off the PMOS device  150 . This increasingly isolates VDD from the output terminal  128 , which reduces the supplemental drive provided to the output terminal  128 . At this point, the rising edge weak path driver PMOS device  124  should be able to drive the output delayed waveform  126 . 
     The rising edge weak path  102 A and the rising edge strong path  102 B are designed with relative delays and transistor thresholds so that, under normal operating conditions, when the input waveform  108  rises, the output delayed waveform  126  rises within a desired delay time. When this occurs, the feedback signal  136  falls quickly enough to couple the node  144  to VDD, disabling the output driver  148  before the input waveform  108  propagates through the NMOS device  140  to the node  144 . When, however, the output delayed waveform  126  does not rise withing the desired delay time, the input waveform  108  propagates through the NMOS device  140  to the node  144  and turns on the PMOS device  150 . The PMOS device  150  remains on until the feedback signal  136  falls in response to the rising output delayed waveform  126 , or until the input waveform  108  falls. 
     When the input signal  108  falls, the PMOS device  150  terminates the output drive from the rising edge strong path  102 B as follows. When the input signal  108  falls, the NMOS device  140  turns off, isolating the node  144  from the low potential VSS. Furthermore, as the input signal  108  falls, the PMOS device  138  turns on, coupling the node  144  to VDD, which turns off the PMOS device  150 . Thus, as the input signal  108  falls, the output driver  148  terminates the output drive from the rising edge strong path  102 B. Similarly, as the input waveform  108  falls, the output of the inverter  114  in rises, turning off the PMOS device  124 , thus terminating the output of the rising edge weak path  102 A. Furthermore, as the input signal  108  falls, falling edge path  104  pulls the output delayed waveform  126  down to the potential of VSS in a similar fashion to the rising edge path  102 , as will be apparent to one skilled in the relevant art(s) based on the description herein. 
     The present invention thus allows use of reduced feature-size technologies for normal operation, while providing back-up circuitry to provide compensation as needed, such as for PVT variations. 
     FIG. 2 is a block diagram of multiple delay blocks  100  coupled in series to obtain a desired overall delay. A first delay block  100 A receives the waveform  108  and outputs a delayed waveform  126   a , substantially as described above with respect to FIG. 1. A second delay block  100 B receives the outputted delayed waveform  126   a  and delays it further and outputs delayed waveform  126   b . This is repeated by subsequent delay blocks through to delay block  120   i , which outputs a final output delayed waveform  126   i.    
     FIG. 4 illustrates another example embodiment of the delay block  100 . 
     In this example, the rising edge weak path  102   a  and the falling edge weak path  104   a  are integrated into a single weak path  402 , while the feedback block  106  is implemented with a rising edge feedback block  106   a  and a falling edge feedback block  106   b . The weak path  402  includes multiple circuit elements  112 , illustrated here as an inverters  114   a - 114   c , and inverting output driver  122 . In this embodiment, the inverter  114   a  is referred to as an initial delay element, and the inverting output driver  122  is referred to as a final delay element. Operation of the delay block  100  illustrated in FIG. 4 is substantially similar to operation of the delay block  100  illustrated in FIG.  1 . 
     In accordance with the invention, one or more of the devices within the weak paths  102 A,  104 A, and  402  are relatively weak devices, and one or more of the devices within the strong paths  102 B,  104 B are relatively strong devices. In the example of FIG. 4, and without limitation, the weak path  402  includes a PMOS device  410  implemented with widths of approximately 0.93 microns and lengths of approximately 0.39 microns, and an NMOS device  418  implemented with widths of approximately 0.49 microns and lengths of approximately 0.39 microns. Within the rising edge strong path  102 B, the PMOS device  150  is implemented with widths of approximately 0.93 microns and lengths of approximately 0.13 microns. Within the falling edge strong path  104 B, the NMOS device  152  is implemented with widths of approximately 0.49 microns and lengths of approximately 0.13 microns. The invention is not, however, limited to these examples. Based on the description herein, one skilled in the relevant art(s) will understand that other widths, lengths, and/or width/length ratios can be implemented as well. 
