Abstract:
A programmable fractional frequency divider enables a finer resolution of output frequency than conventional integer frequency dividers. The programmable fractional frequency divider of this invention allows for the programmability of both an integer divisor as well as a fraction component. The average frequency of the output signal from the fractional divider is dependent upon both the integer divisor and the fraction component, thereby providing for a finer resolution to the average frequency of the output signal. This combination of integer and fractional frequency division is particularly well suited for the generation of signals for systems that are substantially jitter-insensitive.

Description:
This application claims the benefit of U.S. Provisional Application No 60/280410, filed on Mar. 30, 2001. 
    
    
     BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     This invention relates to the field of electronic circuits, and in particular to a circuit for generating lower-speed baud rates, such as used for serial communications, from higher-speed oscillators, such as microprocessor clocks 
     2. Description of Related Art 
     Digital systems typically include a clocking signal for providing periodic signals, and signals of known duration. That is, the clocking signal provides a time reference upon which other timed actions depend. Within the same system, multiple time references are often required. For example, in a microprocessor system, the microprocessor typically requires a high-speed clock that regulates the speed of the central processing unit (CPU), the speed and timing of memory accesses, and other high-speed events. Additionally, for communicating with external devices, such as modems or printers, the microprocessor requires a relatively low speed clock. 
     Because whole-cycle frequency dividers are relatively simple devices, compared to frequency generators, systems with multiple time references typically provide the multiple time references by dividing-down a high-speed clock. FIG. 1 illustrates a microprocessor system that includes a conventional frequency divider  100 . The divider  100  is programmable to provide a lower speed clock signal, the “Baud Rate Clock”, from a higher speed clock signal, the “Xtal Osc Clock” (Crystal Oscillator Clock). The high-speed clock signal is preferably the highest speed clock within a processing system, which is generally the same clock signal used by the core processor, or CPU,  150 . The example divider  100  includes a re-loadable count-down counter  110 , and a register  120  that provides the re-load value to the counter  110 . In operation, the register  120  is loaded with a value corresponding to a ratio of the high-speed clock frequency to the desired low-speed clock frequency, discussed further below, and thereafter the low-speed clock is automatically generated at the desired frequency, without further intervention by the core processor  150 . 
     The count-down counter  110  is clocked by the higher speed clock signal, and generates an output signal each time the count-down counter  110  reaches zero. The output signal is also coupled to the reload input of the counter  110 , and causes the counter  110  to be reloaded with the contents of the register  120  at the next clock cycle of the higher speed clock signal. Thus, if the register  120  contains a value of N, the zero signal will be asserted at each N+1th cycle of the higher speed clock (one cycle to load the value of N, plus N cycles to reduce the count to zero), thereby providing a “divide-by-N+1” frequency divider that can be programmed for a particular division by loading the proper value of N into the register  120 . Illustrated in FIG. 1 is a 12-bit register  120  and 12-bit counter  110 , thereby allowing the frequency divider  100  to provide a division of the high-speed clock by any integer multiple between 2 and 8192. 
     Note that unless the high-speed clock is approximately an integer multiple of the low-speed clock, the frequency divider  100  will not be able to provide an accurate low-speed clock frequency. Note also that the higher the ratio of the high-speed to low-speed clock frequencies, the finer the available resolution. Consider, for example, a 100 MHz high-speed clock, and a desired low-speed clock frequency of 18 KHz. A ratio of 5555 provides a low-speed clock of 18,001.8 Hz, and a ratio of 5556 provides a low-speed clock of 17,998.5 Hz. In this example, 5556 would be selected, and the low-speed clock will be within 1.5 Hertz of the desired 18 KHz (less than 0.01% error). Consider, however, a high-speed clock of 100 KHz, and the same desired low-speed clock frequency. A ratio of 5 provides a low-speed clock of 20,000.0 Hz, and a ratio of 6 provides a low-speed clock of 16,666.7 Hz. In this example, the best option (a ratio of 6) will produce an error of over 1,333 Hz. (over 7% error), and would likely be unacceptable. Generally, for acceptable frequency accuracy, a minimum ratio of 50:1 is required for supporting independent high-speed and low-speed clock frequencies. That is, the high-speed clock frequency is typically selected by the designer of the core processor  150 , or other components within the microprocessor system, and the low-speed clock frequency is typically determined based on existing communications standards, or based on characteristics of a device that is external to the microprocessor system. As such, the high-speed clock frequency and low-speed clock frequency are generally substantially independent of each other, and an integer-factor relationship between the high-speed and low-speed clocks cannot be assumed. To allow for the programming of an arbitrary low-speed clock frequency, a ratio of 50:1 between the high and low clock frequencies is generally considered a minimum requirement. 
