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While reviewing the reference design for the TI CC2541EM 2-layer board, and the associated parts list, I noticed one component (C254) states "DNM - Do Not Mount".

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What is the reason for this? Why even draw this on the schematic if you don't intend the component to be populated? if they go to the trouble to change the schematic to read "DNM", why not just remove the component altogether?

OR, are they using the two unpopulated SMT pads as a capacitor? That seems strange.

OR, is this a placeholder for a cap that would be added later, to properly tune the antenna if needed?

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    \$\begingroup\$ Placeholder for possible cap, to tune. \$\endgroup\$
    – analogsystemsrf
    Sep 10, 2017 at 16:28
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    \$\begingroup\$ It's on the schematic because the location is on the physical board that they're selling as a demo. If somebody buys the board and there's a location that doesn't appear in the schematics, the apps engineers are going to have to respond to a bunch of questions about "what's this location on the board that isn't shown on the schematic?" \$\endgroup\$
    – The Photon
    Sep 10, 2017 at 17:24

1 Answer 1

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Many electronic designs have options, to accommodate different components (like a different antenna for example). Accommodating different components may require some component installed or removed. In any case the PCB has to have a proper placeholder for these optional components.

The PCB layout software therefore must have an information about these placeholders. In modern CAD tools the schematic database is usually integrated with layout tool and its database, and exhaustive check for connectivity is usually conducted, to avoid costly board re-spins. If one wants to remove a component from drawing, it will cause mismatch in databases, and will generate lots of errors. As result, manufacturing/production engineers prefer to keep their databases consistent across all tolls, which is the reason why the DNP (or DNM) components remain in schematics.

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