Abstract:
A clock signal generator varies a frequency of a digital clock over a selected range of frequencies. The generator employs a divider for lowering a frequency of a clock signal. A counter increments synchronously with the signal, and causes a selected sequence of outputs to be generated by a pattern generator. The pattern generator output forms an input to a digitally controllable delay line which receives the lower frequency clock signal. The pattern generator causes the digital delay line to vary a frequency of the lowered frequency clock signal between selected boundaries. The varying frequency clock signal is then raised up again such that a final clock has a varying frequency, and will exhibit less EMI spiking during switching of an associated, synchronous digital data device. The solid state nature of the generator allows for simple fabrication, inexpensive manufacture and ready integration into digital circuitry, such as multifunction integrated circuits.

Description:
This application is a Continuation-In-Part of U.S. Ser. No. 10/647,929 entitled “Spread Spectrum Clock Generator” filed Aug. 26, 2003, which is incorporated by reference in its entirety, herein. 

   BACKGROUND OF THE INVENTION 
   The subject application is directed generally to the art of synchronous digital circuitry, and more particularly to synchronous digital circuitry in which a lessened effect of electromagnetic interference (“EMI”) is desirable. 
   Most digital devices today operate synchronously. That is, data processing operations occur under a timing dictated by a digital clock signal. Such digital clock signals are typically square waves that oscillate at a selected frequency. As improvements are made to digital processing devices, clock frequencies may be increased. Faster clock frequencies allow for improved data processing throughput. Current digital clock frequencies are already in the multi-gigahertz range. As clock frequencies continue to rise, an increased incidence of electromagnetic interference exists. Such EMI requires that special shielding or casing be developed to dampen such interference. EMI can cause data errors in associated data processing devices, as well as provide for radio frequency (“RF”) interference for analog devices such as radios and televisions. 
   Designers have become aware that implementing a spread spectrum clock generator (“SSCG”) works to substantially reduce the high energy spikes associated with digitally-generated EMI. 
   SSCG circuitry functions to vary slightly a frequency of a digital clock signal over time. This is accomplished by reducing “noise” associated with harmonics of a large scale integration (“LSI”) clock signal. SSCG circuitry functions to alter slightly a signal interval and thus diffuses a frequency spectrum and lowers a peak value. 
   A side effect from the use of an SSCG is an introduction of a slight jitter in the system clock. However, such jitter is generally of little consequence other than in particular applications relating to communication network interfaces or input/output interfaces, as well as other applications having varying tolerance to jitter. Thus, it is desirable to be able to vary a degree of frequency shift and associated jitter to accommodate a lessening of peak EMI while simultaneously minimizing the jitter to acceptable application parameters. 
   Current SSCG circuitry employs frequency comparators and voltage controlled oscillators (“VCO”) to accomplish the shifting of frequency to result in a modulated clock signal. While effective, such analog-based implementations render it difficult and expensive to accomplish an SSCG circuitry, particularly in applications when a system is desired to coexist on other standard digital circuitry and in conjunction with a single substrate. 
   The subject invention provides for a digital spread spectrum clock generator which accomplishes selected frequency variation of an associated digital clock while minimizing the required use of extensive or incompatible analog circuitry. 
   SUMMARY OF THE INVENTION 
   In accordance with the subject invention, there is provided a spread spectrum clock generator which includes a divider for lowering a frequency of an input clock signal. A digital counter is incremented synchronously with the clock signal. The counter, in turn, processes through a selected sequence of outputs to be generated by a pattern generator. The pattern generator output, in turn, is communicated to a digitally controllable delay circuit into which the lowered frequency clock signal is provided. Thus, a variation in frequency to the clock signal is controlled by the selected pattern in the pattern generator. This varying frequency clock signal is then multiplied to a higher overall frequency compatible with the original clock signal, and output as a clock signal to remaining, synchronous digital circuitry. 
   In accordance with another aspect of the present invention, the frequency variation of the modified clock signal is toggled between a selected higher limit and selected lower limit. 
   In accordance with another aspect of the present invention, a method is provided for generating a spread spectrum clock signal in accordance with the foregoing. 

   
     SUMMARY OF THE DRAWINGS 
     The subject invention is described with reference to certain parts, and arrangements to parts, which are evidenced in conjunction with the associated drawings which form a part hereof and not for the purposes of limiting the same in which: 
       FIG. 1  is a schematic of a conventional spread spectrum clock generator; 
       FIG. 2  is a diagram of the improved spread spectrum clock generator of the present invention; 
       FIG. 3  is a block diagram of the spread spectrum clock generator of the subject invention inclusive of a master clock, the frequency of which is lowered prior to alteration of a frequency and raised after completion thereof; 
       FIG. 4  is a diagram of the input clock wave form as compared to the output clock which has been processed for spread spectrum frequency modulation; and 
       FIG. 5  is a graph of clock period versus frequency delta associated with the spread spectrum clock generation of the subject invention. 
       FIG. 6  is a diagram of an integrated circuit of the present invention. 
       FIG. 7  is a diagram of used delay line circuit. 
       FIG. 8   a  and  FIG. 8   b  illustrate the difference between “Phase Modulation” and “Frequency Modulation” based on the modulation pattern difference. 
       FIGS. 9   a  and  9   b  illustrate an example of “Phase Modulation Pattern” and its result waveform. 
       FIGS. 10   a  and  10   b  illustrate an example of “Frequency Modulation Pattern” based on this invention and its result waveform. 
       FIG. 11  shows an integrated circuit of the present invention with external peripheral devices such as ATA100, PCI Controller. 
       FIG. 12  shows the contents of pattern generator, which modulates 20 MHz source clock used in a particular application. 
       FIG. 13  shows the modulation waveform in the example of  FIG. 12  pattern table. 
   

   DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT 
   Turning now to the drawings wherein the illustrations are for illustrating the preferred embodiment only, and not for delivering the same,  FIG. 1A  shows a block diagram of a conventional spread spectrum clock generator. In a conventional system, a clock input  10  was provided as one input to a frequency phase comparator  12 . An output of the comparator  12  was provided to a charge pump  14 , the output of which is provided to a voltage controlled oscillator (“VCO  16 ”). Output  18  of the VCO  16  forms a system clock output, as well as a feedback loop into frequency comparator  12  via a  1 /N divider  20 . 
   A conventional spread spectrum clock generator employed an RC circuit  22  as a filter to ground. A signal generator  24  served to generate a waveform (such as that evidenced in  FIG. 1B ) into the input of the VCO  16 . By injecting this signal into the VCO input, an output frequency at output  18  was modulated in conjunction with the waveform of  FIG. 1B . 
   It will be appreciated by the view of  FIGS. 1A and 1B  that the basic circuitry employed in the spread spectrum clock generator was that of a phase lock loop. The system, while functional, relied heavily on analog circuitry and was thus not readily adaptable to implementation in conjunction with digital circuitry. 
   Turning now to  FIG. 2 , the basic architecture of the spread spectrum clock generator of the present invention is described. The SSCG A includes a clock input  30 , which input is provided by the standard clock generated in conjunction with a frequency associated with an associated synchronous digital system. The clock input from  30  is communicated to an input  32  of a digital delay line  34 . The input  30  is also communicated to an input  36  of a counter  38 . The counter  38  is suitably comprised of any simple binary counter. In the preferred embodiment, the counter  38  functions to count an increment on the basis of a number of input clock signals generated at counter input  36 . 
   The counter  38  is in data communication with the pattern generator  40  through its output lines thereof (not shown). In a simple binary counter, a series of binary lines are provided which correspond to a base numeric sequence. In a preferred embodiment, a particular binary number placed on an input to the pattern generator results in the providing of a preselected digital value at an output  50  thereof. A particular pattern of a pattern generator  40  of the preferred embodiment will be detailed in conjunction with Table 1, below. In the preferred embodiment, sequencing the counter  38  will result in a periodically repeating pattern being generated by pattern generator  4  at output  50 . 
   
     
       
             
           
             
             
             
           
             
             
             
           
         
             
               TABLE 1 
             
           
           
             
                 
             
             
               The TRUTH table of Pattern Generator 
             
           
        
         
             
               S 
               D 
               V 
             
             
                 
             
           
        
         
