Electronic – Measuring the waveform of mains voltage using an oscilloscope

The problem is that you are using a MEMS digital accelerometer, and what you are reading is the SCK (serial clock) pin of the serial interface. In order to function, that sensor needs to be interfaced with a microcontroller, that sets it for the sampling frequency, the range and so forth.

So you don't have to expect a square wave with 100Hz frequency, but a fast (depending on the bus bitrate) spike, corresponding to a transmission. Expanding the spike, if the scope is fast enough, you should then see the clock square wave inside the spike.

Moreover, if you don't set the SPI interface correctly, the uC will not generate the clock (the sensor operates in slave mode), and you won't read any value.

If you want to see a 100Hz signal, you could probe the Int pin, which sends an interrupt to the microcontroller every time a measure is available. Then, if you handle the interrupt from the microcontroller properly, you wil see the pulse corresponding to the transmission every 10 ms (100Hz).

But make sure that you're not using motion detection; in that case, only when an acceleration is measured, it will generate the interrupt.

To read the data at the SPI port, the simplest thing is to configure the communication with the sensor; otherwise, it won't send data at all. Then, check if the microcontroller is getting the interrupts and if it's reading the data the sensor gives; you can use a timer to add a timestamp to values and check the frequency they come.

(still WIP)

I guess the USB shield isn't grounded. Strange but not impossible.

Without a ground reference, the scope will measure the difference between its own ground or chassis, and the laptop ground. If the laptop has only a two pin plug, then its ground will be floating at about VSupply/2 (RMS) because of the capacitors in its EMI filter.

Clip the scope ground to the Arduino ground to force the laptop ground to the same as mains ground, and the scope will start to read more normally. A small current will flow, it's limited by law to perhaps 200 uA, but might be a lot less.

In general on an experimental bench, you should connect the equipment ground to mains earth. The 100 V at 100 uA won't hurt you, and your laptop will be protected against it, but the Arduino isn't, and you can easily blow things by touching them.