Re: divide by 25/16

On Fri, 18 Aug 2006 14:01:05 GMT, "colin"
<no.spam.for.me@xxxxxxxxxxxx> wrote:


"Joop" <jojo@xxxxxxxxx> wrote in message
news:tss9e2d8q3dtl6587k1lsvkgp1jshmsihg@xxxxxxxxxx


aha yes that would do, many thanks, shld be able to do in 2 chips a
74'390
dual decade counter and a 74'00.

I had written down the numbers in base 5, and had a big page of possible
sequences looking for easy decodes, I think I just about had this
sequence
but didnt see the wood for the trees and overlooked its simplicity, im
impresed at ariving at this so quickly :)

Colin =^.^=

I actually have such a device. It is a :100 prescaler using the
74LS390 after a U664 1.2GHz :64 counter.

Apparently it was based on a Siemens appnote. Basically there are two
5/4 counters cascaded.
An LS02 is used next to the LS390. The input at pin 12 is also fed via
an inverter and norred with pin 9. The output goes into pin 4. Pin 4
is inverted and norred with pin 7. This nor output is your :100
signal.

Cheers.

Joop

Cool thanks, I tried to find the appnote with google but no luck

Colin =^.^=

I could not find it either, but here is something similar to my
complete circuit. In mine the last two ls02 ports are left out.
http://www.ure.es/gpa/modules.php?name=Downloads&d_op=getit&lid=169#search=%22prescaler%20de%201300%20MHz%2074ls390%22
The original idea was using the 74167, but that chip seems difficult
to get these days. It is also used in another version in the above
article by the way.

Anyway I am not sure if 50MHz input is a bit too high. You might need
an additional :10 counter in front of the LS390. That is something I
would not have mind either, since it would put the decimal point of my
frequency counter back a normal spot.

Joop
.

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