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Discovery board series bus resistors

Question asked by R Letizia on Jun 7, 2017



Comparing Discovery Boards (STM32F769-EVAL and STM32F746-DISCO) the use of 33 ohm series resistors on the FMC and QSPI buses are different.    The STM32769-EVAL FMC address/data lines and QSPI data lines have series 33 ohm resistors, but the same lines on the STM32F746-DISCO don't have resistors.


Hardware Development Application note (AN4661) indicates a required 50 ohm trace impedance for these signals.  Both these boards appear to be controlled impedance design that would implement this 50 ohm characteristic impedance requirement.


So if these boards implement 50 ohm trace impedance, why is the 33 ohm resistor used?

Why are they used on one board and not the other for the same application?


Thanks for helping me understand whey to use this resistors.