             
               0 
               0 
               16 
             
             
               1 
               0 
               16 
             
             
               2 
               0 
               16 
             
             
               3 
               1 
               17 
             
             
               4 
               0 
               17 
             
             
               5 
               0 
               17 
             
             
               6 
               0 
               17 
             
             
               7 
               0 
               17 
             
             
               8 
               1 
               18 
             
             
               9 
               0 
               18 
             
             
               10 
               0 
               18 
             
             
               11 
               0 
               18 
             
             
               12 
               1 
               19 
             
             
               13 
               0 
               19 
             
             
               14 
               0 
               19 
             
             
               15 
               1 
               20 
             
             
               16 
               0 
               20 
             
             
               17 
               1 
               21 
             
             
               18 
               0 
               21 
             
             
               19 
               1 
               22 
             
             
               20 
               0 
               22 
             
             
               21 
               1 
               23 
             
             
               22 
               0 
               23 
             
             
               23 
               1 
               24 
             
             
               24 
               0 
               24 
             
             
               25 
               1 
               25 
             
             
               26 
               0 
               25 
             
             
               27 
               1 
               26 
             
             
               28 
               1 
               27 
             
             
               29 
               0 
               27 
             
             
               30 
               1 
               28 
             
             
               31 
               1 
               29 
             
             
               32 
               1 
               30 
             
             
               33 
               0 
               30 
             
             
               34 
               1 
               31 
             
             
               35 
               1 
               32 
             
             
               36 
               1 
               33 
             
             
               37 
               1 
               34 
             
             
               38 
               0 
               34 
             
             
               39 
               1 
               35 
             
             
               40 
               1 
               36 
             
             
               41 
               1 
               37 
             
             
               42 
               1 
               38 
             
             
               43 
               1 
               39 
             
             
               44 
               1 
               40 
             
             
               45 
               2 
               42 
             
             
               46 
               1 
               43 
             
             
               47 
               1 
               44 
             
             
               48 
               1 
               45 
             
             
               49 
               1 
               46 
             
             
               50 
               2 
               48 
             
             
               51 
               1 
               49 
             
             
               52 
               1 
               50 
             
             
               53 
               1 
               51 
             
             
               54 
               2 
               53 
             
             
               55 
               1 
               54 
             
             
               56 
               1 
               55 
             
             
               57 
               2 
               57 
             
             
               58 
               1 
               58 
             
             
               59 
               2 
               60 
             
             
               60 
               1 
               61 
             
             
               61 
               2 
               63 
             
             
               62 
               1 
               64 
             
             
               63 
               2 
               66 
             
             
               64 
               1 
               67 
             
             
               65 
               2 
               69 
             
             
               66 
               1 
               70 
             
             
               67 
               2 
               72 
             
             
               68 
               1 
               73 
             
             
               69 
               2 
               75 
             
             
               70 
               2 
               77 
             
             
               71 
               1 
               78 
             
             
               72 
               2 
               80 
             
             
               73 
               2 
               82 
             
             
               74 
               2 
               84 
             
             
               75 
               1 
               85 
             
             
               76 
               2 
               87 
             
             
               77 
               2 
               89 
             
             
               78 
               2 
               91 
             
             
               79 
               2 
               93 
             
             
               80 
               1 
               94 
             
             
               81 
               2 
               96 
             
             
               82 
               2 
               98 
             
             
               83 
               2 
               100 
             
             
               84 
               2 
               102 
             
             
               85 
               2 
               104 
             
             
               86 
               2 
               106 
             
             
               87 
               3 
               109 
             
             
               88 
               2 
               111 
             
             
               89 
               2 
               113 
             
             
               90 
               2 
               115 
             
             
               91 
               2 
               117 
             
             
               92 
               3 
               120 
             
             
               93 
               2 
               122 
             
             
               94 
               2 
               124 
             
             
               95 
               2 
               126 
             
             
               96 
               3 
               129 
             
             
               97 
               2 
               131 
             
             
               98 
               2 
               133 
             
             
               99 
               3 
               136 
             
             
               100 
               2 
               138 
             
             
               101 
               3 
               141 
             
             
               102 
               2 
               143 
             
             
               103 
               3 
               146 
             
             
               104 
               2 
               148 
             
             
               105 
               3 
               151 
             
             
               106 
               2 
               153 
             
             
               107 
               3 
               156 
             
             
               108 
               2 
               158 
             
             
               109 
               3 
               161 
             
             
               110 
               2 
               163 
             
             
               111 
               3 
               166 
             
             
               112 
               3 
               169 
             
             
               113 
               2 
               171 
             
             
               114 
               3 
               174 
             
             
               115 
               3 
               177 
             
             
               116 
               3 
               180 
             
             
               117 
               2 
               182 
             
             
               118 
               3 
               185 
             
             
               119 
               3 
               188 
             
             
               120 
               3 
               191 
             
             
               121 
               3 
               194 
             
             
               122 
               2 
               196 
             
             
               123 
               3 
               199 
             
             
               124 
               3 
               202 
             
             
               125 
               3 
               205 
             
             
               126 
               3 
               208 
             
             
               127 
               3 
               211 
             
             
               128 
               4 
               215 
             
             
               129 
               3 
               218 
             
             
               130 
               3 
               221 
             
             
               131 
               3 
               224 
             
             
               132 
               4 
               228 
             
             
               133 
               3 
               231 
             
             
               134 
               4 
               235 
             
             
               135 
               3 
               238 
             
             
               136 
               3 
               241 
             
             
               137 
               4 
               245 
             
             
               138 
               4 
               249 
             
             
               139 
               4 
               253 
             
             
               140 
               3 
               256 
             
             
               141 
               4 
               260 
             
             
               142 
               4 
               264 
             
             
               143 
