[time-nuts] low power divide by 5

[Hal Murray]

> You might try the 74AC161, which works to 73MHz at 3.3V or 103 MHz at 5V, -40
> to 85C.

> Set the data inputs to DCBA = 1011 and connect an inverter from the carry
> output (pin 15) to the Load input (pin 9) to divide by 5. See http://
> www.techlib.com/electronics/74161Divider.htm 

You didn't read the data sheet carefully enough.  That 73 MHz is the bragging 
number for sales people, often not useful.  For something like this, you need 
to add the clock-to-out for the ripple carry, prop time through inverter, and 
setup time at the load input.

I was going to ask whether 73MHz included the delay through the inverter, but 
it's much worse than that.  The clock to out on the RCO pin is 21 ns.  Even 
without the inverter, it won't make 50 MHz.

You can save a few ns if you use a FF with inverting output instead of an 
inverter.  That adds a pipeline stage so you have to adjust the constant that 
gets loaded.  Setup time on a 3V AC74 is 4.3 ns which gets to 40 MHz (actually 
only 39.5).

At 5V,
  AC161 clk-RCO is 15.2
  AC74 setup is 3.1
So that works - 54.6 MHz.

Using an inverter:
  AC161 clk-RCO is 15.2
  AC04 prop 5.9
  AC161 setup 5.3
That's 37.9 MHz

(That's all assuming I didn't fatfinger anything.)

I like Richard Karlquist's trick of using a data bit to reload.  
Unfortunately, for the AC161, the data out isn't significantly faster than the 
carry out.

If I did the numbers correctly, that's 35 MHz at 3.3V and 49.3 MHz at 5V.








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