Patent Publication Number: US-6668334-B1

Title: Apparatus for detecting clock failure within a fixed number of cycles of the clock

Description:
FIELD OF THE INVENTION 
     The invention pertains to the field of integrated circuits. More particularly, the invention pertains to an apparatus for detecting a clock failure in an integrated circuit. 
     BACKGROUND OF THE INVENTION 
     Digital and mixed-signal integrated circuits (ICs) usually depend on the presence of a clock, a signal consisting of a repetitive and ideally unchanging sequence of alternating digital ‘1’s and ‘0’s, to drive the sequential digital logic within these circuits. If the clock signal is disrupted, or stopped altogether, the digital circuits triggered off of this clock may function incorrectly or cease operation altogether. Such a disruption or stopping of the clock is referred to in the art as a “loss of clock.” A loss-of-clock (LOC) detector—a circuit that monitors the clock signal and asserts a warning signal (flag) when the clock is disrupted—is thus clearly important in critical systems. The loss-of-clock detector warning signal may be used to indicate to the system controller that the clock has been disrupted, and that the data processed by the digital subsystems that depend on this clock may be corrupted. Alternatively, the warning signal may be used to force automatic selection of an alternative clock source if the primary clock signal is disrupted. 
     The concept of a loss-of-clock detector is well known in the art. As indicated in FIG. 1, a typical LOC detector  100  is implemented by using pull-up resistor  101  to charge capacitor  102  up to the supply voltage  103 , and by using grounded switch  104  to discharge capacitor  102 . A clock edge detector  105  causes switch  104  to close momentarily after each rising edge (transition from logic ‘0’ to logic ‘1’) and after each falling edge (transition from logic ‘1’ to logic ‘0’) of the input clock, CLK. The voltage across capacitor  102  is input to comparator  106 , the output of which is the loss-of-clock signal (LOCFLG)  107 . Comparator  106  operates in such a way that LOCFLG  107  is logic ‘1’ whenever the comparator input voltage exceeds the comparator trip voltage and logic ‘0’ otherwise. The comparator trip voltage is equal to the voltage of Voltage Reference  108 , V TR . 
     The values of resistor  101  and capacitor  102  are chosen so that the resulting resistor-capacitor (RC) time constant, R PU ×C, is much higher than the expected time between transitions or edges of CLK. Thus, if the input clock is operating normally, switch  104  is closed often enough to prevent resistor  101  from charging capacitor  102  past the comparator trip voltage. However, when the clock stops functioning, switch  104  remains open and capacitor  102  is charged up towards V DD . Once the voltage across capacitor  102  passes the trip voltage, V TR , of comparator  106 , loss-of-clock signal  107  goes to logic ‘1’, indicating that the input clock has stopped. 
     Referring to FIGS. 2A-C, in a typical timing diagram of the LOC detector function, the input clock signal CLK is monitored for rising and falling edges whose presence holds the voltage across the capacitor  102  low, and thus the loss-of-clock signal (LOCFLG) is held low as shown. Following the final transition of the disrupted input clock, after the detection time, T LOC , the output of the loss-of-clock detector  100 , LOCFLG, is asserted high. 
     In an ideal case, the value of T LOC  would be a specific number of clock periods (cycles). However, due to the impreciseness of manufacturing tolerances of integrated circuits, the value of T LOC  varies greatly. Referring to FIG. 2C, the desired detection time window is typically set by system level concerns such as detecting the disruption of a clock within a certain time interval of the disruption occurring. The desired detection time window opens (begins) a small time interval, T DECMIN , after the final transition of the disrupted input clock. The desired detection time window closes (ends) after a larger time interval, T DECMAX , following the final transition of the disrupted input clock. Manifestly, T DECMAX  is greater than T DECMIN , and T DECMIN  may be zero, which means the desired detection window opens coincident with the final transition of the input clock. An operational (functional) loss-of-clock detector  100  asserts LOCFLG so that T LOC  is greater than T DECMIN  and less than T DECMAX . When the input clock signal is restored, transitions or edges resume. As shown, coincident with the first transition of the restored input clock, the loss-of-clock signal is de-asserted low. 