               4 
               268 
             
             
               144 
               4 
               272 
             
             
               145 
               4 
               276 
             
             
               146 
               3 
               279 
             
             
               147 
               4 
               283 
             
             
               148 
               3 
               286 
             
             
               149 
               4 
               290 
             
             
               150 
               3 
               293 
             
             
               151 
               3 
               296 
             
             
               152 
               3 
               299 
             
             
               153 
               4 
               303 
             
             
               154 
               3 
               306 
             
             
               155 
               3 
               309 
             
             
               156 
               3 
               312 
             
             
               157 
               3 
               315 
             
             
               158 
               3 
               318 
             
             
               159 
               2 
               320 
             
             
               160 
               3 
               323 
             
             
               161 
               3 
               326 
             
             
               162 
               3 
               329 
             
             
               163 
               3 
               332 
             
             
               164 
               2 
               334 
             
             
               165 
               3 
               337 
             
             
               166 
               3 
               340 
             
             
               167 
               3 
               343 
             
             
               168 
               2 
               345 
             
             
               169 
               3 
               348 
             
             
               170 
               3 
               351 
             
             
               171 
               2 
               353 
             
             
               172 
               3 
               356 
             
             
               173 
               2 
               358 
             
             
               174 
               3 
               361 
             
             
               175 
               2 
               363 
             
             
               176 
               3 
               366 
             
             
               177 
               2 
               368 
             
             
               178 
               3 
               371 
             
             
               179 
               2 
               373 
             
             
               180 
               3 
               376 
             
             
               181 
               2 
               378 
             
             
               182 
               3 
               381 
             
             
               183 
               2 
               383 
             
             
               184 
               2 
               385 
             
             
               185 
               3 
               388 
             
             
               186 
               2 
               390 
             
             
               187 
               2 
               392 
             
             
               188 
               2 
               394 
             
             
               189 
               3 
               397 
             
             
               190 
               2 
               399 
             
             
               191 
               2 
               401 
             
             
               192 
               2 
               403 
             
             
               193 
               2 
               405 
             
             
               194 
               3 
               408 
             
             
               195 
               2 
               410 
             
             
               196 
               2 
               412 
             
             
               197 
               2 
               414 
             
             
               198 
               2 
               416 
             
             
               199 
               2 
               418 
             
             
               200 
               2 
               420 
             
             
               201 
               1 
               421 
             
             
               202 
               2 
               423 
             
             
               203 
               2 
               425 
             
             
               204 
               2 
               427 
             
             
               205 
               2 
               429 
             
             
               206 
               1 
               430 
             
             
               207 
               2 
               432 
             
             
               208 
               2 
               434 
             
             
               209 
               2 
               436 
             
             
               210 
               1 
               437 
             
             
               211 
               2 
               439 
             
             
               212 
               2 
               441 
             
             
               213 
               1 
               442 
             
             
               214 
               2 
               444 
             
             
               215 
               1 
               445 
             
             
               216 
               2 
               447 
             
             
               217 
               1 
               448 
             
             
               218 
               2 
               450 
             
             
               219 
               1 
               451 
             
             
               220 
               2 
               453 
             
             
               221 
               1 
               454 
             
             
               222 
               2 
               456 
             
             
               223 
               1 
               457 
             
             
               224 
               2 
               459 
             
             
               225 
               1 
               460 
             
             
               226 
               1 
               461 
             
             
               227 
               2 
               463 
             
             
               228 
               1 
               464 
             
             
               229 
               1 
               465 
             
             
               230 
               1 
               466 
             
             
               231 
               2 
               468 
             
             
               232 
               1 
               469 
             
             
               233 
               1 
               470 
             
             
               234 
               1 
               471 
             
             
               235 
               1 
               472 
             
             
               236 
               2 
               474 
             
             
               237 
               1 
               475 
             
             
               238 
               1 
               476 
             
             
               239 
               1 
               477 
             
             
               240 
               1 
               478 
             
             
               241 
               1 
               479 
             
             
               242 
               1 
               480 
             
             
               243 
               0 
               480 
             
             
               244 
               1 
               481 
             
             
               245 
               1 
               482 
             
             
               246 
               1 
               483 
             
             
               247 
               1 
               484 
             
             
               248 
               0 
               484 
             
             
               249 
               1 
               485 
             
             
               250 
               1 
               486 
             
             
               251 
               1 
               487 
             
             
               252 
               0 
               487 
             
             
               253 
               1 
               488 
             
             
               254 
               1 
               489 
             
             
               255 
               0 
               489 
             
             
               256 
               1 
               490 
             
             
               257 
               0 
               490 
             
             
               258 
               1 
               491 
             
             
               259 
               0 
               491 
             
             
               260 
               1 
               492 
             
             
               261 
               0 
               492 
             
             
               262 
               1 
               493 
             
             
               263 
               0 
               493 
             
             
               264 
               1 
               494 
             
             
               265 
               0 
               494 
             
             
               266 
               1 
               495 
             
             
               267 
               0 
               495 
             
             
               268 
               0 
               495 
             
             
               269 
               1 
               496 
             
             
               270 
               0 
               496 
             
             
               271 
               0 
               496 
             
             
               272 
               0 
               496 
             
             
               273 
               1 
               497 
             
             
               274 
               0 
               497 
             
             
               275 
               0 
               497 
             
             
               276 
               0 
               497 
             
             
               277 
               0 
               497 
             
             
               278 
               1 
               498 
             
             
               279 
               0 
               498 
             
             
               280 
               0 
               498 
             
             
               281 
               0 
               498 
             
             
               282 
               0 
               498 
             
             
               283 
               0 
               498 
             
             
               284 
               0 
