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# Developing a Slow Clock Rate from a Master Clock which is Not an Integral Multiple of the Slow Clock

IP.com Disclosure Number: IPCOM000048033D
Original Publication Date: 1981-Dec-01
Included in the Prior Art Database: 2005-Feb-08
Document File: 2 page(s) / 19K

IBM

## Related People

Higgins, PH: AUTHOR [+2]

## Abstract

Background: In designing a clocking system to supply several (m) component clock frequencies (f(i)), a master clock frequency (F) is selected which is the lowest common multiple (LCM) of all the m components f(i) to be generated. Then each separate f(i) is generated by using a single counter to divide the pulse train F by the integer quotient of the LCM divided by f(i). The LCM is equal to the product of the maximum power of each prime number contained in all m of the f(i). However, where one f(i) equals f(max) contains a large prime number such as 193, this technique for generating f(max) may require an F with an unreasonably high frequency.

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Developing a Slow Clock Rate from a Master Clock which is Not an Integral Multiple of the Slow Clock

Background: In designing a clocking system to supply several (m) component clock frequencies (f(i)), a master clock frequency (F) is selected which is the lowest common multiple (LCM) of all the m components f(i) to be generated. Then each separate f(i) is generated by using a single counter to divide the pulse train F by the integer quotient of the LCM divided by f(i). The LCM is equal to the product of the maximum power of each prime number contained in all m of the f(i). However, where one f(i) equals f(max) contains a large prime number such as 193, this technique for generating f(max) may require an F with an unreasonably high frequency.

Solution: Calculate the LCM for all the smaller f(i) and /fmax/ p, and us that value as the master F. To generate the f(max) having the largest prime number, divide the value of F by f(max), let the whole number portion equal a, and the fraction equal x. Use two counters to count the F pulse train, the first generating a count pulse for every a+1 pulse from f and the second counter generating a count pulse for every a pulse from F. Since the relationship (Equation 1) can be shown to be true, then by counting F with the first counter for x times followed by counting F with the second counter p-x times, the average counter pulse rate will be f(max). other cyclic patterns could require with the second counter to make consecutive counts of F.

Hardware: The sequential pattern for selecting either the first or the second counter as the source of the f(max) pulse can be store...