     FIG. 5 illustrates another example embodiment of the delay block  100 , wherein the feedback blocks  106 A and  106 B are designed to sense current at the output terminal  128 . In previous drawing figures, the feedback block  106  was designed to sense primarily voltage levels at the output terminal  128 . 
     In FIG. 5, feedback block  106 A includes a PMOS device  502 , configured as a capacitor, and an NMOS device  504  configured as a diode. As the output delayed waveform  126  voltage increases with time (dV/dt), a current flows from a gate of the PMOS device  502  to a node  510 . This current flows through diode connected NMOS device  504  to a relatively low potential, illustrated here as ground. The current flow thought the diode connected NMOS device  504  generates a voltage at the node  510 , proportional to the dV/dt of the output delayed waveform  126 . 
     The feedback block  106 A further includes an inverter formed by a PMOS device  506  and an NMOS device  508 . The inverter inverts the signal at the node  510  and outputs the inverted signal at a node  136   a . In operation, when the dV/dt of the output delayed waveform  126  is sufficiently high, the voltage at the node  510  increases. As the voltage at the node  510  increases, the voltage at the node  136   a  decreases. As the voltage at the node  136   a  decreases, the PMOS device  146  increasingly turns on, which turns off the output driver PMOS device  150 . In other words, when the output delayed waveform  126  rises at or greater than a desired dV/dt, the output driver  150  does not provide supplemental output drive. 
     Another way of analyzing the operation of the feedback block  106 A is to consider the current flow. The NMOS device  504  forms a current mirror with the NMOS device  508 . The width/length ratios of the NMOS devices  504  and  508  determine the current ratio between the NMOS devices  504  and  508 . As the current through the NMOS device  508  increases, it pulls down the node  136   a.    
     An optional enable/disable feature is provided by a line  514  coupled between an output of the first inverter  114   a  and a gate terminal of an NMOS device  512 . When the input waveform  110  falls, the output of the first inverter  114   a  rises. This turns on the NMOS device  512 , which couples the node  510  to ground. This turns on the PMOS device  506 , which couples the node  136   a  to VDD. This turns off the PMOS device  146 , which effectively prevents the feedback block  106 A from disabling the PMOS device  150 . 
     The falling edge feedback block  106 B operates in a similar fashion to the rising edge feedback block  106 A, taking into account that the falling edge strong path  104 B pulls the output terminal  128  down when the input waveform  108  falls. 
     FIG. 6 illustrates another example implementation of the delay block  100 , wherein the delay block  100  includes multiple circuit elements  112   a ,  112   b , and wherein the feedback block receives feedback from a point prior to the output terminal  128 . In the example of FIG. 6, the feedback blocks  106 A and  106 B receive an interim delayed waveform  602  from the circuit element  112   a . Where the feedback blocks  106 A and  106 B receive an interim delayed waveform  602  from a subsequent circuit element  112 , additional delay circuitry can be included in the feedback blocks  106 A and  106 B, and/or in the strong paths  102 A and  102 B, to compensate for the additional delay encountered in the subsequent circuit elements  112 . The rising edge strong path  102 B and the falling edge strong path  104 B provide compensation  604  to the output terminal  128 , substantially as described above. Delay in subsequent delay elements, illustrated here as circuit element  112 B, can be accounted for with one or more compensation-path delay elements  606 . 
     An advantage of the delay block  100  illustrated in FIG. 6 is that the single set of feedback blocks  106 A and  106 B, and a single set of rising edge strong path  102 B and falling edge strong path  104 B are required for a plurality of circuit elements  112 . One or more of the delay blocks  100  illustrated in FIG. 2 can be implemented as illustrated in FIG.  6 . 
