Frequency synthesizer.

The present invention relates to a frequency synthesizer which may be used in a data processing system to provide a constant frequency clock signal from a plurality of different input frequencies.

For example, a data processing system may include an input/output (1/0) interface unit which utilizes one or more counter signals, each of which has a substantially constant frequency and is derived from a system input clock signal. The system I/0 interface unit, however, may receive a system input clock signal which has any one of a plurality of different but known, frequencies. Accordingly, such a unit can include a frequency synthesizer which can respond to any of the plurality of different frequency input signals and can still derive the desired one or more counter signals therefrom at the desired substantially constant frequencies involved.

The frequency synthesizer according to the invention is defined in the appended claims.

The invention will now be described in more detail, by way of example, with reference to the accompanying drawings, in which: <ul id="ul0001" list-style="none"><li>Fig. 1 is a block diagram of a frequency synthesizer unit;</li><li>Fig. 2 is a chart which depicts a parsing algorithm used for the operation of the frequency synthesizer of Fig. 1; and</li><li>Fig. 3 is a timing diagram showing how the synthesizer 'output signal is generated from two different input signals.</li></ul>

The frequency synthesizer may be used in a system I/0 unit such as that described in EP 0 035 334 (and in application No. which, like the present application, is divided out of 81 300 560, published as EP 0 035 334).

The system I/0 interface unit is designed so as to be capable of operation at more than one input system clock frequency and is, therefore, adaptable for use with a system wherein the system clock is not expected to remain at a single fixed frequency for all operations. The frequency synthesizer unit is provided so as to be responsive to one of a plurality of different selected input frequencies to produce the desired internal clock frequency required for operation of the internal devices of the interface unit and the desired master baud rate signal for operation of the asynchronous terminal system. In a particular embodiment, for example, the internal clock signal must have a substantially constant frequency of 1 MHz so as to provide for correct operation of the internal, or local, devices (such as a programmed interval timer or a real time clock), and a substantially constant selected baud rate derived from a substantially constant master baud rate signal for the asynchronous terminal input/output logic.

The apparatus for effecting such frequency sythesis is shown in Fig. 1. In a particular embodiment thereof, the clock rate of the selected input clock signal 0 to the frequency synthesizer unit may be one of eight different clock signals having clock periods which vary from 300 nanoseconds (nS) to 600 nanoseconds (see chart of Fig. 2). For example, in a particular embodiment eight different signals having clock periods, T, of 300 nS, 325 nS, 350 nS, 375 nS, 400 nS, 450 nS, 500 nS and 600 nS are utilized. The input frequendy is identified by a 3-bit input signal on an internal bus U of the I/0 interface unit, namely as U-bus bits U 9, 10, 11. This signal is supplied to a clock rate latch and decode ROM 80, which identifies which of the eight clock rates must be latched for supply to the frequency synthesizer. The selected clock rate so latched is supplied to a high-count position and parsing ROM 81, to a denominator select ROM 82, and a high-low integer select ROM 83. The functions thereof can be understood better with the help of an example explained with reference to the chart of Fig. 2.

This chart depicts an example of the above frequency synthesizer technique for generating the pseudo-constant 1.0 MHz output clock for each of the eight different input clock signals.

As can be seen therein, except for the 500 nS clock signal, the clock periods for each of the eight frequencies involved are non-integral multiples of the clock period for the desired 1.0 MHz signal. For example, an input clock signal having a 350 nS clock period completes 2 6/7 periods in the total 1000 nS clock period of a 1.0 MHz signal. Thus, for every 2 6/7 clock pulses of the 350 nS input clock signal, one clock pulse must be produced for the 1.0 MHz output clock signal, i.e., for any 20 input-tlock pulses, 7 output clock pulses are produced.

The chart of Fig. 2 depicts the parsing algorithm associated with each of the input clock signals. As can be seen therein, groups of input clock pulses are produced over repeatable or recycle time intervals the number of pulses in each group being selected as one of two numbers. Thus, for an input clock signal having a 350 nS_:period, seven groups of input pulses are utilized having either 3 pulses (high counts) or 2 pulses (low count). The position of the high count groups is depicted by the "high-count position" cclumn in accordance with the lettered column of the algorithm. Thus, the high count (3-pulse) groups are at the A and B positions of the seven group cycle (the low count group being at position C) so that the seven groups are as follows: <ul id="ul0002" list-style="none"><li>3 3 3 2 3 3 3</li></ul>