     This minimum ratio of 50:1 for the general-purpose application of a programmable frequency divider had not been a significant constraint in typical microprocessor designs, because the master CPU clock has conventionally been substantially faster than the clocks required for serial communications, or other derived-clock applications. The increased need for low-power processors (lower processor clock speed), and the increased need for faster communications (higher interfacing clock speeds), however, has substantially narrowed the gap between CPU clock speeds and derived-clock speeds. 
     BRIEF SUMMARY OF THE INVENTION 
     It is an object of this invention to provide a frequency divider that can provide accurate low-speed clock frequencies with a minimal difference between the low-speed clock frequency and the clock frequency that provides the input to the frequency divider. It is a further object of this invention to provide a programmable frequency divider that has a resolution that is substantially less dependent upon the difference between the high and low clock frequencies than conventional frequency dividers. 
     These objects and others are achieved by providing a programmable fractional frequency divider that is configured to enable a finer resolution of output frequency than conventional frequency dividers. The programmable fractional frequency divider of this invention allows for the programmability of both an integer divisor as well as a fraction component. The average frequency of the output signal from the fractional divider is dependent upon both the integer divisor and the fraction component, thereby providing for a finer resolution to the average frequency of the output signal. This combination of integer and fractional frequency division is particularly well suited for the generation of signals for systems that are substantially jitter-insensitive. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The invention is explained in further detail, and by way of example, with reference to the accompanying drawings wherein: 
     FIG. 1 illustrates an example microprocessor system with a conventional programmable frequency divider. 
     FIG. 2 illustrates a microprocessor system with an example programmable fractional frequency divider in accordance with this invention. 
     FIG. 3 illustrates an example logic structure for adjusting an output period of a fractional frequency divider in accordance with this invention. 
     FIG. 4 illustrates an alternative example programmable fractional frequency divider in accordance with this invention. 
     FIG. 5 illustrates an alternative example logic structure for adjusting an output period of a fractional frequency divider in accordance with this invention. 
     Throughout the drawings, the same reference numerals indicate similar or corresponding features or functions. 
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     This invention is based on the observation that a number of derived-clock systems, and in particular, derived serial-port-clocks on conventional microcontrollers and microprocessors, are designed for use in jitter-insensitive applications. As noted above, conventional asynchronous serial communications systems, such as Universal Asynchronous Receiver-Transmitter (UART) systems, are designed to allow for noisy communication paths that may introduce substantial intermediate frequency changes or anomalies. For example, attenuation in the communications path may “round-off” transition edges of transmitted signals, thereby causing a delayed detection of one or more of each of the edges of the transmitted signals. At a receiving system, this rounding of the edges may produce a foreshortened, or elongated, pulse when the round-off edge triggers, or fails-to-trigger, a threshold detector in the receiver that is configured to identify each state transition of the input signal. Each affected transition corresponds to an effective change in the period, or pulse width, of the received signal, as interpreted by the receiver. Depending upon the degree of attenuation at any point in time, different pulses of the received signal will exhibit different received pulse widths, and conventional receiving systems are designed to accommodate these varying pulse widths. On the other hand, because asynchronous communication systems do not, by definition, have a reference clock that is common to the transmitter and receiver, such systems rely upon the transmitter and receiver having substantially the same pulse frequency. That is, conventional asynchronous communications systems assume a common pulse rate, but allow for substantially varying pulse durations. Alternatively viewed, conventional asynchronous communications systems assume a common average frequency, but allow for substantial jitter about this common average. This invention is particularly well suited for systems that require an accurate average frequency, but allow for jitter about this average. 
