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