Browse Prior Art Database

AUTOMATIC REFERENCE SWITCH AND TEST PORT

IP.com Disclosure Number: IPCOM000006648D
Original Publication Date: 1992-Dec-01
Included in the Prior Art Database: 2002-Jan-18
Document File: 2 page(s) / 99K

Publishing Venue

Motorola

Related People

Timothy Smith: AUTHOR

Abstract

BACKGROUND OF THE INVENTION SUMMARY OF THE INVENTION This invention relates to the selection of reference frequencies. Many electronic machines use an internally generated reference frequency for their operation and it is sometimes possible to lock this reference to an exter- nal reference. This is usually done through a port which may be used to either monitor the internal reference or used to inject the external reference.

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MOTOROLA INC. Technical Developments Volume 17 December 1992

AUTOMATIC REFERENCE SWITCH AND TEST PORT

by Timothy Smith

BACKGROUND OF THE INVENTION SUMMARY OF THE INVENTlON

  This invention relates to the selection of reference frequencies. Many electronic machines use an internally generated reference frequency for their operation and it is sometimes possible to lock this reference to an exter- nal reference. This is usually done through a port which may be used to either monitor the internal reference or used to inject the external reference.

  There is an electronic switch (or buffer) which is used to disable the internally generated reference fre- quency. Two pulse generators (such as monostable multivibrators) which may be triggered, re-triggered, reset or disabled control the automatic disabling of the inter- nal reference frequency and the selection of the external reference frequency.

  In this case, the internal reference is not locked but switched off and the external reference switched in its place when external reference is selected. The same con- nection serves as both monitor and injection point.

DESCRIPTION OF THE DRAWING

  This aim of this invention is a low cost method of automatically switching t&n an internal reference clock to an externally generated reference clock when the exter- nal reference is applied and to switch back to the inter- nal reference when the external reference is removed.

SUMMARY OF THE PRIOR ART

In the previous application, a switch was used to manually select between internal and externl reference.

  The circuit comprises of the buffer (1) and of 2 monostable multivibrators (2 & 3). Each monostable has an "R" input which may be used to reset or hold in reset the device. The period of the first monostable (2) is set to be equal to or longer than the propagation delay through the buffer (1) but much shorter than the period of the internal reference. The period of the second monostable (3) is set to be longer than the period of the external reference frequency. The second monostable must be re-triggerable. Providing these criteria are met, exact timing of the monostables is not critical. Each monostable, when triggered, disables the other.

1. Buffer To circuit

External

Internal Reference

1

A C

3. Monostable

24 0 Motorola, 1°C. 1992

R C

2....