22:59 < karlp> BrainDamage: yeah, proper questions from you of course. building 
               test  harness host board to plug all the nucleo64 boards into 
               for locm3 testing.  Adding a port for plugging a cheap fx2 logic 
               analyser on, so I can do auto testing with sigrok, 
23:00 < karlp> I'm estimating like 20-30mm on the side tap to the logic 
               analyser port, plus 30-40mm or so between target/host.
23:00 < karlp> how fast can I go on the signals before I need to do anything I 
               guess?
23:00 < karlp> i2c/uart are probably fine, spi might have an upper limit?
23:06 < BrainDamage> the highest frequency for squarewaves is 2/trise for 
                     signal integrity, assuming say, 30MHz spi that gives us 
                     30ns period, trise would be say, 1/5th of a semiperiod so 
                     30/2/5 ~3ns, so that gives us a frequency of approx 
                     ~700MHz for signal integrity, in turn that has a 
                     wavelength of c/700MHz ~430mm, now a common ballpark is 
                     that when your circuit size is comparable to 1/10th of the
                     wavelength, you start seeing
23:06 < BrainDamage> effects, since you said it's overall 60mm it's more than 
                     40, so yes, it'll distort the signal a bit leaving it 
                     dangling there
23:07  * karlp loves you
23:08 < karlp> so, is the solution "throw small series resistors right by the 
               stub off to the dangling trace, so that it dissipates any 
               reflections on that line?
23:08 < karlp> I guess this also then changes when the LA is plugged in/not in 
               too.
23:09 < BrainDamage> the LA tends to be a very high impedance, so it doesn't 
                     change a lot when it's in compared to the open circuit case
23:09 < BrainDamage> series termination should work
23:09 < karlp> adds capacitance though, I've foudn that out the hardware with 
               insufficient i2c pullups, but that's already taken care of.
23:09 < karlp> and yeah, just like 22ohm right on the beginning of the stub, by 
               the T-junction?
23:12 < BrainDamage> i forgot the exact value for series termination, it was 
                     either 2x or 1/2 of the nominal impedance, if 22 is what 
                     you've seen before then i guess it was 1/2 and yes, that 
                     should fix it
23:14 < BrainDamage> if you feel brave there's faster terminations using 
                     clamping diodes and caps, buuut, i'd say to keep it simple 
                     first