     A drawback to this prior art LOC detector is that the detection time, T LOC , is set by the R PU ×C time constant. Accordingly, manufacturing variations in R PU  or C, which can be for example as large as 50% of the nominal component values, or variation of R PU  with chip temperature, which can be for example as large as 20% over the specified operating temperature range of most integrated circuits, limit how precisely T DECMIN  and T DECMAX  can be controlled, thus leading to a large detection time window. In addition, since R PU  and C are fixed quantities, the time constant R PU ×C does not vary with the frequency of the input clock, F CLK . Thus when F CLK &gt;&gt;1/(R PU ×C), a significant number of clock periods pass before the circuit detects that the clock has been lost. 
     Alternative LOC detector architectures that partially overcome these drawbacks are also known from the prior art; however, each of these architectures introduces a new set of drawbacks. For example, pull-up resistor  101  in FIG. 1 can be replaced by a constant current source; the resulting time constant that controls T LOC  is insensitive to resistor variation. However, this circuit requires a well-controlled current source, which may require a manual tuning process; further, this improvement fails to mitigate the effect of capacitor variation, or the variation of frequency of the input clock. 
     An LOC detector can also be implemented without any precision analog components by using a secondary clock and digital circuitry to sample the primary input clock, keeping track of how much time has elapsed since the previous transition. Obviously, this requires a second clock, something that may not be available in every system. Further, if the secondary clock itself fails, the detector that monitors the primary clock will also fail. 
     SUMMARY OF THE INVENTION 
     Briefly stated, an apparatus for detecting a failure of an input clock and generating a loss-of-clock signal includes a frequency-to-current converter for generating a charging current substantially proportional to a frequency of the input clock, capacitor for accepting the charging current and providing a terminal voltage that changes in response to the charging current, an edge detector receiving the input clock signal as an input and producing an output pulse on an edge of the input clock signal, a first switch coupled to the capacitor such that the capacitor is discharged to a reference potential when the first switch is closed, and wherein the first switch is controlled by the edge detector to close when the edge detector output pulse is asserted, and comparator for generating a loss-of-clock signal when the voltage on the capacitor passes (i.e., exceeds or drops below) a specified value of trip voltage. 
     The frequency-to-current converter of the loss-of-clock detector of the present invention avoids the disadvantages of prior art circuits by providing a value of output current substantially linearly proportional to the frequency of the input clock and to a second capacitor whose capacitance tracks the capacitance of the first capacitor. As a result, T LOC  is substantially equal to a fixed number of clock periods and virtually independent of varying clock frequency, manufacturing process variations, and chip temperature, allowing for a significant narrowing of the desired detection time window over current state of the art circuits. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 shows a schematic overview of a loss-of-clock (LOC) detector according to the prior art. 
     FIG. 2A shows a timing diagram for an input clock that is temporarily disrupted. 
     FIG. 2B shows a timing diagram for a loss-of-clock signal set in relation to the input clock of FIG.  2 A. 
     FIG. 2C shows a timing diagram for the desired detection time window in relation to the input clock of FIG.  2 A and the loss-of-clock signal set in FIG.  2 B. 
     FIG. 3 shows a schematic overview of a loss-of-clock (LOC) detector according to the present invention. 
     FIG. 4A shows a schematic diagram of an embodiment of a frequency-to-current converter according to the present invention. 
     FIG. 4B shows a schematic diagram of an alternative embodiment of a frequency-to-current converter according to the present invention. 
     FIG. 5 shows a schematic diagram for an embodiment of a clock generator according to the present invention. 
     FIG. 6 shows a schematic diagram of a frequency-to-current converter according to the present invention. 
     FIG. 7 shows a schematic diagram of a comparator, a capacitor, and a switch, according to the invention. 
     FIG. 8 shows a schematic diagram of a voltage reference according to the invention. 
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENT 
     To assist in a better understanding of the invention, a specific embodiment of the present invention will now be described in detail. Although such is the preferred embodiment, it is to be understood that the invention can take other embodiments. This detailed description will include reference to FIGS. 1 through 8. The same reference numerals will be used to indicate the same parts and locations in all the figures unless otherwise indicated. For example, various portions of the description make reference to MOS transistors. Although such transistors are preferable, as is apparent to one skilled in the art another type of transistor could be used without substantively altering the invention, including without limitation bipolar junction transistors or junction field-effect transistors. 
     Referring to FIG. 3, a loss-of-clock (LOC) detector  200  includes a frequency-to-current converter  202 , to which the clock signal CLK is input. 
