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Is series resistor necessary in SDMMC CLK line for impedance matching?

Question asked by David Harrison on Sep 25, 2017
Latest reply on Sep 26, 2017 by John F.

Hi, I am using the STM32746NG on the STM32746NG-DISCO discovery board. I am accessing the on-board SDMMC connector at double bus speed, i.e. 48 MHz. According to the schematic for the DISCO board there is no series resistor in between port C12 and the SDMMC microSD connector. Everything seems to work well, although admittedly only tested at room temperature. I have seen some schematics on-line that recommend a 47 Ohm series resistor in the clock line, especially when clocking it at double speed. I assume that is to get a better impedance match and help reduce line reflections. I see the distance on the DISCO board is quite long, about 7cm or 70mm.

 

My production board will have the microSD card connector coupled quite closely to the STM32746NG with the trace length being no more than about 6mm. In that case can I get away with no series resistor as I would think line reflections at that short length would be negligible?

 

This is a cost sensitive product and every elimination of unnecessary parts helps, especially since I don't use 47 Ohm resistors anywhere else.

 

Thanks.

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