nRF51822 2 Layer Layout

[Mike McCormack]

I have been looking at the nRF51822 for a project, and have looked at several designs available on the web for reference (I prefer to plagiarize working stuff rather than invent new errors myself.) When comparing the Nordic layout recommendations and the layout of the BBC Micro:Bit I am wondering just how much leeway there is in working with the part. Looking at the Micro:Bit

I see they have shrunk the center pad considerably, routed signals under the part, used very long power feeds and violated a couple other "rules." The press releases indicate Nordic worked very closely on the design, so I was hoping to get some insight on the real design rules and / or feedback on the design practices used.

Cheers

Mike

[MartinBL]

Hi.

Have you seen our reference layout files? We strongly recommend "plagiarizing" these layouts. You might also be interested in this blog post: General PCB design guidelines for nRF51

EDIT: Answers to questions in comment below:

I guess you are right in that the reference files don't really provide any context. The blogpost however, is a set of general recommendations which hasn't changed since february.

When using the words "lie at the heart" I think our marketing department was merely pointing out that our chip is the main component used in the kit. Although we followed the design process closely it is not our design. When reviewing layouts we at Nordic mainly focus on the nRF and the components in its immediate surrounds and we felt comfortable endorsing BBC's design. As a side note: I guess you found the image posted in your initial post here, but note that the design shown in the blog is not the final one.

I cannot in good conscience say that Nordic's recommendations in our blogs or reference designs are "rules made to be broken". Especially not on a public forum. But of course, you are rarely able to design electronics without doing some compromising. You will probably need to bend the rules a bit here and there, but our reference designs are proven to work and if you have to bend the rules that is on you. However, please feel free to submit your design on our MyPage support portal and we will be happy to do a review of it. We can also do tuning of the antenna and matching network. We strive to give you an answer within 24 hours and your questions and/or designs are handled with complete confidentiality.

I don't think any of the recommendations you mention is overly restrictive or ambitious. As i mentioned, they are just general recommendations and you are of course free to use your own discretion.

1. The ground plane is important for several reasons. E.g counterweight for the antenna (a small ground plane makes the antenna less efficient), return path for noise, and shielding of the RF part (under the matching network). Try to route as many traces as possible on the top layer to get a ground plane as coherent as possible on the bottom layer.

2. Lots of vias are imortant. On large ground planes we recommend a via every 0.5-1.0 cm. Any local ground planes should be connected with several vias.

3. A star network, with a center close to the power supply or a large capacitor, should be used to route the supply voltage to the nRF and other components on the board. Good filtering and decoupling is important as noise on the nRF's Vdd pins can be modulated into the carrier

4. Again, it is a general rule of thumb. If not possible then maybe you can add a large capacitor close to the nRF.

5. Nordic does not generally recommend routing any signals under the nRF (on any layer), but rather fill the area with a ground plane. As you can see, BBC chose to do it differently, which is their prerogative.

[Mike McCormack]

Thanks for the detailed response. While I tend defer to manufacturer' recommendations, looking over the material that is available on this high profile design brought many questions to mind.