[time-nuts] low power divide by 5

[ed breya]

Jim,

At the risk of knocking over another bucket of worms, if your definition 
of "low power" can be extended to just beating the needs of the current 
part (circa 1W?), then you can look at ECL and its more modern 
derivatives, which are quite extensive. The classic bi-quinary ECL 
counter is the 10138 - another of my favorites. It's also long obsolete, 
along with a lot of others, and 10H series too. But, a lot of more 
modern ECL parts exist in a number of families, even going down to 3.3V 
and below, and with fairly low power requirements.

I used to be familiar with a lot of the choices, but always forget if 
not revisited often enough. I've seen all sorts of parts in the 100E and 
10Exx families. The fixed-decade counter may be long gone, but there are 
programmable counters in these families that are so fast that it should 
be no sweat to do a divide by 5, with little concern for external prop 
delays in a 50 MHz system. The problem then may be finding something 
small (preferably 4 bits if such exist) enough and slow enough (family 
dependent) to minimize the power, and fast enough to get the job done.

One example is the 10/100E016, which is a power hog because it's 8-bits, 
but can run below 1W at 5V.. I don't know or recall if there are a lot 
more counters (probably not many) to choose from, or which may be at 
risk of going extinct (everything does ultimately). If there are no 
4-bit counters, one trick that may help a little with power is to leave 
off the terminators/loads of all unused outputs, as long as it doesn't 
upset things internally. Also, running as low a supply as possible 
helps, so a 3.3V family should be better, for a given speed class. As 
you can see, it's about ten times as fast as you need, so if any slower 
families can be found, you can save some power. Especially in the "10" 
series, I think the one or two letters that follow, are indicative of 
both brand and speed class - it's a bit confusing to me. Look for the 
base part numbers first, then figure out the letters. A 10EP016 is much 
faster than a 10E016. A 10KP016 is just another brand, as far as I know.

Here's a data sheet:
http://ww1.microchip.com/downloads/en/DeviceDoc/sy10-100e016.pdf

Ed