SPI vs IO interface

Question

Hi, 
 Background: I am looking at interfacing nRF21540 (FEM) with nRF52833 (Host) - hoping there's Zephyr support available for this(?) 
 Question: Product spec for nRF21540 reads “ Control interface via I/O, SPI, or a combination of both ” – I think that means, I can use SPI bus exclusively for accessing All of the 21540 functions – i.e. no I/O needed, except for PDN (and SPI CSN)? If so, how are unused nRF21540 pins 4, 7, 8, 9 supposed to be terminated – left unconnected, tied to VDD, or GND? 
 In may case "SPI-only" interface (no I/Os needed) is convenient for sure. But in your experience/ opinion, is there any potential downside, such as performance and/or (OS/ driver/ example code) support penalty in taking this route? 
 Please advise. Thanks.

Andreas · Accepted Answer

Hi, 
 
 I would avoid routing and thus changing the layout underneath the nRF21540. This will cause mismatch and/or alter the grounding of nRF21540 while that is assembled, which should be avoided to minimize the risk of any surprises down the road. 
 I would add a transmission line around the device, something like this: This way you would mount the two green caps instead of C1 and the new blue cap, to disconnect nRF21540 without any shunt stubs in the RF line. Similarly, if nRF21540 is mounted, the bypass path will not introduce any shunt stubs, the blue cap must still be mounted but if using a rather large value (~10pF or larger) it will not cause any mismatch. This obviously requires a 4-layer stackup, or more, as the RF paths must have solid ground underneath, the control lines for nRF21540 must then be routed at the bottom layer or similarly far down the stackup. 
 
 Best regards, Andreas

SPI vs IO interface

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