On 5/27/2017 12:21 PM, Gene Heskett wrote: > On Saturday 27 May 2017 11:26:18 Peter C. Wallace wrote: > >> You might try lowering the series termination resistor value since >> this looks like a possible SI issue (and the clock signal will be very >> sensitive to SI issues). > > SI? Acronym for what?
Signal integrity. Source, cable, and load impedance all need to match pretty well, but you knew that already. :) If you're running any distance, I'd recommend a buffer on the SPI lines. The SoC parts are designed to drive short PCB traces and typically only have a few mA of drive, not really enough to properly drive a cable and it's capacitance. For series termination to work well, the driver needs enough current output to drive the full signal across the series termination resistor. Otherwise, you wind up needing two full cable round-trip times to get a reliable signal at the far end, and you leave the load sitting halfway through the transition for a cable round-trip time. I'd wager if you just stick a reasonably fast driver (AHCT, LVC, or just about any 3.3V logic family) on the clock like at the RPi end to drive the cable (with a suitable[1] series resistor, probably 25-33 ohm), your problems will go away. [1] The driver output impedance plus the series termination resistor should equal the characteristic impedance of the cable. Most cables (ribbon with alternating ground, twisted pair Ethernet) are going to be around 100-120 ohms. The driver needs to be able to drive a full step (3.3V or 5V, depending on your logic family) into the effective impedance of the cable impedance plus the driver & series terminator (so 200-240 ohms). The I/O drivers on most SoC parts just aren't big enough to be able to do that effectively, so it takes two round-trip flight times to bring the load end to the final voltage, which also typically leaves the load end sitting in the transition region for one round-trip flight time. A recipe for problems when you're talking about a clock line. -- Charles Steinkuehler char...@steinkuehler.net ------------------------------------------------------------------------------ Check out the vibrant tech community on one of the world's most engaging tech sites, Slashdot.org! http://sdm.link/slashdot _______________________________________________ Emc-users mailing list Emc-users@lists.sourceforge.net https://lists.sourceforge.net/lists/listinfo/emc-users
