[time-nuts] low power divide by 5

[dschuecker]

Hi,

a divide by five should possible with a synchronous state-machine made 
of 3 ( sufficiently fast-) JK-FlipFlops.

All 3 FFs are clocked with the input freq. , the outputs of the FFs are 
fed back to the the JK-inputs,  the divided freq. is output of one of 
the FFs.

Additional constraints: no external ANDs or ORs or NOTs, the 
state-machine does not get stuck in the 3 unused states.

This turned out to be a very interesting problem and I do not yet come 
up with a solution. Maybe there is none. Analytical solutions all 
failed, I will try a brute force enumeration attack tomorrow.

lots of fun !

Cheers

Detlef



Am 30.06.2020 um 08:37 schrieb 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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