I’m laying out a Zigbee end node for soil probes, based on a TI reference design for the CC2652. The nodes talk to a coordinator in the shed that forwards their readings to Hermetry.
Several components on the reference schematic are marked «NM»: two capacitors and an inductor near the antenna, a 32 kHz crystal and its two load caps, and a 0 Ω resistor next to a jumper footprint. The BOM lists them but with quantity 0. Do I put them on my board or not? And if not, why are they drawn at all?
NM is «not mounted», same as DNP (do not populate). The footprint is on the board but no part goes in it.
Data person asking: so the board has empty pads on purpose?
That’s what I don’t get either. Why put pads for parts you don’t fit?
I always read NM as nanometre on RF drawings, is that wrong?
Neha’s right, and Emily, on a schematic it’s not mounted. I’ve laid out a few of these TI and Silicon Labs radio boards, and the NM parts each have a reason:
Antenna matching (your two caps and the inductor): the match depends on your PCB, enclosure and antenna, so TI leaves a pi network footprint. You build the board, measure with a VNA, and then fit the values that bring the antenna to 50 Ω. Some designs fit a 0 Ω in series and leave the shunt parts empty until tuning.
32 kHz crystal: optional. The chip has an internal RC oscillator that’s good enough for many applications. If your end node sleeps a lot and needs accurate wake timing, fit the crystal; otherwise leave it.
0 Ω and jumper footprints: alternatives, like choosing between two power paths or connecting a debug interface. The reference design keeps both options.
For your soil probes I’d keep every NM footprint on the layout, fit the crystal (sleepy end nodes benefit), and plan one tuning session with a VNA on the final enclosure before you order a batch. In your BOM, mark them DNP so the assembler doesn’t fit anything there.
That clears it up. Kept the footprints, fitting the crystal, and borrowed a NanoVNA for the matching once the enclosures arrive. The first prototype with only the 0 Ω in series already reaches the coordinator from 60 m, so the tuning should only make it better.
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