[time-nuts] Frequency divider design critique request

Hal Murray hmurray at megapathdsl.net
Wed Jul 16 05:12:27 EDT 2008


[Context is filtering on inputs from switches.]

Could somebody tell me why we are doing anything fancy at all?  What's wrong 
with just a simple pullup?

I can think of two cases that might be interesting.

One is signal integrity.  There might be enough crosstalk to cause troubles.  
This is a 4 layer board.  Right?  Unless the traces from the switch to the 
mux chip wrap around some high speed signal there isn't likely to be a 
problem.

I'm not a signal integrity wizard, but the ballpark is that you only need a 
few trace widths of separation between agressor/source and victim/receiver.  
This case is slightly ugly since the pullup is not low impedance like a 
typical driver.

So the question becomes how small a pullup do we need to maintain good signal 
integrity?  Or how far from a nearby trace do we have to be with a given 
pullup?

If the coupling is primarily capacitive, then we have a C-R high pass filter. 
 I'm not sure that's valid, but it is easy to analyze.  I should be smart 
enough to work this out, but it's late.  HC is pretty slow.  AC might be fast 
enough to make things interesting.  But this only matters if the switch 
traces run parallel to a trace that is active for a significant length.

What is the (ballpark) output impedance of a CMOS driver?  What's the input 
capacitance on a CMOS gate?  (It's in parallel with the R, making a C-C 
divider at high frequencies.)


The other possible complication is trying to keep a clean output signal when 
the switch changes state.  I'm assuming we don't really care what happens 
while the contacts are bouncing between switch settings.  If we do, then we 
need serious switch debouncing and synchronization.  I don't think anybody 
has mentioned this tangle.


All the switch inputs I've worked with recently have been a simple 
pullup/down.  They usually go into a small CPU using the internal pullups and 
software debouncing.

I have heard war stories of cross talk on reset signals.  They often wander 
all over the board and since they are slow, they often get overlooked when 
checking for crosstalk.







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