From: Michael S. <mi...@sc...> - 2013-09-24 16:29:26
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On 09/24/2013 03:30 PM, Paul Fertser wrote: >> If you have signal integrity issues, increasing the drive strength of the >> outputs may actually make the problem worse due to increased ringing. > So true. Do you probably have any suggestions on how to use > raspberrypi without additional buffers? What series resistors > should be attached and what would be the best drive strength in this > case? I would guess maximum drive strength combined with the right series resistors - you will have to attach a scope to determine these, as the proper matching depends on the output impedance of the RPi's drivers, and the cable impedance. Using a flat cable with every second pin grounded will help to reduce crosstalk and get a constant impedance. A ribbon cable with every second pin grounded should have around 100 Ohm impedance, so 80-100 Ohm resistors should provide a good starting point. Observe proper high-speed probing techniques when attaching the scope - you will probably need 60MHz or more bandwidth, and a *short* gound lead on the probe (best none at all) to get useful results. Measure close to the target - a source-terminated signal will show reflections everywhere except at the end of the line. (see http://www.signalintegrity.com/Pubs/straight/probes.htm if you don't know already). cu Michael |