Correct TWI Sensor data only when programmer connected

[Iulian Mate?ica]

Hello,

I have made a custom board using nrf51822 QFAC that has an IMU on TWI interface. For programming I am using ST-LinkV2 and OpenOCD. The board is working just fine. I have successfully programmed it.

I want to transmit the readings from the IMU (rotation angles on each axis x, y and z). For this I have made a custom service with a read-only characteristic where I put the readings from the sensor. The notification property is enabled. Everything is ok, the service gets created and the characteristic is present just as I have configured it. I am using nRF Connect on Android to read the characteristic.

For reading the sensor I have made a library that use APP_TWI, just as I have seen in the examples from SDK v12.2.0. While writing this library I have checked every single one return value of the app_twi_perform function, and I advance in the code only if I get NRF_SUCCESS.

In the main loop, on every iteration I read the sensor and update the characteristic.

The Problem: if I let the ST-LinkV2 (plugged in usb) programmer connected to my board, after flashing, the results are ok - the characteristic gets updated and I can see the angles in nRF Connect. If I unplug the programmer from my custom board after flashing, the characteristic in nRF Connect is 0. All angles are 0.

I do not understand the link between the SWD interface and TWI. Why when SWDIO, SWDCLK and GND are connected to ST-LinkV2 the data from the sensor are correct, and when they are disconnected why do I get 0?

In order to identify that the problem is with TWI, I have made a variable I increment every iteration and I update the characteristic with this variable, instead of the values read from the sensor. The result: the characteristic gets updated no matter what the state of the SWDIO, SWDCLK pins is (ST-Link connected or disconnected). I can see the variable value gets incremente in nRF Connect.

[Iulian Mate?ica]

Okay, I have found the problem.

The IMU I am using accepts an external 32kHz crystal oscillator for better performance instead of the internal one. When I was initializing the TWI and my sensor I was configuring it to use external crystal. Now I have configured it to use the internal one and it works perfectly (with or without OpenOCD open/with or without ST-LinkV2 connected).

I do not understand why it works with the external crystal for IMU only with OpenOCD running. Any ideas what could be the connection between those two is welcome. The wires I connect to the programmer are: SWDIO, SWDCLK and GND. No VCC - the board is powered from batteries.

I will be using the internal oscillator but I will still investigate the problem. If I find anything I will let you know.

[Jørgen Holmefjord]

Have you connected the same 32.768 kHz crystal to both the nRF51822 and the IMU? Connecting the debugger to the SWD interface might put the chip in debug mode, which will force the clock sources to run. If this is the case, you can try starting the 32.768 kHz crystal from your nRF51822 device before initializing the IMU to use the external crystal.