               498 
             
             
               285 
               −1 
               497 
             
             
               286 
               0 
               497 
             
             
               287 
               0 
               497 
             
             
               288 
               0 
               497 
             
             
               289 
               0 
               497 
             
             
               290 
               −1 
               496 
             
             
               291 
               0 
               496 
             
             
               292 
               0 
               496 
             
             
               293 
               0 
               496 
             
             
               294 
               −1 
               495 
             
             
               295 
               0 
               495 
             
             
               296 
               0 
               495 
             
             
               297 
               −1 
               494 
             
             
               298 
               0 
               494 
             
             
               299 
               −1 
               493 
             
             
               300 
               0 
               493 
             
             
               301 
               −1 
               492 
             
             
               302 
               0 
               492 
             
             
               303 
               −1 
               491 
             
             
               304 
               0 
               491 
             
             
               305 
               −1 
               490 
             
             
               306 
               0 
               490 
             
             
               307 
               −1 
               489 
             
             
               308 
               0 
               489 
             
             
               309 
               −1 
               488 
             
             
               310 
               −1 
               487 
             
             
               311 
               0 
               487 
             
             
               312 
               −1 
               486 
             
             
               313 
               −1 
               485 
             
             
               314 
               −1 
               484 
             
             
               315 
               0 
               484 
             
             
               316 
               −1 
               483 
             
             
               317 
               −1 
               482 
             
             
               318 
               −1 
               481 
             
             
               319 
               −1 
               480 
             
             
               320 
               0 
               480 
             
             
               321 
               −1 
               479 
             
             
               322 
               −1 
               478 
             
             
               323 
               −1 
               477 
             
             
               324 
               −1 
               476 
             
             
               325 
               −1 
               475 
             
             
               326 
               −1 
               474 
             
             
               327 
               −2 
               472 
             
             
               328 
               −1 
               471 
             
             
               329 
               −1 
               470 
             
             
               330 
               −1 
               469 
             
             
               331 
               −1 
               468 
             
             
               332 
               −2 
               466 
             
             
               333 
               −1 
               465 
             
             
               334 
               −1 
               464 
             
             
               335 
               −1 
               463 
             
             
               336 
               −2 
               461 
             
             
               337 
               −1 
               460 
             
             
               338 
               −1 
               459 
             
             
               339 
               −2 
               457 
             
             
               340 
               −1 
               456 
             
             
               341 
               −2 
               454 
             
             
               342 
               −1 
               453 
             
             
               343 
               −2 
               451 
             
             
               344 
               −1 
               450 
             
             
               345 
               −2 
               448 
             
             
               346 
               −1 
               447 
             
             
               347 
               −2 
               445 
             
             
               348 
               −1 
               444 
             
             
               349 
               −2 
               442 
             
             
               350 
               −1 
               441 
             
             
               351 
               −2 
               439 
             
             
               352 
               −2 
               437 
             
             
               353 
               −1 
               436 
             
             
               354 
               −2 
               434 
             
             
               355 
               −2 
               432 
             
             
               356 
               −2 
               430 
             
             
               357 
               −1 
               429 
             
             
               358 
               −2 
               427 
             
             
               359 
               −2 
               425 
             
             
               360 
               −2 
               423 
             
             
               361 
               −2 
               421 
             
             
               362 
               −1 
               420 
             
             
               363 
               −2 
               418 
             
             
               364 
               −2 
               416 
             
             
               365 
               −2 
               414 
             
             
               366 
               −2 
               412 
             
             
               367 
               −2 
               410 
             
             
               368 
               −2 
               408 
             
             
               369 
               −3 
               405 
             
             
               370 
               −2 
               403 
             
             
               371 
               −2 
               401 
             
             
               372 
               −2 
               399 
             
             
               373 
               −2 
               397 
             
             
               374 
               −3 
               394 
             
             
               375 
               −2 
               392 
             
             
               376 
               −2 
               390 
             
             
               377 
               −2 
               388 
             
             
               378 
               −3 
               385 
             
             
               379 
               −2 
               383 
             
             
               380 
               −2 
               381 
             
             
               381 
               −3 
               378 
             
             
               382 
               −2 
               376 
             
             
               383 
               −3 
               373 
             
             
               384 
               −2 
               371 
             
             
               385 
               −3 
               368 
             
             
               386 
               −2 
               366 
             
             
               387 
               −3 
               363 
             
             
               388 
               −2 
               361 
             
             
               389 
               −3 
               358 
             
             
               390 
               −2 
               356 
             
             
               391 
               −3 
               353 
             
             
               392 
               −2 
               351 
             
             
               393 
               −3 
               348 
             
             
               394 
               −3 
               345 
             
             
               395 
               −2 
               343 
             
             
               396 
               −3 
               340 
             
             
               397 
               −3 
               337 
             
             
               398 
               −3 
               334 
             
             
               399 
               −2 
               332 
             
             
               400 
               −3 
               329 
             
             
               401 
               −3 
               326 
             
             
               402 
               −3 
               323 
             
             
               403 
               −3 
               320 
             
             
               404 
               −2 
               318 
             
             
               405 
               −3 
               315 
             
             
               406 
               −3 
               312 
             
             
               407 
               −3 
               309 
             
             
               408 
               −3 
               306 
             
             
               409 
               −3 
               303 
             
             
               410 
               −4 
               299 
             
             
               411 
               −3 
               296 
             
             
               412 
               −3 
               293 
             
             
               413 
               −3 
               290 
             
             
               414 
               −4 
               286 
             
             
               415 
               −3 
               283 
             
             
               416 
               −4 
               279 
             
             
               417 
               −3 
               276 
             
             
               418 
               −3 
               273 
             
             
               419 
               −4 
               269 
             
             
               420 
               −4 
               265 
             
             
               421 
               −4 
               261 
             
             
               422 
               −3 
               258 
             
             
               423 
               −4 
               254 
             
             