     FIG. 2 illustrates a microprocessor system with an example programmable fractional frequency divider  200  in accordance with this invention. The programmable fractional frequency divider  200  includes the counter  110  and register  120  of the prior art divider  100  of FIG. 1, but also includes an adder  240  that is configured to selectively vary the value that is loaded into the counter  110 , depending upon the state of a Carry input signal to the adder  240 . If the carry input signal is asserted, the value provided to the count-down counter  110  is one more than the value that is currently in the register  120 . That is, if the value in the register  120  is N, the Baud Rate Clock output signal from the counter  110  will occur after N+1 cycles of the Xtal Osc Clock input signal (as discussed above with regard to FIG. 1) when the Carry input signal to the adder  240  is zero, and will occur after N+2 cycles of the Xtal Osc Clock when the Carry input signal to the adder  240  is one. In this manner, the output signal from the counter  110  is either a “divide-by-N+1” or a “divide-by-N+2” of the input clock signal. 
     Consider, for example, an assertion of the Carry input signal to the adder  240  every other time that the output Baud Rate clock is asserted. When the Carry input is zero, the clock input is divided by N+1; when the Carry input is one, the clock is divided by N+2. This corresponds to an average division of the clock input signal by a factor of N+1+0.5. If the Carry input signal is asserted every fourth cycle of the output clock, the average division is by a factor of N+1+0.25; if the Carry input signal is asserted every eighth cycle, the average division is by a factor of N+1+0.125; and so on. Note that the assertion of the Carry input signal stretches the pulse duration, relative to the duration when the Carry input signal is not asserted. Relative to the average pulse frequency, the assertion of the Carry input signal introduces a slower intermediate pulse frequency, and the non-assertion of the Carry input signal introduces a faster intermediate pulse frequency. That is, the Carry input signal introduces a positive or negative jitter about the average pulse frequency. 
     The sizes of the registers  120 ,  220  and counters  110 ,  210  (twelve bits for the integer division and four bits for the fractional division) are illustrated as typical sizes, although it would be evident to one of ordinary skill in the art that the principles of this invention are applicable regardless of the particular sizes of the registers and counters. 
     Any of a variety of techniques can be employed to determine when to assert the Carry input signal to provide an average output frequency that differs from an integer division of the input clock frequency. Illustrated in FIG. 2 are a counter  210 , a fraction register  220 , and a “+1” logic block  230  that determines whether the Carry input to the adder  240  is asserted. In this example embodiment, the register  220  is a four-bit register that contains a value, F, between 0 and 15, and the +1 logic block  230  is configured to assert the Carry input signal every F/16 cycles of the output Baud Rate Clock. In this manner, the resolution that is achievable in the average output frequency of the divider  200  is {fraction (1/16)} as fine as the resolution that is achievable by the prior art divider  100  of FIG.  1 . Thus, if a minimum ratio of high to low clock frequencies is 50:1 for the general purpose application of the programmable divider  100 , the programmable divider  200  in accordance with this invention requires a mere 3:1 (50/16:1) ratio to achieve the same resolution in average output frequency. 
     In the example of FIG. 1, wherein the high-frequency clock is 100 KHz and the low-frequency clock is 18 KHz, the register  120  is loaded with a value of four, and the fraction register  220  is loaded with a value of nine, thereby providing an effective division by 4+1+9/16, for an average output frequency of 17,977.5 Hz (100 KHz/5.5625), which is within 23 Hz (0.13%) of the desired low-frequency clock output frequency, as compared to an error of 1333 Hz (7.4%). In operation, the counter  110  provides an output every six cycles of the input clock for 9 out of 16 of the output cycles, and every five cycles of the input clock for the remaining 7 out of 16 output cycles. Preferably, the +1 logic  230  is configured so that the six-cycle and five-cycle periods occur as uniformly as possible within each 16 output cycles, to prevent long periods of one frequency (100 KHz/5) followed by long periods at another frequency (100 KHz/6). 
     FIG. 3 illustrates an example logic structure for adjusting an output period of a fractional frequency divider  200  in accordance with this invention, to uniformly distribute the different frequency periods as much as possible. F 3 -F 0  represent the contents of the fraction register  220 , and C 3 -C 0  represent the contents of the counter  210 , from most-significant bit to least-significant bit. A “b” suffix (“bar”) indicates a complement of the indicated signal. 
