I have some demo code and it works , but when i look up the documentation , some registers used in the code arent even listed.
For example writeByte(MPU_ADDR, MOT_THR, 5); //Write the desired Motion threshold to register 0x1F
Register 0x1F ( 31 ) is not even mentioned in the register map documents i found.
Anyone have some experience that can explain this?
When the MPU-6050 was released, only parts of it were documented.
Even the 'dmp' processing unit was kept a secret 
At Invensense they thought that was a good idea 💩
Then someone reverse-engineered firmware for the 'dmp', so the 'dmp' could be used by Arduino users. Others found that some registers were offset values.
After some years, Invensense also released the rest of the documentation of the chip. You are probably looking at the original documents. If I remember it well, there were four documents.
If you bought your MPU-6050 from AliExpress/Amazon/Ebay then it is a counterfeit.
Can you buy a more recent and more normal sensor ?
I checked out different versions of the document some registers are never mentioned.
For example for using and setting motion detect threshold function for the int pin.
I guess i have a clone it is the GY521 but it seems to work fine.
Any other good alternatives for motion detect with int function and esp support?
Look for the DMP documentation, but who knows if the counterfeits actually implement it?
There are better sensors these days, and they are much better documented, for example the LSM6DS3.
I tried to locate the official DMP documentation but i failed , all broken links.
I did find that it was officially released in 2014 by TDK.
I got the clone to do what i need it to do , without using a library , but i would have liked to tweak some settings.
I understand the mpu6050 is old and obsolete , but i have a few lying around to play with.
What it does now is generate a 50us pos pulse when one of the acc values reaches a threshold ( motion detect ) so it can wake up my esp to send a post request.
Does anyone know if the gyroscope is needed for motiondetect to work with the mpu ?
Otherwise i could disable gyroscope to save on batt power
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