               424 
               −4 
               250 
             
             
               425 
               −4 
               246 
             
             
               426 
               −4 
               242 
             
             
               427 
               −4 
               238 
             
             
               428 
               −3 
               235 
             
             
               429 
               −4 
               231 
             
             
               430 
               −3 
               228 
             
             
               431 
               −4 
               224 
             
             
               432 
               −3 
               221 
             
             
               433 
               −3 
               218 
             
             
               434 
               −3 
               215 
             
             
               435 
               −4 
               211 
             
             
               436 
               −3 
               208 
             
             
               437 
               −3 
               205 
             
             
               438 
               −3 
               202 
             
             
               439 
               −3 
               199 
             
             
               440 
               −3 
               196 
             
             
               441 
               −2 
               194 
             
             
               442 
               −3 
               191 
             
             
               443 
               −3 
               188 
             
             
               444 
               −3 
               185 
             
             
               445 
               −3 
               182 
             
             
               446 
               −2 
               180 
             
             
               447 
               −3 
               177 
             
             
               448 
               −3 
               174 
             
             
               449 
               −3 
               171 
             
             
               450 
               −2 
               169 
             
             
               451 
               −3 
               166 
             
             
               452 
               −3 
               163 
             
             
               453 
               −2 
               161 
             
             
               454 
               −3 
               158 
             
             
               455 
               −2 
               156 
             
             
               456 
               −3 
               153 
             
             
               457 
               −2 
               151 
             
             
               458 
               −3 
               148 
             
             
               459 
               −2 
               146 
             
             
               460 
               −3 
               143 
             
             
               461 
               −2 
               141 
             
             
               462 
               −3 
               138 
             
             
               463 
               −2 
               136 
             
             
               464 
               −3 
               133 
             
             
               465 
               −2 
               131 
             
             
               466 
               −2 
               129 
             
             
               467 
               −3 
               126 
             
             
               468 
               −2 
               124 
             
             
               469 
               −2 
               122 
             
             
               470 
               −2 
               120 
             
             
               471 
               −3 
               117 
             
             
               472 
               −2 
               115 
             
             
               473 
               −2 
               113 
             
             
               474 
               −2 
               111 
             
             
               475 
               −2 
               109 
             
             
               476 
               −3 
               106 
             
             
               477 
               −2 
               104 
             
             
               478 
               −2 
               102 
             
             
               479 
               −2 
               100 
             
             
               480 
               −2 
               98 
             
             
               481 
               −2 
               96 
             
             
               482 
               −2 
               94 
             
             
               483 
               −1 
               93 
             
             
               484 
               −2 
               91 
             
             
               485 
               −2 
               89 
             
             
               486 
               −2 
               87 
             
             
               487 
               −2 
               85 
             
             
               488 
               −1 
               84 
             
             
               489 
               −2 
               82 
             
             
               490 
               −2 
               80 
             
             
               491 
               −2 
               78 
             
             
               492 
               −1 
               77 
             
             
               493 
               −2 
               75 
             
             
               494 
               −2 
               73 
             
             
               495 
               −1 
               72 
             
             
               496 
               −2 
               70 
             
             
               497 
               −1 
               69 
             
             
               498 
               −2 
               67 
             
             
               499 
               −1 
               66 
             
             
               500 
               −2 
               64 
             
             
               501 
               −1 
               63 
             
             
               502 
               −2 
               61 
             
             
               503 
               −1 
               60 
             
             
               504 
               −2 
               58 
             
             
               505 
               −1 
               57 
             
             
               506 
               −2 
               55 
             
             
               507 
               −1 
               54 
             
             
               508 
               −1 
               53 
             
             
               509 
               −2 
               51 
             
             
               510 
               −1 
               50 
             
             
               511 
               −1 
               49 
             
             
               512 
               −1 
               48 
             
             
               513 
               −2 
               46 
             
             
               514 
               −1 
               45 
             
             
               515 
               −1 
               44 
             
             
               516 
               −1 
               43 
             
             
               517 
               −1 
               42 
             
             
               518 
               −2 
               40 
             
             
               519 
               −1 
               39 
             
             
               520 
               −1 
               38 
             
             
               521 
               −1 
               37 
             
             
               522 
               −1 
               36 
             
             
               523 
               −1 
               35 
             
             
               524 
               −1 
               34 
             
             
               525 
               0 
               34 
             
             
               526 
               −1 
               33 
             
             
               527 
               −1 
               32 
             
             
               528 
               −1 
               31 
             
             
               529 
               −1 
               30 
             
             
               530 
               0 
               30 
             
             
               531 
               −1 
               29 
             
             
               532 
               −1 
               28 
             
             
               533 
               −1 
               27 
             
             
               534 
               0 
               27 
             
             
               535 
               −1 
               26 
             
             
               536 
               −1 
               25 
             
             
               537 
               0 
               25 
             
             
               538 
               −1 
               24 
             
             
               539 
               0 
               24 
             
             
               540 
               −1 
               23 
             
             
               541 
               0 
               23 
             
             
               542 
               −1 
               22 
             
             
               543 
               0 
               22 
             
             
               544 
               −1 
               21 
             
             
               545 
               0 
               21 
             
             
               546 
               −1 
               20 
             
             
               547 
               0 
               20 
             
             
               548 
               −1 
               19 
             
             
               549 
               0 
               19 
             
             
               550 
               0 
               19 
             
             
               551 
               −1 
               18 
             
             
               552 
               0 
               18 
             
             
               553 
               0 
               18 
             
             
               554 
               0 
               18 
             
             
               555 
               −1 
               17 
             
             
               556 
               0 
               17 
             
             
               557 
               0 
               17 
             
             
               558 
               0 
               17 
             
             
               559 
               0 
               17 
             
             
               560 
               −1 
               16 
             
             
               561 
               0 
               16 
             
             
               562 
               0 
               16 
             
             
               563 
               0 
               16 
             
             
                 
             
           
        
       
     
   