It should be noted that the non-integral count (i.e., 2 6/7) identifies the algorithm to be used. Thus for the 350 nS input clock, the low/high counts vary between "2" (the whole number) and "3" (the next highest integer). The numerator of the fraction identifies the number of high-count posiiton groups (i.e., "6" high count position groups), and the denominator identifies the total number of position groups ("7" position groups), some times referred to as the recycle time period. As another example the 300 nS clock signal requires a count of 3 3/9 and utilizes low/high counts of "3" and "4", a recycle time period (the denominator of the fraction) of "9" (i.e., 9 groups), three of which groups (the numerator of the fraction) are "high count" groups. The 3 high-count groups are placed in positions B and C. The other algorithms can be similarly described. In Fig. 1 the denominator select ROM 82 selects the appropriate denominator, or recycle time period, depending on which input clock RATE has been latched, the selected denominator (i.e., "7" for the 350 nS input clock) being identified by a 4-bit output of ROM 82. The ROM 81 identifies which of the eight parsing algorithms shown in the chart of Fig. 2 is to be used. In the example being discussed the parsing algorithm A.B. representing seven groups of 350 nS pulses having 3 3 3 2 3 3 3 pulses in each group is selected.

It should be noted that in each parsing algorithm one of two numbers of pulses is used in each group. Thus, for the 350 nS algorithm either 3 pulses (the high count) or 2 pulses (the low count) are used, in the 300 nS algorithm either 4 pulses (the high count) or 3 pulses (the low count) are used. The parsing ROM 81 effectively determines for each denominator count from denominator counter 84 which of the pulse counts (high or low) is to be used. Thus, for the 350 nS clock the parsing ROM 81 determines that the high count (3) is selected for the first three and the last three of the seven denominator counts and that the low count (2) is selected from the middle denominator count in the overall recycle counter period.

The high or low integer is then selected by high-low integer select ROM 83 to identify which of the counts on integer counter 85'must count so in order to supply one output pulse therefrom for each of the seven groups (i.e., whether the group contains 3 350 nS pulse counts or 350 nS pulse counts) so that for every 7 groups (making up 20 350 nS pulses in total) seven output pulses are supplied. Accordingly, as seen in the timing diagram of Fig. 3, the output of integer counter 85 is a 1 MHz clock signal, i.e., every 7000 nS (20 x 350 nS) the output counter 85 produces 7 pulses which is, of course, equivalent to one pulse every 1000 nS. A similar analysis is shown in Fig. 3 for the 450 nS input clock and can be made for each of the eight input clocks and associated parsing algorithms shown in the chart of Fig. 11.

As can be seen in Fig. 3 for the 350 nS input clock, while the pulse spacing of the output signal is not uniform, the number of pulses over each seven microseconds (i.e., 7000 nanoseconds) recycle time period is the same. Thus, while a timing error may occur during each recycle time period the error reduces to zero at the end thereof. For the 350 nS clock signal the maximum error which occurs during the recycle period is ¹ 150 nS, as shown in Fig. 3, and the error at the end of the recycle period is zero. For the 450 nS input signal maximum error is 300 nS and at the end of the recycle time period of 9000 nS, the error is zero. The maximum error for all of the input clock signals are shown in the chart of Fig. 2. Such errors are of little significance over the time periods required for operation of the internal units which utilize the lMHz signal so that for all practical purposes, the 1 MHz signal has an essentially constant frequency. It should be noted that in no case is the maximum error greater than the time period of the input clock signal involved.

The above frequency synthesis unit provides an extremely adaptable system I/0 interface unit which can supply the desired master clock signal for the local or internal devices and the desired master baud rate signal for asynchronous terminal operation even when the system input clock signal has one of a plurality of different selectable frequencies. The overall frequency synthesizer circuitry includes substantially two versions of the general unit shown in Fig. 1, one for the 1.0 MHz output signal and one for the master baud rate output. A baud rate selection circuit can be utilized to provide one of a plurality of 16 different baud rates from the master baud rate by suitable baud selection circuitry which provides integer multiples of the master baud rate as desired. The circuitry for dividing the master baud rate to obtain each of 16 different baud rates is well known to those in the art. Moreover, the 1.0 MHz clock rate can also be appropriately divided down to produce clock rates which are integer submultiples of the 1.0 MHz master clock rate for various internal devices as required. Lists of the exemplary baud rates which can be derived from a master baud rate of 614,400 Hz and exemplary local counter output signals (decode counter clocks) which can be derived from a master 1.0 MHz clock signal are provided below. <tables id="tabl0001" num="0001"><img file="EP0097977A2_D0001.tif" /></tables>