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Diffstat (limited to 'hw1/signal.integrity.wisdom')
| -rw-r--r-- | hw1/signal.integrity.wisdom | 41 |
1 files changed, 0 insertions, 41 deletions
diff --git a/hw1/signal.integrity.wisdom b/hw1/signal.integrity.wisdom deleted file mode 100644 index 47dff01..0000000 --- a/hw1/signal.integrity.wisdom +++ /dev/null @@ -1,41 +0,0 @@ -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 - |