Timeline for answer to Guidance on medium-speed SPI bus over backplane by Justme
Current License: CC BY-SA 4.0
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| when toggle format | what | by | license | comment | |
|---|---|---|---|---|---|
| yesterday | comment | added | Justme | @AaronKimball The connector is so short that the mismatch caused by connector is not that significant. Assuming electrical length for connector is 2cm, that's ~10x shorter than the 22cm rule of thumb for a 4ns signal. Assuming you pick slow and weak enough types of chips, or use source termination to reduce slew rate, you barely need to consider impedance. Pin headers are used for multi-hundred MBps differential communications at around 100 ohms differential impedance, and e.g. few hundred MHz of analog video adequately runs over D-Sub connectors at 75 ohm single-ended. | |
| 2 days ago | vote | accept | Aaron Kimball | ||
| 2 days ago | comment | added | Tony Stewart EE since 1975 | 74AHC245 are 125 Ω max @ 25 ℃ so a 100 Ohm bus will be well damped. Alternative also 74AHCT245 with input threshold at 1.4V and 55 Ω max @ 25'C | |
| 2 days ago | comment | added | Aaron Kimball | Thank you. Why is the pin header impedance not an issue? Board-to-board pins just won't make meaningful reflections? Is there a rule of thumb for picking a "good" board impedance? Absent other constraints at this point I could go to 100 Ohm... or higher? SPI will be running at 3.3V. And, by default I reach for AHC logic b/c the relatively weak drive strength means less ringing to damp... but I could use something else like 74AC245 if the +/-12 mA (rated@ 3V) strength is more appropriate for this application. | |
| 2 days ago | history | answered | Justme | CC BY-SA 4.0 |