     The current output of frequency-to-current converter  202 , labeled I CH  in FIG. 3, is substantially directly proportional to the frequency of the input clock, F CLK , and thus substantially inversely proportional to the period of the input clock, T CLK . That is,                  I   CH     =         K   F2I     ×     F   CLK       =       K   F2I       T   CLK           ,           (   1   )                         
     where K F2I  is a proportionality constant. This output current charges a capacitor  206  having a capacitance designated C 2 . This capacitor is preferably a MOS gate capacitor. However, other capacitor types could be used in the present invention, as is readily apparent to one skilled in the art. These capacitor types include without limitation, a semiconductor junction capacitor, a linear analog capacitor, or a capacitor formed by two or more layers of interconnect separated by interlayer dielectric. 
     The input clock signal CLK is also received by an edge detector  208 , which on every edge of CLK outputs a pulse DISCH that momentarily closes a switch  210 . The duration of this pulse is sufficient to allow switch  210  to completely discharge capacitor  206  to ground. Between CLK edges, capacitor  206  is charged toward V DD  by I CH . If the time between successive CLK transitions is too long, the voltage across capacitor  206  becomes greater than a trip voltage, V TR , of a comparator  212 , set by a voltage reference  214 , thus causing signal LOCFLG to transition to logic ‘1’. The time between the final transition of CLK and the assertion of LOCFLG, T LOC , is equal to the time required to charge capacitor  206  to the comparator trip voltage. Since this charging time is proportional to the capacitance C 2  and inversely proportional to I CH , it can be seen that T LOC  is proportional to the period of the input clock, T CLK :                T   LOC     =           C   2          V   TR         I   CH       =           C   2          V   TR         K   F2I            T   CLK                 (   2   )                         
     where V TR  is the trip voltage of comparator  212 . The goal is to make the equation for T LOC  independent of the constants C 2 and V   TR . The solution of the present invention is to make I CH  substantially proportional to V TR  and to a second capacitor whose value tracks C 2 , as is further explained below. 
     LOC detector  200  is capable of maintaining a narrow desired detection time window over a wide range of frequencies; however, the range is ultimately limited by nonidealities in the transfer function of the frequency-to-current converter. One source of nonideality which is not necessary to the practice of the invention, but which is contained in the preferred embodiment, is a small constant current, also known as a trickle current, which is independent of the clock frequency. This trickle current is added to the output of converter  202  to ensure that the LOC signal will eventually be asserted even if no input clock is ever applied to the circuit. 
     FIG. 4A illustrates the preferred embodiment of frequency-to-current converter  202 . In this circuit, a noninverting input terminal of an operational amplifier  216  is driven by voltage reference  215 , the voltage of this voltage reference is substantially proportional to the trip voltage of comparator  212  in loss-of-clock detector  200 . Operational amplifier  216  drives the gate of an n-type transistor configured as a source follower  218 , and the drain current of this transistor flows through a diode-connected transistor  220 . Transistor  220  and a transistor  222  form a current mirror with a ratio of N:1 between the left and right sides of the mirror as shown. A low-pass RC filter formed by a resistor  224  and a capacitor  226  effectively passes the DC component of the gate voltage of transistor  220  to transistor  222 , and the drain current of transistor  222  is the output current, I CH . The source of transistor  218  is coupled to a switched-capacitor circuit  228 , and also to the inverting input terminal of operational amplifier  216 . This connection forms a negative feedback loop that causes the voltage of this node to return to substantially V TR  if it is disturbed. 
     Two clock phases, Phi 1  and Phi 2 , are generated from the main input clock, CLK, by a clock generator circuit  204  such that they have equal frequencies but are non-overlapping; that is, Phi 1  is asserted only after Phi 2  has been de-asserted, and Phi 2  is asserted only after Phi 1  has been de-asserted. The period of Phi 1  and Phi 2 , T Ph1,2 , is either equal to or is a whole number multiple of the period T CLK  of CLK. The ratio        M   =       T       Phi                 1     ,   2         T   CLK                       
     depends upon the implementation of clock generator  204 . 
     Circuit  228  is a switched-capacitor circuit that includes a grounded capacitor  230 , of value C 1 , and a pair of switches  232 ,  234  respectively controlled by complementary clock signals Phi 1  and Phi 2  derived from CLK by clock generator  204 . When Phi 1  is active, switch  232  is closed, and the voltage across capacitor  230  is charged to V TR . When Phi 2  is active, switch  234  is closed and capacitor  230  is discharged to ground. Thus, captor  230  is alternately charged to V TR  and discharged to ground at a frequency equal to          F       Phil                 1     ,   2       =       1     T       Phi                 1     ,   2         .                     
