How do I take an nrf52840 Devkit prototype to custom PCB

[davidm]

Hi all

I am working on Bluetooth application using NRF52840 kit. The application is very simple, the device would have about 7 push buttons which when pressed data will simply be  sent to the nearby connected peer device. I am planning to take this to production soon, so I am trying to figure out the how to get started on the schematic and BOM list. But I am not very clear with the process of taking it from Devkit prototype to custom PCB for Nordic/nrf52 products. I came across a  reference layout for nrf52840  and I know the next step is to create the schematic but still, I am not sure how to proceed from here. In addition to nrf52840 SoC, the other components needed for my custom device  are 7 push buttons, 8 10K ohm resistors, and 1 led.

For now, I have one question, based on my needs how do I proceed from here(create schematic and find BOM list)?

Thank you in advance

[Andreas]

Hi,

 

You can download the reference layout and schematic and use that, so you do not have to add the nRF52840 with support passives manually. There is a download in the link you referenced. Buttons, and resistors are added to the GPIOs just as you did in your devkit prototype. There are also some Eagle libraries here, if you prefer that.

You can also use a 3rd party module, simplifying PCB design.

 

Best regards,

Andreas

[davidm]

Thank you, Andreas, for the response and sharing the resources.The pre-certified modules sounds good, I was not aware of that.

So If I decide to go with pre-certified modules for example Fanstel BT840,that means I have to use their(third party) schematics/design not reference design provided by Nordic, is that correct?(Because I didn't see any schematic files on their website)

In both cases(Nordic IC and third party/pre-certified) how does the Antenna comes into the picture?

[Andreas]

Hi,

 

The 3rd party modules feature all necessary passives, the manufacturer will provide some reference design guidelines usually, but you mostly have to worry about power, positioning on your board, grounding of the module and keepout for the antenna. All this is described in the datasheet from the manufacturer.

If you use our SoC you have to implement the antenna yourself, this can be a little tricky if you do not have RF experience, but we have some material and try to offer as best support we can to lead you through this. It is not rocket science, just barely. This means you have more flexibility, which can be useful for a compact and/or densely populated design. With a 3rd party module you have to follow the mounting considerations/requirements from the manufacturer, if not you risk RF performance being degraded. If you follow the recommendations there should not be a whole lot of difference to performance.

There are different kinds of modules for different kind of mountings, some have an antenna onboard, some have a coaxial connector for use with an external off-the-shelf antenna. I suggest you spend a couple minutes browsing, and find a module that fits well with your plans and product.

 

Best regards,

Andreas

[davidm]

Thank you, Andreas.

Based on your explanation , it sounds like if you want to go to market sooner,and you are not very experienced in RF, it's better to go with third-party modules.Is that correct?

Are there any well known good modules?

Fanstel sells their module for $8 @ 1000pcs,with that price bump compared with Nordic SoC price per unit, do you think it's worth it ,from what I would have paid for certification costs with Nordic SoC ?

[Andreas]

Hi,

 

davidm said:

Based on your explanation , it sounds like if you want to go to market sooner,and you are not very experienced in RF, it's better to go with third-party modules.Is that correct?

Yes, this is quite accurate. You save yourself some design work, some testing and some certification (work and money), at the cost of each module probably being more expensive than if you do the design yourself. Just make sure you follow any design and layout recommendations from the manufacturer for the specific module you are using, there are differences.

 

All the modules on that list should be good, and there should be little difference in performance between them. For low volumes, you will save in total even when the module comes at a higher cost, when you take into consideration what you save on certifications and engineering. Exactly at what volume the threshold is impossible to say, it will depend on the specific module - how much it costs, what certifications are covered etc. You need to compare this for any interesting modules if you want to be sure.

A couple of factors you might want to consider:

• Size: Do you need the smallest one? Usually comes at a higher cost and potentially slight hit to performance
• SW: Any restrictions? Do you need to use module vendor SW and what do you prefer?
• Documentation: Good or lacking? Design recommendations well covered?
• Antenna: SMD, PCB or external (coaxial connector), and what do you want/prefer?
• Re-usable certifications, what guidance and information does the manufacturer provide?

 

Best regards,

Andreas

[davidm]

Thank you so much Andreas. Those are very good factors you brought up.It is really pushing me to think through about this.

To briefly give you the background info of the product, it will be about 7-inch radius, the PCB will have an ABS hard plastic enclosure, very low power, 2 AA Battery powered(with expectation of at least 2 yrs battery life), the range is very important (longer the better but at least 300 meters ), very small data packets.The device environment will always be inside the buildings.

So to respond to the factors you brought up, size is not an issue in this case I think.

Based on the details above, which type of Antenna you think I should consider?

After looking into 3rd party vendors Fanstel products stood out the most to me specifically BT840.But I am not sure if they require to use their own software(I wouldn't want to use module vendor software).And another thing, I was not able to find their module schematic.

[Andreas]

Hi,

 

Size should not be an issue no, 7 inches (radius) is more than enough for every nRF52 series device, depending on how much else you plan on putting on there.

 

As for type of antenna, PCB antenna is generally recommended if you can accommodate it, it seems to align well with the range results fanstel claim in their datasheet. As for exact range in number of meters, this is very dependent on the environment and their numbers do look quite optimistic.  In order to compare apples to apples you need to get some samples or evaluation kits and compare the range in the same environment, preferably an environment that will be typical for your end product.

 

I am not sure why they have not put a schematic (other than page 3) in their datasheet, but as everything is integrated in the module itself it's pretty much plug and play.

 

Best regards,

Andreas

[Mojo]

beware, those modules don't have everything integrated.  The 32768Hz crystal and the DC/DC coils are not builtin.  You need to add them externally. 

[davidm]

Thank you so much, Andreas. You provided very helpful information. I will continue to research  and I will open another thread if I have more questions. Thank you again.