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I am in the process of designing a breakout-board style PCB for a project that uses the ORG1411-PM01 module (datasheet here:https://www.origingps.com/wp-content/uploads/2016/11/Nano-Hornet-ORG1411-Datasheet.pdf) and I am trying to figure out if a suggested component is required. enter image description here

In the Typical Application Circuit (Figure 9, pg 26) a 3 Terminal RC Filter Array is placed on the comms lines between the GPS module and the host MCU. It is a Murata module NFA31GD1004704.

Main question - What is this unit (the RC filter) really meant to do? Is it really necessary?

I have come across other example schematics where it is not included - for example Mikro Nano GPS click (I will have to attach the link in comments as I do not yet have sufficient reputation).

Any advice greatly appreciated!

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The RC filter is a low-pass filter and is meant to get rid of high frequency noise on the bus due to reflections. The edges on an SPI bus can be fast enough that the resulting ringing can look like additional clock pulses, incorrect data, etc.

Is it really necessary?

It depends. The bus may communicate fine without it. It's easier to place some sort of filtering in the comms lines before spinning the board rather than after. You can always depopulate unneeded components, and/or replace with a 0-ohm until the footprint can be removed.

If the question is whether it will damage anything if omitted, the answer is almost certainly 'no'. You just run a higher chance of noisy SPI lines, and the fix will be messier if you don't have some sort of footprint for a filter already in place.

If in the end all you want is a more generic alternative rather to this specialized RC filter part, I've had some success with just putting a 22-Ohm resistor in series with each SPI line. Place the resistor nearest where the signal originates.

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  • \$\begingroup\$ Many thanks for the clear and comprehensive response! \$\endgroup\$
    – R. Fitz
    Feb 10, 2017 at 10:54

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