nRF52840 Reference Design

[mavenboard]

Hello community,

I was hoping to ask 2 questions about my design choices when designing around the nRF52840, specifically the QIAA-R branch.

First, I am confused on as to which reference design I should use, out of the 7 provided. I was going with 1, but I am reading some conflicting things about how ref. 1 is for the pcb antenna. In my application, I will be running it as a meshtastic node, with a battery as the main supply and usb as fallback/charger when connected (so I'm also looking to have a power priority type of thing, I don't know if this can be done internally or I need an external ic for this), and also as a programmer. I was looking to use an external ipex antenna or a chip antenna for the BT. Which layout should I go with in this case?

Also, the reference states I dont need the DEC5 cap (C9 820pF), for revisions Fxx and newer, but on the my distributors listing, there is no current stock revision code. I am buying components from LCSC and getting it manufactured by JLCPCB. When I manufacture with JLC, they charge extra for "extended components", their way of saying irregular values or components, and 820pF is considered a irregular value. I was hoping I could just avoid the extra charge, but I don't know if my distributor has the right hw revision. Do I have to inquire about that or is there a way I can tell? 

Thanks advance for your help.

[Asbjørn]

Hello,

Q1: There are seven configurations given in the reference design to show combinations possible with regard to power supply and setup. You want to start on configuration #3 probably. It has USB and a single voltage supply on VDD otherwise. You can combine as you think is useful for you. There's a little table for every configuration showing what is available and what is not:

https://docs.nordicsemi.com/bundle/ps_nrf52840/page/ref_circuitry.html#ariaid-title4

The RF match from the ANT pin to the antenna is the same for each variant of the package. So there's a component selection and layout for aQFN, WLCSP  and for the QFN package, but all seven configuration using the aQFN will have the same component selection and layout from ANT in to the 50Ohm point. With the antenna we recommend following the guides from the antenna manufacturer, but most of the time we see customers adding a pi network of tuning components after the C22 in this example:

This screen shot is from the aQFN73-QIAA package variant. After C22 we recommend components for tuning the antenna input impedance to 50Ohm. See the antenna section here: https://devzone.nordicsemi.com/guides/hardware-design-test-and-measuring/b/nrf5x/posts/general-pcb-design-guidelines-for-nrf52-series

Q2: You can skip the C9 as long as you ensure that the nRF52840 components you are buying is from version F or later. Version F was started in 2021, so unless you have old stock on inventory you are buying from, it's not a problem. You should specify with HW revision you want from the source and then the C9 component can be skipped.

Best regards

Asbjørn

[mavenboard]

Wow, thanks for the detailed answer. I will follow the design ref. 3, and skip C9. Thank you yet again.