nRF51288 custom board cannot be discovered

[Michal Zacharias]

Hello, everybody. We have a custom board with nRF51288. We also have code based on ble_template that is working on Nordic devboard and dongle. Code is also working on our custom board (verified through debugging and led-based visual verification :)) but device cannot be discovered (devboard and dongle CAN be discovered).

What we think the problem could be one of these:

1. Crystal is giving inaccurate frequency
2. Matching circuit is wrong

Crystal is giving inaccurate frequency

Is it possible to somehow check the frequency in firmware?

We used app_timer module to periodically blink the LED and measure the time of ON/OFF states. Can this method be used or it's completely wrong to assume something like this?

Matching circuit is wrong

This is our matching circuit. It should be same as in Product Specification.

LINK TO IMAGE

LINK TO IMAGE

There is list of components we used:

L1 - link

L2 - link

L4 - link

C15 - link

C2 - link

C16 - link

C17 - link

ANT - link

So, what do you think is the problem? Could it be something different then wrong crystal freq or matching circuit?

We are quite desperate right now, so any suggestions would help. Thank you!!

[FormerMember]

The schematic looks good, the only thing that is missing is a pi-network for the antenna, for matching the antenna to 50 Ohm, see this post regarding why matching to 50 Ohm. However, that should only decrease the range.

The crystal also seems to be fine, at least with respect to the given information, frequency tolerance and load capacitance. The requirements for the 32 MHz crystal can be found in the nRF51822 product specification, chapter 8.1.3. Therefore, I don't think the crystal is the root of the problem.

How did you do the layout? Did you follow our reference layout? In the RF part of the layout, the layout should be a copy of our reference layout. This means that all component values, distances, and relative placement of the components with respect to each other should be the same as in our reference layout. In addition, it is also important to have a lot of via holes. Each grounded component in the matching network should preferably be grounded with at least tow via holes. For where to place the via holes, see our reference layout.

In the RF part of the layout (the components between the chip and the antenna), the component size has to be the same as in our reference layout, 0402. The reason for this is the high frequency, 2.4 GHz.

The reference designs for the nRF5822 can be downloaded from the nRF51822 webpage in the "downloads" tab.

If you don't want to upload the layout here, you can create at support case in the Nordic techinical support system, for a review. If you create a support case, you should upload both schematic and layout (gerber or altium files).