     Since the current delivered to capacitor  230  flows through transistor  220 , the average current in this transistor is substantially equal to                    I   220     _     =         C   1          V   TR         M   ×     T   CLK           ,           (   3   )                         
     where        M   =         T       Phi                 1     ,   2         T   CLK       .                     
     A characteristic of switched-capacitor circuit  228  is that it creates a current spike in transistor  218  immediately after switch  232  is closed; a capacitor  236  has been included to limit the effect of this current spike on the gate-source voltage of transistor  220 . 
     It is understood by one skilled in the art that the noninverting input terminal of operational amplifier  216  may be coupled instead to a voltage proportional to V TR . In such a case, the equations described herein are modified with the appropriate proportionality factor. 
     The output of frequency-to-current converter  202 , I CH , is a current substantially equal to the average current {overscore (I 220  )} of transistor  220  scaled by k/N                I   CH     =       (     k     M   ×   N       )                         C   1          V   TR         T   CLK                 (   4   )                         
     where T CLK  is the period of the input clock signal CLK, 1/N is the ratio of the width/length ratio of transistor  222  to the width/length ratio of transistor  220 , and k is a proportionality constant to incorporate other circuit factors. Thus, the time to assertion of LOCFLG (FIG. 2) is given by                T   LOC     =           C   2          V   TR           [     k     M   ×   N       ]                         C   1          V   TR         T   CLK           =           (     M   ×   N     )     k          [       C   2       C   1       ]            T   CLK                 (   5   )                         
     Based on equation (5), T LOC , measured in units of clock periods, in FIG. 2B is given by            M   ×   N     k            (       C   2       C   1       )     .                     
     Thus, the number of clock periods to assert the loss-of-clock signal can be adjusted by changing the current mirror ratio, N, or by changing the clock divider ratio between CLK and Phi 1  and Phi 2 , M, or by changing the capacitor ratio C 2 /C 1 , or by changing k. In the preferred embodiment, all three ratios are chosen to minimize the area occupied by transistors  220  and  222  and by capacitors  230  and  206 . 
     An additional control signal, RANGE (not shown) can be added as an input to the frequency-to-current converter  202 . The RANGE control is used to select the sizes of the current mirror transistors  220  and  222  in FIG. 4A, adjusting their operating point for optimum matching. In most integrated circuit manufacturing processes, the degree to which transistors can be matched is limited. Thus it is widely known in the art that current mirrors composed of transistors with large aspect ratios (the ratio of width to length of a transistor) should not carry low currents, since such mirrors are often poorly matched. Referring to FIG. 4A, transistors  220  and  222  must have aspect ratios large enough to accommodate the large values of I 220  and I CH  at the maximum specified input clock frequency. Thus, if the range of possible input clock frequencies is very large, these transistors carry an unfavorably low level of current when F CLK  is very low, and the matching is poor, compromising performance. In the preferred embodiment, the range of input clock frequencies is split into two pieces, and the additional control input, RANGE, is used to select the sizes of transistors  220  and  222  which are appropriate for each piece. When the input clock frequency is large, the RANGE control input selects transistor sizes with larger aspect ratios. When the input clock frequency is small, the RANGE control input selects transistor sizes with smaller aspect ratios. 
     The embodiment for RANGE control as described above uses two (in this case) distinct sets of transistors for two different frequency ranges. This type of range approach can be referred to as “distinct mode”. 
     Alternatively, RANGE control may be accomplished with a different circuit architecture (hereinafter, “accumulative mode”) in which one set of transistors is always selected to be active. The second set of transistors may be selected into the circuit (i.e., “accumulated” in) for the higher frequency range. 
     Although the embodiment described here divides the input frequency range into only two pieces or sets, it is apparent to one skilled in the art that the concept of selecting current mirror sizes for optimum matching can be extended to as many sets of frequency ranges as is deemed appropriate. 
     An advantage of the accumulative mode is that a smaller overall circuit area may be achieved because some circuitry is reused for more than one frequency range. This advantage in area reduction may become more important when the number of frequency ranges is increased from two. 
