Possible to use nrf51822 without antenna (like simple MCU)?

RE: Possible to use nrf51822 without antenna (like simple MCU)?

AmbystomaLabs — Fri, 17 Aug 2018 14:25:43 GMT

Sorry if I was overly critical. I didn't mean to discourage you.

You mentioned there is a ground pour. Watch out for what happened in your top layer where the traces barricade the ground pour and isolate it to essentially just the die.

Your design will be more robust if you just keep all the components on the same layer. Then you can focus on keeping the ground pour under the SoC as complete as is possible. This more important if you use the RF. Then you have to have a uniform ground to launch the wavefront from.

If you wish, please post your updated layout for review before you send it out for fab.

RE: Possible to use nrf51822 without antenna (like simple MCU)?

the_summer — Thu, 16 Aug 2018 14:13:29 GMT

Thank you very much for your comments. I think I have to go back to the drawing board then. I had a ground fill on both layers (they were hidden when I made the screenshot. Sorry for that), but after some additional reading I think they weren't too well connected anyways.

So far I mainly used AVRs when doing simple custom designs. I suppose they are more forgiving regarding amateurish design.

RE: Possible to use nrf51822 without antenna (like simple MCU)?

AmbystomaLabs — Mon, 13 Aug 2018 18:08:31 GMT

You have got a lot of problems with this board. I actually just helped out someone who made similar mistakes.

For starters the caps need to be at the pins specified in the reference layout. When you place them far away the parasitic inductance in the trace combined with crosstalk between different parts of the IC create one big mess on the supply rails.

Second you should never share ground connections. Above you share crystal load caps with general ground connections with the nRF. All bad. There are other shared grounds, I just pointed out those. You should always strive to have a via for each ground. Barring that you can dump the current to the ground pour, but on your board there isn't a ground pour.

Finally, where is the ground pour? There should be a ground pour at a minimum on the bottom layer(top layer for you since you place the nRF on the bottom) to give all the ground currents easy paths back to the origin. Same as on #1, all the parasitics from the traces just makes for a mess on Vdd. The ground flood should include Epad otherwise Epad isn't really doing anything.

Your via pattern on Epad is minimal and would be insufficient for RF. But since you aren't using the radio you might not care. I would recommend you copy the ref design 4x4 pattern. On the devices we make we use a 5x5 pattern. You may as well add the 3 inductors that provide power to the RF output. Isn't much room and who knows you may decide to turn on the radio after all. Doesn't really need an antenna for close range comms. On the Ref design this is L1, L2, C3.

Sorry, your board is really messy.

I would recommend on your schematic you make the schematic object look like the part. When you have it as a generic box with 49 pins it invites the kind of mistakes you made since there isn't any awareness of how the layout should look when you put down the traces. This is probably less important when it is basic logic circuits but on a complicated SoC it is very important to get the parts physically where they belong.

RE: Possible to use nrf51822 without antenna (like simple MCU)?

the_summer — Mon, 13 Aug 2018 17:39:04 GMT

Here is the bottom layer:

And here the top layer:

RE: Possible to use nrf51822 without antenna (like simple MCU)?

AmbystomaLabs — Mon, 13 Aug 2018 16:15:57 GMT

If you wish, post your layout and I can review/comment.

RE: Possible to use nrf51822 without antenna (like simple MCU)?

the_summer — Mon, 13 Aug 2018 15:54:29 GMT

Thanks you for your help. The pin 49 is actually connected to GND. The wire must have gone missing when I rotated schematic for a better screenshot.

I will check out your suggestions from your other answers.

RE: Possible to use nrf51822 without antenna (like simple MCU)?

AmbystomaLabs — Mon, 13 Aug 2018 15:10:38 GMT

In fact I notice, pin 49 EPad is not connected. That's a big mistake. You need to have one stable ground for the entire device even when you are not using the radio.

So, that is your problem, but you may have other mistakes. I recommend you have someone look over your layout before you have the pcb made again.

RE: Possible to use nrf51822 without antenna (like simple MCU)?

AmbystomaLabs — Mon, 13 Aug 2018 15:03:42 GMT

Schematically it looks fine(except pin 49, see comment below). But how does the layout look?

People often make mistakes by putting all the caps on one side of the board instead of near the pins they need to be next to, also I have seen where the ground slug under the nRF wasn't placed and they assumed one or two pins for ground is good enough, it's not.

Also make sure you didn't rotate your crystal. Occasionally people rotate it 90 degrees. Functionally it doesn't need the external crystal. It can work on HF_RC, but if it is rotated the crystal shorts out the driver pins.

RE: Possible to use nrf51822 without antenna (like simple MCU)?

the_summer — Mon, 13 Aug 2018 14:24:36 GMT

Thank you, good to know. I use 3.3 V for the power supply and the SWD. I guess that should be sufficient.

I used the following schematic based on the reference circuits of the product specification. Does anyone see grave mistakes?

RE: Possible to use nrf51822 without antenna (like simple MCU)?

AmbystomaLabs — Sun, 12 Aug 2018 19:52:24 GMT

Normally Ant1 and Ant2 DC float and are connected to VDD_PA across the output balun. It shouldn't be required to provide power to the final stage of the radio for the MCU to fire up. Leaving these floating should be fine.

Likely something else is wrong, such as insufficient local capacitance, too low of a supply voltage for SWD to operate, etc.