Browse Prior Art Database

REAL TIME CLOCK TIMEBASE WITH AUTOMATIC 50/60 HERTZ SYNCHRONIZATION

IP.com Disclosure Number: IPCOM000005530D
Original Publication Date: 1984-Apr-01
Included in the Prior Art Database: 2001-Oct-12
Document File: 2 page(s) / 84K

Publishing Venue

Motorola

Abstract

The frequency characteristics of commercially generated A.C. line supplies are generally known to be extremely precise over a long period of time. Many consumer clock circuits use this fact to achieve a high degree of accuracy in their time keeping chore. Unfortunately, this high stability is lost when the power line fails.

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(fi) MOTOROLA Technical Developments Volume4 April 1984

REAL TIME CLOCK TIMEBASE WITH AUTOMATIC 50/60 HERTZ SYNCHRONIZATION

   The frequency characteristics of commercially generated A.C. line supplies are generally known to be extremely precise over a long period of time. Many consumer clock circuits use this fact to achieve a high degree of accuracy in their time keeping chore. Unfortunately, this high stability is lost when the power line fails.

   Oftentimes, microprocessor control systems will implement a real-time clock feature. A signal with an exceptionally stable frequency is used as an interrupt source to the processor. This timebase interrupt is counted by the processor allowing it to keep an extremely accurate accounting of real time.

   The stable line frequency is an obvious choice .as a timebase reference. With simple signal conditioning, the line frequency may be used directly as the interrupt source. However, some circuit alteration is usually necessary should the system be implemented in an environment using a different line frequency (i.e. 50 Hertz versus 60 Hertz).

   The circuit outlined in Figure 1 shows a timebase generator which is chiefly driven from a high frequency source such as a crystal oscillator. Because the crystal frequency changes with aging and temperature, its frequency characteristic alone is not stable enough to serve as a clock timebase. As shown in Figure 1, the crystal frequency is divided down to a relatively low frequency. This low frequency input (node A) is routed to the clock input of a presettable counter. The output timebase signal is derived from the most si...