AUTOMATIC REFERENCE SWITCH AND TEST PORT
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Del.icio.us| IP.com Number | IPCOM000006648D |
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| Dated | Dec 1, 1992 UTC | ||
| Size | 2 page(s) (99.4 KB) | ||
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Publication Summary
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.| Country | United States |
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| Language | English (United States) |
| Related Person(s) |
(AUTHOR) Timothy Smith |
| Copyright | Motorola Inc. December 1992 |
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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....