     Referring to FIG. 4B, in an alternative embodiment to that illustrated in FIG. 4A capacitor  430  has its top plate switched between nodes VC 0  and VC 1  by clocks Phi 1  and Phi 2  respectively, and its bottom plate switched between nodes VC 2  and VC 3  respectively by clocks Phi 2  and Phi 1  respectively. It is apparent to one skilled in the art that the average current  1420  flowing through the transistor  420  is substantially equal to              I   420     _     =     k   ×     C   1                         (       VC   0     -     VC   3       )     -     (       VC   1     -     VC   2       )         M   ×     T   CLK             ,                   
     where k is a proportionality constant. 
     Hence the presence of voltages VC 0  to VC 3  allows a larger average current for the same capacitance C 1 , or alternatively, a smaller capacitance (hence a saving in chip area) for the same average current  1420 . 
     Moreover, one can make node VC 1 , still a node separate from node VC 0 , but substantially equal in average voltage to the average voltage of node VC 0 . This results in a parasitic insensitive implementation, as is apparent to one skilled in the art. Namely, the parasitic capacitance CP 1 —e.g., comprised of wiring and other parasitic capacitance from the “top” (it is assumed that the top plate is switched between nodes VC 0  and VC 1 ) plate of C 1  to ground—will no longer contribute any substantial average current to I 420  when switched between nodes VC 0  and VC 1 . Parasitic capacitance CP 2  (comprised of wiring and other parasitic from the bottom plate of C 1  to ground) never contributes substantial average current to I 420 . 
     The parasitic implementation still allows a magnification of available current I 420 , if voltage VC 2 -VC 3  is chosen to be larger than V TR .          I   420     =     k   ×     C   1                       (       VC   2     -     VC   3       )       M   ×     T   CLK                           
     It may be desirable for either embodiment to contain circuitry that provides a DC level shift between the output of the operational amplifier and the gate of the source follower transistor, by techniques known to those skilled in the art. A DC level shift may be desirable in that it helps set the DC quiescent point of the operational amplifier output to be such that the average input referred DC offset voltage of the operational amplifier is substantially zero. As a result, the DC voltage at the source of the source follower can be made to be substantially closer to V TR . 
     FIG. 5 shows the preferred embodiment of clock generator  204  that includes a D flip-flop  238  and a nonoverlapping clock generator  240 . D flip-flop  238  is configured to generate a secondary clock, CLK_HALF, which has a frequency that is half the frequency of the primary input clock, and from which nonoverlapping clock generator  240  generates the complementary nonoverlapping phases Phi 1  and Phi 2 . The use of D flip-flop  238  ensures that the clock signal from which these phases are derived has a duty cycle near 50%, thus freeing the main input clock, CLK, from this restriction. In this implementation, Phi 1  and Phi 2  have a frequency equal to one half the frequency of CLK; thus M=2 in equations (3), (4), and (5). Implementations required for generating other values of M are widely known to those skilled in the art. 
     The circuit may be optimized for different frequency ranges not only for more appropriate biasing and associated matching behavior (as described above), but also to permit more flexibility in adjusting circuit parameters (e.g., in response to widely diverse frequencies of operation). 
     Alternative methods of adjusting circuit operation may include programmability of any of the terms in equation 5. For example, M may be adjusted by inserting a programmable amount of clock division when generating T PHI  from T CLK . N may be adjusted by selecting in or out appropriate subsections of transistor circuitry on either side of the current mirror. C 2  may be adjusted by programmably selecting in or out appropriate amounts of capacitance. C 1  may be adjusted in a similar way. All such changes can be implemented in distinct mode or accumulative mode (or combination). 
     Moreover, the source follower transistor may be broken up into two or more parallel transistors whose gates are coupled together and whose sources are coupled together. Their separate drains may be optionally coupled to the current mirror or to VDD programmably. 
     This allows more flexibility in biasing the transistors configured as source followers, and provides an additional design factor in circuit operation. Namely this added design factor is the ratio of total average current coupled to the current mirror (by the source follower devices) divided by the total average current coupled by the switched-capacitor circuit to the sources of the source follower devices. 
     Finally, it may be desirable to scale the trickle current programmably in response to different frequency ranges. This may be done for example by adjusting programmably the appropriate current mirror used to realize the trickle current. 
