FREQUENCY DIVIDER

TECHNICAL FIELD

The invention relates to a method and apparatus for achieving in a computer an optional division of a clock frequency Cl in a ratio A/B, where the quotient between B and A is a whole number C and a remainder D.

BACKGROUND ART

In synchronous operation, e.g. between computer systems or between input/- 5 output means in digital telephone exchanges, a timing accuracy of better than plus/minus 0,5% is required, where the clock pulse frequency in the trans¬ mission of data may vary from 64 kHz to 1200 Hz. Particularly with high clock pulse frequencies there is a problem, when data is received by a computer for processing and distribution, this being to keep a constant data output flow

10 within given accuracy limits for a continuous data input flow. It is known to use a buffer store, where outgoing data are stored for feeding out at a rate dependent on the space occupied in the buffer store. It is also known, with D/A signal conversion, to control a voltage-controlled oscillator depending on the degree of occupation in the buffer store, and subsequently acheive frequency

15 variations within the permitted plus/minus 0,5% via controllable division of the oscillator frequency. The method requires a complicated circuit structure.

DISCLOSURE OF INVENTION

The present invention, the distinguishing features of which are disclosed in the accompanying claims, relates to a method and an apparatus with the aid of which an optional division of the internal clock frequency of a computer may be 20 performed in an easiliy integratable way in a system.

BRIEF DESCRPTION OF DRAWINGS

The invention will now be described in detail with reference to the accompanying drawings, where Figure 1 is a block diagram showing, inter alia,

included registers and a dividing unit, Figure 2 illustrates the shape of a divided clock pulse and Figure 3 illustrates an application of an apparatus in accordance with the invention.

BEST MODE FOR CARRYING OUT THE INVENTION

Figure 1 is a block diagram where three registers 1, 2 and 3, each with its register content, are connected to a dividing unit 4. The internal clock of a computer is also connected to the mentioned dividing unit 4. The clock has a pulse frequency Cl, which is to be divided in a ratio A/B, where the quotient between B and A is a whole number C and the remainder D. With this clock pulse, the half pulses of which are equally as long, a first counter 5 is stepped forward one step for each half pulse. The count on this counter is compared with the register content C in the first register 1 in a first comparator circuit 7. For equality between the count and register content C the circuit 7 sends a signal which sets a bistable multivibrator 10, the divided clock pulse being sent from its output. The signal also resets the counter 5 to zero. In a first embodiment the signal also steps a second counter 9. In a second embodiment the counter 9 is stepped by the half pulses on the output of the bistable multivibrator 10, as denoted by the dashed line in Figure 1.

The count on the second counter 9 is compared in a second comparator circuit 12 with the register content (A-D) in the second register 2. For equality between this count and the register content (A-D) the comparator circuit 12 sends a signal setting the counter 9 to zero and also resetting two switches 8, 11. In its reset state the first switch 8 switches an adder circuit 6, which adds a "one" to the register content C in the first register 1. In another embodiment, unillustrated in Figure 1, the adding circuit 6 may be controlled in such a way that the mentioned "one" can also be subtracted from the content C of register 1. In its reset state the second switch 11 switches the third register 3 instead of register 2 so that the content D of the third register is compared with the count on the second counter 9. For equality the two switches 8, 11 are returned to their previous state, and the sequences described above are repeated. There is thus obtained a pulse train containing as many half pulses as the content (A-D) in the second register 2. The pulse length will be as many times as long as the pulse length of the clock pulse frequency Cl as is the

register content C in the first register 1. The pulse train furthermore contains just as many half pulses as the content D in the third register 3, the pulse length here being as many times as long as the pulse length of the clock pulse frequency Cl as is the content of the first register 1 plus "one", i.e. (C + 1).

A division of the clock pulse frequency Cl to 8/17 of Cl is illustrated in Figure 2. The quotient of 17 and 8 gives the whole number 2 and the remainder 1. The register 1 is then charged with the number 2, the register 2 with the number (8 -1) = 7 and the register 3 with the number 1. Seven half pulses are generated cyclically with a pulse length of 2 clock pulses and 1 half pulse with a pulse length of 3 clock pulses. The deviation in pulse length thus obtained is acceptable in certain applications. Such an application is described below in connection with Figure 3.

A number of input means supplying data synchronously with a pulse frequency of 64 kHz are connected to a computer 13 having a clock frequency of 4 MHz. The frequency accuracy of the means is better than plus/minus 0,5%. The data is processed in the computer and intermediately stored in a buffer store 14, from which data is fed out to a number of means connected to the computer, these latter means operating synchronously under the same conditions as the input means. It is essential for communication with the output means that the transmission speed is such that the buffer store is neither overfilled nor emptied. It is thus arranged that a first signal is sent when the level to which the store has been filled reaches a predetermined lower value, and that a second signal is sent when the filling level has reached an upper value. The rate at which data are supplied from the buffer store is controlled by these signals in the following manner. When the buffer store has reached the lower filling level the mentioned first signal is sent. The first register 1 is then charged with the number 62, which is the whole number of the quotient between 64 kHz and 4MHz. The second register 2 is charged with the number 1 and the third register 3 with the number 4. There is thus obtained a clock frequency divided down to 63.694 kHz (64kHz minus 0,48%). This clock frequency controls the feedout from the buffer store until the upper filling level is reached. The second signal then sent causes a change in the contents of the registers such that the second register 2 is charged with the number 4 and the third register 3 with the number 1. The divided pulse frequency is thus increased to 64,309 kHz

(64kHz plus 0,48%). The procedure is repeated when the buffer store has been emptied to the lower level. The deviation 0,5 from the mean pulse length 62,5 is 0,8% per pulse, and the total deviation is 4 x 0,8 = 3,2%.