Differences in nRF51 chip versions

[Hieu]

We are moving from a PCB using a nRF51822 QFAB module to a newer PCB using an nRF51822 QFAC module.

We observed that with the new QFAC PCB we have a significant connection drop rate, >20 times an hour. Logging of disconnection shows that most of the time it is reason 08, BLE_HCI_CONNECTION_TIMEOUT. Sometimes it is reason 62, BLE_HCI_CONN_FAILED_TO_BE_ESTABLISHED. Meanwhile the same firmware runs very well on the old QFAB PCB, no undesired disconnection.

Is there any difference between the QFAB and QFAC version of the chip that could cause this issue? Perhaps a change in the internal RC oscillator? In our test we use NRF_CLOCK_LFCLKSRC_RC_250_PPM_250MS_CALIBRATION.

[Hung Bui]

Have you followed the reference layout for the QFAC ? The matching network components values are slightly different between QFAB and QFAC. Please take a look at the reference design here.

Also make sure your antenna is tuned properly to have good performance.

[Hieu]

We are using modules that integrate both the nRF51 and PCB antenna, so I think those issues cannot be helped in our case. But if there are still things that has to be adjusted, please advise me, and I will forward it to our PCB designing engineer.

From your answer, I take it the RC oscillators are the same in the two chips? My supervisor kind of want me to be sure about that, aside from finding out what else is wrong with the current design.

[Hung Bui]

Yes, the RC oscillators shouldn't be a problem here. Same firmware should be able to run on both chip (if the size fits). I don't understand what you meant here "modules that integrate both the nRF51 and PCB antenna, so I think those issues cannot be helped in our case"

The reference design is made to be used with PCB antenna (as well as other antenna). It should be followed. You should only modify the component values if you do have equipment to tune and match them with the design and material of the product.

[Hieu]

By module I meant one like those listed here. The ones we used are designed and manufactured by a local business and is not listed there, however.

I also reviewed the reference you linked and this blog post. If I understand the procedure right, I think we could only hope that the module manufacturer followed reference design and tuned their antenna. But if there is any guidelines that we should followed in this situation, or if I am having any misconception, please advise me.

[Hung Bui]

You may want to request support from the module maker. A sniffer trace would tell if there are a lot of packets drop causing connection timeout.