nRF9151 SMA DK power input questions

[m4rkw]

Hi

I'm working on a new project using the nRF9151 SMA DK and I have some questions that I haven't been able to answer by searching this forum or reading through the documentation.

1. The hardware guide says the board should be powered from either USB or the VIN pin (pin 9 on P20). However it would be advantageous for my project if I could instead create a shield during prototyping that supplies 5v via pin 5 of P1. SB35 (closed by default) connects the P1/P3/P7 5V pins to VDD_5V, so electrically they appear to be the same net as VIN_5V on P20. Is this a supported way to power the DK, or are there caveats I should be aware of (e.g. the power switch SW6, power-input selection FETs, etc.)?

2. The guide says only one source should be applied at a time but doesn't say why. My assumption is that it's to prevent reverse current flow between the two sources. For my application I'd like the DK to remain powered from the vehicle's 12 V (via a buck) while also being able to connect USB for programming/debugging, without having to disconnect either side. I'm planning to put a Schottky in series with my buck output (cathode toward the DK's 5V net) to prevent USB current from flowing back into my buck. Looking at the power input section of the schematic, it appears that Q9 (MCM2301-TP) provides similar reverse-blocking on the VIN_5V -> VBUS path via its body diode. Is that the intended behaviour, and if so, would my Schottky + the existing Q9 be sufficient to safely connect both sources simultaneously? Are there any other failure modes (transient behaviour during USB plug-in, SW6 interaction, USB_DETECT logic, etc.) I should be aware of?

Thanks,
Mark

[Simon D-M]

Hi,

Simon D-M said:

I'll ask one of my colleague to review what I say.

I got confirmation from a colleague that what I said was correct.

m4rkw said:

I realise it’s just a dev board but for a beginner getting used to the platform being able to use it as an initial prototype is very desirable. My plan is to prototype using the dev board at first and then later develop the solution with a ready made nRF9151 module and then eventually a full blown board of my own.

Sure, this is one of the reasons we got rude of the "Arduino-style" header in the more recent devkits (for example the nRF54 family). With the new headers it should be easier to make some custom modules that go on top of the DKs. 

m4rkw said:

It sounds like it’s probably ok to have it powered through VIN and connect a macbook for debugging, since a laptop isn’t going to act badly on the usb bus and send silly voltage through it and will handle incoming transients sensibly. I think as long as that’s relatively safe to do then this should all be fine, hopefully your hardware person can clarify. I was initially thinking of creating a shield type board to use for the prototype where using the other header would have been handy but now I’m thinking it would probably make more sense to just have it be a separate thing with a connector.

Yes, as far as I understand your setup, you are most likely fine with doing this. 

m4rkw said:

Oh one other thing - it would be good to know if it’s likely that the board will reset when switching between VIN and USB

Yes, the board might reset during the switch. If the current consumption during the switching is very low (sleep, ...), you might get away with it, but this really depends on a lot of factors. So as a rule of thumb, you can consider that the board will reset when switching power.

Best regards,

Simon D-M

[m4rkw]

Many thanks both