   As will be appreciated by one of ordinary skill in the art, a feed digital delay line  34  functions to provide a selected delay to an input signal, the duration of which delay is dictated by an input thereto such is provided by the output of pattern generator  50 . Thus, a clock signal  30  will be provided with a selected delay, as dictated by the output of the pattern generator  40 , and this delay will be provided on output  52 . It will be appreciated, therefore, that interaction between the counter  38 , pattern generator  40  and digital delay line  34  will serve to provide a selected delay sequence to respective pulses of the clock signal at input  30 , as it is output to output  52 . In this fashion, the entire sequence of delay is suitably fabricated from digital elements and avoids implementation of the VCO/PLL circuitry as provided in connection with  FIG. 1A , above. 
   Turning now to  FIG. 3 , the SSCG A of  FIG. 1A  is shown in connection with additional support circuitry. Conventional switching circuitry currently operates in the multi-gigahertz range. It will be appreciated that implementation of the counter, pattern generator and digital delay line, such as described herein, is more readily adapted to perform at lower frequencies than this. The additional structure of  FIG. 3  accomplishes the beneficial advantages of the subject invention while facilitating use in connection with substantially higher clock frequencies. An input from a master clock  60  has communicated to a divider  62  to divide the frequency thereof. In the preferred embodiments, divider  62  is a ⅓ divider. By way of example, an input master clock frequency of 48 MHz provided at input  60  would result in a 16 MHz signal being provided at the output of divider  62 , which forms the clock input  30 . Thus, a period of 20.83 microseconds can be extended to a period of 62.5 microseconds. The function of the SSCG A is as described in connection with  FIG. 2 , above. 
   Turning now to the output  52  of digital delay line  34  in  FIG. 3 , in this embodiment the output forms an input to a phase lock loop  70 . As will be appreciated by one of ordinary skill in the art, the PLL  70  suitably serves as a signal conditioner to clean an output pulse, as well as a system for stepping up an input frequency. The PLL  70  suitably takes an input of 16 MHz, as provided from the output  52  of the digital delay line  34 , and outputs a substantially higher frequency, 400 MHz in the preferred embodiment and which output is provided at  72 . Also, an internal divider  74  suitably provides feedback at terminal  76  to allow for the enhanced output at  72 . 
   Turning now to  FIG. 4 , a comparison of an input clock and an output clock  82  is described as a function of time. The input clock shows a suitable system clock input, such as may be provided at digital delay line input  30  ( FIG. 1A ) or master clock input  60  ( FIG. 2 ). An output waveform  82  evidences a skew in frequency as provided by the SSCG circuitry described above. 
   Turning now to Table 1, disclosed is a suitable true table of the content of a pattern generator such as described herein. In the preferred embodiment, the decoder content of the subject invention will be applied with every 564 clock cycles. In this fashion, a modulation frequency of around 28 KHz is provided. As used in Table 1, S refers to “step”, D “delay value”, and V refers to “decoder value”. The step value S is incremented with every input clock pulse, such as that provided at input  30  ( FIG. 1A  or  FIG. 2 ). A specified delay value and decoder value follows every increment of the counter  38 . While the values of  FIG. 1A  are provided in the preferred embodiment, it will be appreciated that other suitable values may be implemented to accomplish the delays of the subject invention. 
   Turning now to Table 2, an example output of the pattern generator  40  is detailed. As evidenced in Table 2, the counter will increment at every input clock. At such point as a counter shows a value of 16, the next value will be reset to a 0. Thus, the pattern generator will decode a counter value to appear in the column “Pattern” and feed it to the delay line ( 50 ) ( FIGS. 2 and 3 ). As noted above, the delay line  34  will delay an input clock by the value given from its input  50 . By way of example, when a counter value is set at 0, delay value is 0. When a counter achieves  1 , the delay is 1. Next, the delay value will skip 1 and the result will be 3. As evidenced in  FIG. 2 , the values of column DELTA P show the difference between each adjacent account. This sequence of delta values, up and down in the preferred embodiment, is evidenced therein. 
   
     
       
             
           
             
             
             
             
             
           
             
             
             
             
             
           
         
             
               TABLE 2 
             
           
           
             
                 
             
             
               Example of pattern generator table 
             
           
        
         
             
                 
               Count 
               Pattern 
               DELTA T 
               Delta T 
             
             
                 
                 
             
           
        
         
             
                 
               0 
               0 
               0 
               0.00% 
             
             
                 
               1 
               1 
               1 
               0.05% 
             
             
                 
               2 
               3 
               2 
               0.10% 
             
             
                 
               3 
               6 
               3 
               0.15% 
             
             
                 
               4 
               10 
               4 
               0.20% 
             
             
                 
               5 
               13 
               3 
               0.15% 
             
             
                 
               6 
               15 
               2 
               0.10% 
             
             
                 
               7 
               16 
               1 
               0.05% 
             
             
                 
               8 
               16 
               0 
               0.00% 
             
             
                 
               9 
               15 
               −1 
               −0.05% 
             
             
                 
               10 
               13 
               −2 
               −0.10% 
             
             
                 
               11 
               10 
               −3 
               −0.15% 
             
             
                 
               12 
               6 
               −4 
               −0.20% 
             
             
                 
               13 
               3 
               −3 
               −0.15% 
             
             
                 
               14 
               1 
               −2 
               −0.10% 
             
             
                 
               15 
               0 
               −1 
               −0.05% 
             
             
                 
                 
             
           
        
       
     
   
   Referring back to  FIG. 3 , when an input to the SSCG A is at a value T, a first period and its corresponding output is T 1 −T 0 , which is T+Δ. As used herein, Δ is a unidelay of the delay line. As used herein:
 