     Referring to FIG. 6, a detailed schematic for frequency-to-current converter  202  includes operational amplifier  216  whose output drives the gate of MN 5 , which comprises source follower  218 . Operational amplifier  216  consists of transistors MP 1 , MP 2 , MN 1 , MN 2 , MN 3 , and MCCOMP. Transistors MN 1  and MN 2  form the source-coupled input pair of operational amplifier  216 , with the gate of MN 1  acting as the noninverting operational amplifier input terminal, and the gate of MN 2  acting as the inverting input terminal. Transistor MN 3  is the tail current source for operational amplifier  216 . Transistor MCCOMP is biased in accumulation mode and acts as a compensation capacitor for operational amplifier  216 . 
     In FIG. 6, switched-capacitor circuit  228  is comprised of transistors MN 7 , MN 8 , MP 7 , MP 8 , MC 1 A, MC 1 B, and MC 1 C. Transistors MN 7  and MP 7  comprise switch  232  (shown in FIG.  4 A), and are closed when Phi 1  is asserted high and Phi 1   b  is asserted low. Transistors MN 8  and MP 8  comprise switch  234  (shown in FIG.  4 A), and are closed when Phi 2  is asserted high and Phi 2   b  is asserted low. Inverters  301  and  302  generate Phi 1   b  and Phi   2   b, the logical complements of Phi 1  and Phi 2 . Transistors MC 1 A, MC 1 B, and MC 1 C are MOS capacitors, which together comprise capacitor  230  in FIG.  4 A. MC 1 B and MC 1 C are added in parallel with MC 1 A to account for the parasitic input capacitance of comparator  212  that acts in parallel with capacitor  206  in FIGS. 3 and 7. 
     The drain terminal of MN 5  is coupled to a pair of current mirrors, each of which is split into two halves, corresponding to transistors  220  and  222  in FIG.  4 A. The additional control signal RANGE, described above, and its complement RANGEB, generated by inverter  303 , are shown here. Circuit  220   a , comprised of transistors MP 3  and MP 3 C, and  222   a , comprised of MP 4  and MP 4 C, are the left and right halves, respectively, of the first mirror, while  220   b , comprised of MP 5  and MP 5 C, and  222   b , comprised of MP 6  and MP 6 C, are the left and right halves of the second mirror. When RANGE is logic ‘1’, transistors MP 3 C and MP 4 C are turned on and thus current mirror  220   a - 222   a , which is assumed to be comprised of large aspect-ratio transistors, is active. When RANGE is logic ‘0’, transistors MP 5 C and MP 6 C are turned on and thus current mirror  220   b - 222   b , which is assumed to be comprised of smaller aspect-ratio transistors, is active. 
     An accumulative implementation is achieved by keeping current mirror  220   b - 222   b  to be always on during circuit operation. For example, this may be achieved by connecting the gates of devices MP 5 C and MP 6 C to ground. Moreover, it may be desirable to resize current mirror  220   a - 222   a  to preserve the total effective current mirror size (i.e., width/length ratio) for the RANGE=1 setting. 
     As shown in FIG. 4A, the left and right halves of the current mirrors are separated by an RC low-pass filter, that includes resistor  224  and capacitor  226 . In the preferred embodiment, transistor MC 226 , configured as an inversion capacitor, comprises capacitor  226 , while resistor  224  is created by using a linear region MOS transistor, MR 224 . Transistor MC 236 , biased as an inversion capacitor, corresponds to capacitor  236  of FIG.  3 . It is understood by one skilled in the art that both capacitors  226  and  236  can be implemented using any of a variety of capacitor types, and resistor  224  can be implemented using any of a variety of resistor types, including without limitation those typically available in integrated circuit manufacturing processes. 
     It is also understood by one skilled in the art that the RC low-pass filter has sufficiently low bandwidth relative to clock frequency that substantially only the DC component is present in the charging current. Moreover, the RC low-pass filter has sufficiently high bandwidth as dictated by other design considerations such as chip area. The bandwidth of the RC low-pass filter may be adjusted programmably or in response to use in different frequency ranges. 
     Subcircuit  242  consists of transistors MP 9 , MP 10 , MP 11 , and resistor RIB 1 . A trickle current, which is mirrored by MP 10  and MP 11 , is set up by resistor RIB 1  and diode-connected transistor MP 9 . The current from MP 10  is sunk by transistor MN 4  and mirrored by transistor MN 3  to act as the tail current of operational amplifier  216 . 
