Ble connection drops out on button press

[ilkobg]

I have a custom board with nrf51422-QFAC, used with SD 130 and SDK 12.3.0. My firmware consist of ble_app_hids_keyboard example modified. Schematics are attached below.

When I connect the board to a central, all four buttons are working fine until I put the pcb in my device body. When the pcb is inside the device body, pressing buttons on pins P0.26 and P0.01 cause a disconnect every 3/5 times. The connection remains stable only when I connect my board with nRF Connect app for Android. Device body is made of 1-2 mm plastic and a rubber keyboard on top. The configuration for the buttons is the default from the examples and they are working fine.

I want to clarify that the board is powered from a 3.7V LiPo battery, which is connected to J3 and J5, which go directly to the chip (the battery is charged to 3.6-3.8 V and skips the power circuit). The reason is that I have a big mistake with the power circuit with the tps61220, which is about to be redesigned. Before I make the new design I want to know if I have to do some hardware changes about the disconnecting issue and what is causing this behaviour. 

Thank you!

Hancy_1.pdf

[Susheel Nuguru]

Hi,

My colleague reviewed your schematics and wants to clarify that the board is powered from 3.7 LiPo batter, which is connected to J3 and J5, which go directly to the chip This os out of specification. Can you test with a stable, external power supply and never connect the LiPO directly?

 

 

[ilkobg]

I powered a new pcb from a stable 3.3V source. The problem is still present, nothing changed.

[Susheel Nuguru]

have you verified to see if this is a software bug? an Hardfault exception could also cause this behaviour

[ilkobg]

The software is working fine. I suspect that the antenna should be retuned by the T-network, because of the plastic enclosure. According to Section 11.2 (page 22) of the following document by Cypress, putting the pcb in a plastic enclosure cause a shift in the resonant frequency.

Antenna Design and RF Layout Guidelines

To sum up, the following details should be taken into consideration:

1. This behaviour is caused when the pcb is inside the device body.
2. This behaviour is caused only by 2 of 4 buttons on the pcb.
3. We have only one pcb from the production series (25 pcbs in total) that is working properly and don't have any issues.

[Susheel Nuguru]

ilkobg said:

The software is working fine. I suspect that the antenna should be retuned by the T-network

lliev, Did you see any better results after the retuning?