     Consider a fraction value of “1000” (F 3 =1, F 2 -F 0 =0), which indicates that eight of sixteen output cycles are “divide-by-N+1” and the remaining eight are “divide-by-N+2”, for an average dividing ratio of N+1.5. Gate  310  is an AND of F 3  (logic-1) and the least significant bit of the counter  210  (logic-1 at every other cycle). Each of the other AND gates  320 ,  330 ,  340  have a logic-0 output, because F 2 -F 0  are at a logic-0. The “+1 output” of the gate  350 , therefore, is assert at logic-1 at every other cycle of the counter  210 , thereby providing alternating divide-by-N+1 and divide-by-N+2 cycles, rather than a series of divide-by-N+1 cycles followed by a series of divide-by-N+2 cycles. In like manner, if bit F 2  is set to a logic-1, the “+1 output” of the gate  350  is asserted each time the least significant bits C 1 -C 0  of the counter  210  are “ 10 ”, via the gate  320 , thereby providing a logic-1 output every four out of sixteen counts of the counter  210 . If both bits F 2  and F 3  are set, and F 1  and F 0  are clear (“1100”), the divide-by-N+1 occurs at every odd cycle, and at every fourth cycle, for a total of twelve cycles (8+4) out of sixteen. In like manner, gates  330  and  340  enable the assertion of the “+1 output” every eighth cycle and every sixteenth cycle, respectively. 
     Other logic structures for selectively causing a longer-duration period between output pulses will be evident to one of ordinary skill in the art in view of this disclosure. 
     FIG. 4 illustrates an alternative example programmable fractional frequency divider  400  in accordance with this invention. This alternative example comprises logic gates  410 ,  420 , and  430  that replace the function of the adder  240  in FIG.  2 . The same “+1 logic” block  230  asserts an output signal whenever a longer-duration period between output pulses is required to achieved the desired average output frequency. In this alternative embodiment, however, when the “+1 output” is asserted, a logic-1 is provided to the input of the delay flip-flop  420 , via the AND gate  410 . The output of the delay flip-flop  420  corresponding to this asserted “+1 output” is provided one cycle later to the OR gate  430 . This causes the counter  110  to be “re-loaded” twice in succession, once when the output Baud Rate Clock is asserted, and immediately again when the delayed output from the flip-flop  420  is asserted. Because each re-load consumes one clock cycle, the successive reload introduces an additional clock cycle to the down-count cycle of the counter  110 , thereby providing the selective longer-duration period between output pulses. 
     FIG. 5 illustrates an alternative example logic structure  500  for adjusting an output period of a fractional frequency divider in accordance with this invention. The accumulator  530  with cyclic feedback replaces the counter  210  and “+1 logic”  230  of FIGS. 2 and 4. In this embodiment, the value of the fraction register  220  is repeatedly accumulated at each cycle of the output Baud Rate Clock. Each time the accumulator  530  overflows, and generates a carry output, a longer-duration period between output pulses is provided, corresponding to the “+1 output” signal&#39;s function detailed above. The accumulator  530  is of the same width as the fraction register  220 , so that the overflow occurs with every accumulation that exceeds the maximum value of the register  220 . If, for example, the fraction register  220  contains a value of “1000”, an overflow will occur at every half cycle. If, on the other hand, the fraction register  220  contains a value of “0001”, an overflow will only occur at every sixteenth cycle; if the fraction register  220  contains a value of “1010”, an overflow will occur at every ten out of sixteen cycles. Using the convention of “(C) Sum” to indicate the output of the accumulator  530  with an input of 1010 from the register  220 , the output sequence will be: {(0)1010, (1)0100, (0)1110, (1)1000, (1)0010, (0)1100, (1)0110, (1)0000, (0) 1010, (1)0100, (0)1110, (1)1000, (1)0010, (0)1100, (1)0110, (1)0000}, for a total of ten carry (1) assertions, and six carry (0) assertions, that are fairly uniformly distributed. 
     The foregoing merely illustrates the principles of the invention. It will thus be appreciated that those skilled in the art will be able to devise various arrangements which, although not explicitly described or shown herein, embody the principles of the invention and are thus within the spirit and scope of the following claims.