     FIG. 7 illustrates a process flowchart  700  in accordance with an aspect of the invention. The process flowchart  700  is described with reference to the example delay block  100  illustrated in FIGS. 1 through 6. The process flowchart  700  is not, however, limited to the example output block  100  illustrated in FIGS. 1 through 6. Based on the description herein, one skilled in the relevant art(s) will understand that the process flowchart  700  can be implemented with other circuits as well. Such other implementations are within the spirit and scope of the present invention. 
     The process begins at step  702 , which includes, receiving a waveform. In the example of FIG. 1, the waveform  108  is received at the input terminal  110 . 
     Step  704  includes delaying the waveform. In the example of FIG. 1, rising edges of the waveform  108  are delayed by the circuit element  112  in the rising edge weak path  102 A, which outputs the delayed waveform  120 . Falling edges of the waveform  108  are delayed by the circuit element  112  in the falling edge weak path  104 A. 
     Step  706  includes outputting the delayed waveform to an output terminal using at least one lower-power driver transistor. In the example of FIG. 1, the PMOS device  124  outputs rising edges of the delayed waveform  108  as an output delayed signal  126  to the output terminal  128 . Falling edges of the delayed waveform  108  are output to the output terminal  128  by the falling edge weak path  104 A. The invention is not, however, limited to this example embodiment. 
     Step  708  includes providing supplemental output drive to the output terminal after an expected period of delay, using at least one higher-power driver transistor. In the example of FIG. 1, supplemental output drive is provided by the strong paths  102 B and  104 B, after an inherent delay of the strong paths  102 B and  104 B. The invention is not, however, limited to this example embodiment. 
     Step  710  includes sensing a level of the delayed waveform. Step  710  can be performed by sensing voltage and/or current levels. In the examples of FIGS. 1,  4 , and  6 , the feedback block  106  senses primarily voltage levels. In the example of FIG. 5, the feedback block  106  senses primarily current levels. 
     Step  712  includes reducing the supplemental output drive as the sensed level rises above a threshold. In the example of FIG. 1, the feedback block  106  disables the output drivers in the strong paths  102 B and  104 B when the output delayed signal  106  rises above a threshold. For example, the feedback block  106  disables the PMOS device  150  when the output delayed signal  106  causes the feedback signal  136  to fall low enough to turn on the PMOS device  146 , as described above. 
     Steps  704 - 712  are performed for rising edge and falling edge portions of the received waveform, as illustrated in FIGS. 1-6. Steps(1) through (5) can be repeated using the first output delayed signal as a second input signal, thereby further delaying the received waveform while compensating for PVT variations, as illustrated in FIG.  2 . 
     The process flowchart  700  can be implemented to delay a received waveform with multiple delay operations, as illustrated, for example, in FIG.  6 . In this embodiment, step  704  includes performing a plurality of serial delay operations, including at least one initial delay operation and a final delay operation, on the received waveform. Step  708  includes providing supplemental output drive to the output terminal through one or more compensation-path delay elements, as illustrated by the compensation-path delay elements  606  in FIG.  6 . Step  710  includes sensing a level of the delayed waveform output from one of the initial delay operations, as illustrated in FIG.  6 . 
     The present invention has been described above with the aid of functional building blocks illustrating the performance of specified functions and relationships thereof. The boundaries of these functional building blocks have been arbitrarily defined herein for the convenience of the description. Alternate boundaries can be defined so long as the specified functions and relationships thereof are appropriately performed. Any such alternate boundaries are thus within the scope and spirit of the claimed invention. One skilled in the art will recognize that these functional building blocks can be implemented by discrete components, application specific integrated circuits, processors executing appropriate software and the like or any combination thereof. 
     When used herein, the terms “connected” and/or “coupled” are generally used to refer to electrical connections. Such electrical connections can be direct electrical connections with no intervening components, and/or indirect electrical connections through one or more components. 
     While various embodiments of the present invention have been described above, it should be understood that they have been presented by way of example only, and not limitation. Thus, the breadth and scope of the present invention should not be limited by any of the above-described exemplary embodiments, but should be defined only in accordance with the following claims and their equivalents.

Technology Classification (CPC): 7