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I am referring to CC2531 reference design from TI (http://www.ti.com/tool/CC2531USB-RD) In the schematic diagram (and in the BOM), it is saying ‘ Not Mount’ for some components (resistors, capacitors, testpins etc.). I am new to PCB design and wondering what that means. How it impacts the PCB design?

Please suggest.

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The board is laid out with footprints for the components indicated, but they are not populated by default. Sometimes the acronym DNP (Do Not Populate) is used.

Some variant of the board can be made with some or all of the indicated parts populated and possibly some other parts not populated. It allows various options of a board to be made with a single PCB pattern.

Sometimes this makes a great deal of sense- since the PCB is a custom part, you can minimize the number of custom parts by allowing for options that can be purchased by the customer for additional features, or for different models. Sometimes it doesn't, for obvious reasons of increasing complexity, more PCB area and reduced yield, more difficult PCB assembly because parts are crammed in and so on.

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    \$\begingroup\$ Also worth noting that it can be used as a contingency, especially on prototypes. Not sure if you need extra filtering on the power rail? Put the pads there, and populate if necessary. \$\endgroup\$ – Matt Young May 28 '14 at 20:34
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    \$\begingroup\$ DNL (do not load) is another acronym for this. \$\endgroup\$ – Nick Alexeev May 28 '14 at 21:05

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