     The drain current of MP 11  is mirrored by transistor MN 6  to transistors MN 9  and MN 10 . The resulting trickle current in MN 9  passes through source-follower transistor  218 , ensuring that this transistor is always on, even if the input clock stops and switched-capacitor circuit  228  is not switching. 
     This also means that during normal operation the drain current of MN 9  is added to the current from switched-capacitor circuit  228  and mirrored by current mirror  220   a - 222   a  or  220   b - 222   b , resulting in an error in the value of I CH . To reduce this error, an additional subcircuit  244 , which includes transistors MN 10  and MN 10 C, subtracts from I CH  a current substantially equal to the trickle current added in by transistor MN 9 . This correction is necessary at low input clock frequencies, as the drain current from MN 9  may be roughly the same order of magnitude as the charging current I CH . 
     Note that slightly reducing the size of transistor MN 10  ensures that some excess trickle current is present in I CH , and thus that there is sufficient current to charge capacitor  206  and assert the loss-of-clock signal even if the input clock is not running upon power-up of LOC detector  200 . Alternatively, the excess trickle current can be added specifically by using a circuit such as subcircuit  146 . In this circuit, the excess trickle current is generated by resistor RIBX and diode-connected transistor MP 12 , and mirrored by transistor MP 13 , thus adding excess trickle current to the primary output current, I CH . Because the current mirror formed by MP 12  and MP 13  carries only a small-magnitude current, the aspect ratios of these transistors may be made small for optimal matching. 
     FIG. 7 shows the preferred embodiment of comparator  212 , capacitor  206  and switch  210 . In this circuit, capacitor  206  is implemented by using a transistor, MC 2 , configured as an accumulation-mode capacitor. Implementing this capacitor with an MOS device permits the use of a simpler MOS process technology; by implementing the MOS capacitor in accumulation-mode, the total integrated charge is less dependent on threshold voltage when compared to some other modes of implementation. Switch  210  is comprised of transistor MN 23 . Input DISCH is driven by the output of edge detector  208 , turning on transistor MN 23  to discharge the capacitance of MC 2 . The output of frequency-to-current converter  202 , current I CH , is coupled to input I CH  in FIG. 7, and thus this current charges MOS capacitor MC 2  towards the trip voltage of comparator  212 . 
     In the preferred embodiment; comparator  212  is implemented as a simple CMOS inverter, comprised of transistors MP 21  and MN 21 . When the voltage across MOS capacitor MC 2  passes the trip voltage of the CMOS inverter, the inverter output, LOCFLGBAR, which is the logical complement of the loss-of-clock signal, transitions to a logic ‘0’. A second CMOS inverter, comprised of transistors MP 22  and MN 22 , generates the loss-of-clock signal, LOCFLG, by logically inverting the LOCFLGBAR signal. If the first inverter&#39;s output does not swing completely to power supply or ground, the second inverter also functions to provide an output signal which is more “squared up” (i.e., output levels are closer to power supply and ground). 
     Unlike the two-input analog comparator shown in FIGS. 1 and 3, the CMOS inverter is a one-input device. The inverter trip-voltage, V TR , which is defined as the value of the input voltage at which the inverter input and output voltages are equal, is a function of the dimensions of transistors MP 21  and MN 21 , as well as the characteristics of the manufacturing process. Within the limits imposed by manufacturing variations, a second CMOS inverter comprised of transistors having dimensions which match or are scaled to those of MP 21  and MN 21  should have an identical trip-point voltage. This fact is used in the circuit of FIG. 8, which is the preferred embodiment of voltage reference  215 , whose output is substantially equal to the trip voltage of comparator  212 . This circuit consists of a second inverter in which the dimensions of transistors MP 34  and MN 34  in FIG. 8 match or are scaled to the dimensions of transistors MP 21  and MN 21 . The input and output of this inverter are coupled together; this connection forces the input and output voltages to substantially equal each other, thus forcing the output to the desired inverter trip point voltage, V TR . This voltage drives the noninverting input of operational amplifier  216  in frequency-to-current converter  202 . 
     Accordingly, it is to be understood that the embodiments of the invention herein described are merely illustrative of the application of the principles of the invention. Reference herein to details of the illustrated embodiments are not intended to limit the scope of the claims, which themselves recite those features regarded as essential to the invention.