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A SIMPLE TECHNIQUE FOR FRACTIONAL DIGITAL DIVISION

IP.com Disclosure Number: IPCOM000005939D
Original Publication Date: 1990-Oct-01
Included in the Prior Art Database: 2001-Nov-16
Document File: 2 page(s) / 83K

Publishing Venue

Motorola

Related People

Mark J. Johnson: AUTHOR

Abstract

This invention relates to digital divider circuits, and specifically dividers whose divide ratios are not integer sub multiples of the input clock.

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MOrOROLA Technical Developments Volume 11 October 1990

A SIMPLE TECHNIQUE FOR FRACTIONAL DIGITAL DIVISION

by Mark J. Johnson

This invention relates to digital divider circuits, and specifically dividers whose divide ratios are not integer sub multiples of the input clock.

   In many applications, it is desirable to generate a clock which is a noninteger submultiple of an existing clock. While many circuits exist to accomplish this, most are unwieldly and complex due to an attempt to either reduce the jitter or spread it out over a large bandwidth, eliminating spurs. In most digital applications, such as serial clocks or regular microprocessor interrupts, some jitter is tolerable as long as the rate is exact. This type of clock may even be used judiciously to drive some analog circuits such as switched capacitor filters, as long as the spectral consequences are understood and compensated for.

   This circuit proposes to create a usable fractional clock. This clock will obviously have some jitter, but it will be predictable and will not accumulate more than one input clock cycle over any period of time. This is adequate for most strictly digital applications such as data clocks and periodic processing interrupts, and may be used with care in some analog applications such as switched capacitor filters. This divider architecture is as shown in Fig. 1.

f1- Conditional divider If2

Figure 1. Bask structure of fractional divider.

This divider produces a clock having the frequency f,=f,/(n+B/A), where B is the order of Xu[O..A-11. The purpose of the match logic in the above diagram is to create the B aspect of the divide ratio...