 T 1− T 0= T+Δ 
 
 T 2− T 1= T+ 2*Δ
 
 T 3− T 2= T+ 3*Δ
 
 T 4− T 3= T+ 4*Δ
 
 T 5− T 4= T+ 3*Δ
 
   As shown in the above, the frequency modulation can be achieved because the period during each clock cycle is changed. 
   Turning now to  FIG. 5 , discloses a graph evidencing the frequency modulation scheme of the preferred embodiment. With the implementation described in the preferred embodiment, detailed above, it will be appreciated that the frequency modulation scheme employed by the circuitry of the subject invention provides for modulation analogous to that provided in conventional circuitry, as evidenced by  FIG. 1B . Thus, the subject system provides for spread spectrum clock generation so as to provide all the advantages of the earlier system, but in a substantially improved, digital structure that is readily adaptable to integration and low cost and effective applications. 
     FIG. 6  shows, as an example, of a diagram of an integrated circuit of the present invention. An integrated circuit  100  includes an SSCG  110  of the preset invention and a microprocessor  120 . The SSCG  110  receives a constant clock (A) and provides a varying frequency clock (B) to the microprocessor  120 . The microprocessor  120  includes at least a program counter  121 , an instruction fetch unit  122 , an instruction decoder  123 , and an execution unit  124 . The program counter  121  increments its stored value in response to the varying frequency clock. The microprocessor  120  can be either of a Reduced Instruction Set Computer (“RISC”), Complex Instruction Set Computer (“CISC”), or a Very Long Word Instruction computer (“VLIW”). A center frequency of the varying frequency clock to the microprocessor is, preferably, from 300 MHz to 900 MHz. 
   The integrated circuit shown in  FIG. 6  can be manufactured by a semiconductor process technology with a design rule of 0.13 um or less. In other words, a gate length of a transistor element is of 0.13 um or less. The design rule of less than 0.1 um can be used employed, too. Further, copper can be used for an interconnection or wiring of the integrated circuit. 
     FIG. 7  shows a diagram of the digital delay line  34  of the present invention. The digital delay line includes a plurality of delay elements ( 341 ), a one-hot decoder ( 343 ), and a set of delay value input ( 342 ), in response to the output of the decoder  40 . Each of the delay elements consists of three NAND gates and it has clock injection input ( 344 ). The source clock will be injected in to the point where the rest of delay elements numbers is corresponding to the delay value. Because of this structure, the delay value input can be changed whenever the input clock is low level without hazard. 
   Earlier Systems allowed on circuit to generate “phase modulated” waveform if the content of pattern generator designed as such.  FIG. 8   a  shows an example of an earlier “Phase Modulation” system. On the other hand,  FIG. 8   b  shows the case of “Frequency Modulation” as is described herein. 
     FIG. 9   a  shows an example of “phase modulation pattern”. By using this pattern, the output clock shows triangle waveform in its phase domain and square waveform in its frequency domain. From the spread spectrum viewpoint, this frequency spectrum is split to two frequencies, such as f 0 +Delta and f 0 −Delta. This result is shown in  FIG. 9   b.    
     FIG. 10   a  is an example of “Frequency Modulation pattern” based on this invention. The result is shown in  FIG. 10   b , where the phase modulation waveform is “integral waveform”, which is resemble to Sine wave. 
   As illustrated in  FIG. 8   b , a clock input is received at terminal  400 . A received clock signal is provided as an input a phase shifter  402 . The phase shifter  402 , in turn, receives frequency modulated pattern data from pattern generator  404  via interface  406 . Thus, phase shifting is accomplished, suitably via a delay, at the phase shifter  402  in connection with the frequency encoded modulation pattern data. 
   As far as “Frequency Domain waveform concern, it shapes triangle waveform, means the frequency is sweeping between f 0 − 5 ×Delta and f 0 +5×Delta. From the spectrum view point, it has been split out up to 11 kind of frequencies, such as f 0 −5×D, f 0 −4×D, f 0 −3×D, f 0 −2×D, f 0 −1×D, f 0 , f 0 +1×D, f 0 +2×D, f 0 +3×D, f 0 −4×D, and f 0 −5×D. 
   In  FIG. 10 , another type of implementation has been described. In this implementation, single non-SSCGed 20 MHz clock is used. 
   Turning to  FIG. 11 , disclosed is a sample embodiment of a circuit employing the spread spectrum clock generator of the subject application. A suitable clock signal is provided as an input  500  into a spread spectrum clock generating unit  510 . As taught above, a unit  510  includes a delay line  512  adapted to receive pattern output from a pattern generator  514 . The pattern generator  514 , in turn, is incremented in connection with a counter  516  connected operatively thereto. The counter  516  increments in connection with an input clock signal received on input  500 . In the disclosed embodiment, the delay line  512  is comprised of  512  stage lines. The output of the spread spectrum clock generator  510  forms input  520  to a phase lock loop (PLL)  522 . In the disclosed embodiment, PLL  522  multiplies the input signal at  520  by 20 times. The resultant 400 MHz signal is communicated to a microprocessor  524 , to form a clock input to allow the microprocessor to run at its selected rate. The 400 MHz signal also forms an input to dividers  526 ,  530  and  534  which are, in the disclosed example ⅓, ¼ and ⅙ dividers. The representative divisions allow for selected clock inputs to be placed into several illustrated components. The 400 MHz signal in the disclosed example, when divided by 3, provides a 133.33 MHz clock signal into DRAM controller  528 . The same signal, when divided by 4 at the divider  530 , provides a requisite 100 MHz signal to a representative structure of an ATA 100 MHz interface. Finally, the divide by 6 divider  534  provides a requisite 33.33 MHz to the illustrated PCI interface. 
   Thus, it would be appreciated by a review of the example structure of  FIG. 11 , the spread spectrum clock generator advantageously provides a means by which a suitable spread spectrum clock signal may be generated to several digital synchronous devices operating at various clock frequencies. 
   The foregoing description of a preferred embodiment of the invention has been presented for purposes of illustration and description. It is not intended to be exhaustive or to limit the invention to the precise form disclosed. Obvious modifications or variations are possible in light of the above teachings. The embodiment was chosen and described to provide the best illustration of the principles of the invention and its practical application to thereby enable one of the ordinary skilled in the art to utilize the invention in various embodiments and with various modifications as are suited to the particular use contemplated. All such modifications and variations are within the scope of the invention as determined by the appended claims when interpreted in accordance to the breadth to which they are fairly, legally and equitably entitled.