NetNews Usenet Archive 1993 #3/NN_1993_3.iso/spool/sci/electron/23540

Internet Message Format | 1993-01-28 | 1.7 KB

Path: sparky!uunet!ogicse!news.u.washington.edu!carson.u.washington.edu!whit
From: [e-mail] (John Whitmore)
Newsgroups: sci.electronics
Subject: Re: Counter
Message-ID: <[e-mail]>
Date: 27 Jan 93 23:23:00 GMT
Article-I.D.: shelley.1k75gkINNebf
References: <[e-mail]>
Organization: University of Washington, Seattle
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NNTP-Posting-Host: carson.u.washington.edu

In article <[e-mail]> [e-mail] (John Kamchen) writes:

> [I need] a counter that counts to 1818, then resets back to zero (with
>some kinda \RCO when it hits 1818).
>I've looked through all my TTL data books, and nothing really strikes me
>as being 'right' for this application.
>The input clock pulse is 28mhz, so that rules out CMOS parts.

    This is well within 74HC series frequency capabilities;
only the old 4000 series CMOS need be ruled out.

    The number you're counting to is less than twelve binary
bits, so three four-bit counter chips will do it.  74HC163 or
equivalent would be my recommendation.  You can save a bit of
electric power by prescaling (so the high-speed clock doesn't
clock all the stages of a synchronous counter), and your
divisor has a factor of six: you might consider using a
74LS92 for divide-by-six function, followed by a counter
set for divide by 303.

    The general scheme for a fixed-modulus counter is to use the
CARRY output connected to the LOAD input, and to strap the
JAM data input to the appropriate number (in your case,
2048 - 1818).  When 1818 counts have elapsed, the counter
CARRY gives you your pulse (and restarts the counter at
its initial value).  The synchronous LOAD function on the
74163 is ideal for this.


    John Whitmore