Connecting RM3100 Magnetometer



  • I am trying to connect the PNI RM3100 magnetometer to the flight core it requires 3.3v. Would you recommend pulling 3.3v from an unused connector such as J12? Also do the SCL and SDA lines on the J10 connector have internal pull-ups somewhere on the flight core?


  • Dev Team

    Hi Ryan,
    We provided several 3.3V and 5V tap-offs for exactly these reasons. For 3.3V, you have 2 options. J12.1 (with J12.4 as the return) as you pointed out, but be aware that this is the R/C port power which has the option to be disabled by software for binding. By default, I believe it is ON. You can pull 250mA safely from this port without harming the rest of the board. The other port that has 3.3V is J13.1 (and J13.2 respectively for the return). That can also easily provide 250mA. However, Be advised, we did not design either of those 3.3V PTO's (power tap-offs) to the tight ripple specification/requirement of the RM3100. That spec calls for only 50mV of ripple. We designed for a much wider range. So, the RM3100 board should have a lot of capacitance to help reduce this potential spec violation, and I would certainly measure it at the RM3100 if you are expecting to meet their performance specs.
    For the I2C interface, all of our I2C busses are at 3.3V logic levels. The port on J6 pins 4/5, J10 4/5, and J13 3/4 all have pullups to 3.3V of 4.7K-Ohms installed on-board. The RM3100 specifies an IOL of just 1mA, so we should be ok there, but make sure you do not have any other pull-ups in the system or you will start to exceed 1mA. I would recommend you try with the J13 port since you get all you need with one cable, and the pull-ups are not on the potential switched rail of the R/C port (in other words, if you use J12 for power only, and J10 or J13 for I2C, there is a chance the SW will disable the power rail, or during startup before it is ON, the pullups are back-feeding into the magnetometer device).
    That connector is a BM05B-SRSS-TB(LF)(SN), and you can find the mating shrouds SHR-05V-S-B on digikey with pre-crimped wires as well, or even a 1:1 cable assembly A05SR05SR30K152B.
    Let us know if you have any other questions.
    Thanks!
    Vinny



  • Thanks for this detailed response! This would be good information to put in the technical documentation when searching in google for "modalai flight core connections for magnetometer" there wasn't much information on how to setup a magnetometer with the flight core.

    I was able to setup the the RM3100 by using J13.[1,2] for 3.3 VCC and GND and the J10.[4,5] for the I2C protocol. This allowed me to set

    SYS_HAS_MAG=1 
    

    in PX4 and successfully calibrate the compass


Log in to reply