MPU 9250 via SPI

[mirkix]

Hi, is it an option to connect the MPU 9250 via SPI and not via I2C? With Ardupilot we can sample at 4 kHz / 8 kHz (acc gyro) in this case I2C is not fast enough for the data transfer. It is possible to sample slower too, than the transfer speed via I2C should be okay.

[StrawsonDesign]

Hi Mirko, Sadly the 9250 is on the I2C bus on both Blue and Cape. In my flight code I use the 9250's DMP for quaternion orientation estimation and also use the DMP's fixed-rate interrupt function (up to 200hz only) to generate the timing for the discrete feedback loop. Pulling accel, gyro, mag, and quaternion data at 200hz puts the i2c bus at about %15 use when taking advantage of the 9250's FIFO buffer to do one big read at a time. I have no doubt sampling just the accel, and gyro 1khz would be possible over I2C with intermittent reads to the magnetometer. However, even on a small quadcopter the crossover frequency for the roll/pitch feedback controllers is only around 10-20hz so I've found 200hz sample rate to be more than sufficient for smooth feedback control. Anything faster is difficult to achieve in userspace linux without multiple cores anyway. Best, James Dept. Mechanical & Aerospace Engineering University of California, San Diego

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On Fri, Dec 23, 2016 at 10:10 PM, Mirko Denecke ***@***.***> wrote: Hi, is it an option to connect the MPU 9250 via SPI and not via I2C? With Ardupilot we can sample at 4 kHz / 8 kHz (acc gyro) in this case I2C is not fast enough for the data transfer. It is possible to sample slower too, than the transfer speed via I2C should be okay. — You are receiving this because you are subscribed to this thread. Reply to this email directly, view it on GitHub <#41>, or mute the thread <https://github.com/notifications/unsubscribe-auth/AF9HoPNNacVHFURymcCZ7Avhnf3maJzVks5rLEbigaJpZM4LVFkp> .

[mirkix]

@StrawsonDesign Hi James, is there any documentation free available about the DMP? I think ArduPilot try to sample as fast as possible to avoid problems caused by vibrations (aliasing).
I am working on ArduPilot support for the Blue and made good progress so far.

Regards
Mirko

[jadonk]

@mirkix @StrawsonDesign shall I try to ping Invensense on the DMP documentation issue?

[StrawsonDesign]

Hi Mirko, Jason, Free documentation on the DMP is limited to their example code and a pamphlet with a brief overview of functionality. I've played with the DMP a lot over the past few years and I think I've gotten as much performance out of it as I can having written my implementation %75 from scratch. I even managed to get it to package the magnetometer data into the same FIFO packet as the DMP, accel, and gyro data which a comment in Pansenti's mpu9250 driver said was "impossible". The sparkfun library is horrifically slow and does a lot of unnecessary register modifications every time it swaps between reading mag and dmp. The DMP is internally limited to 200hz, presumably because it has to do a LOT of floating point arithmetic for it's quaternion estimate. It always samples and runs its filter at 200hz, even if you set up the interrupt signal to run as a divisor of that. Sadly, that also means that the accel and gyros have to be set to sample at 200hz too, so pinging them between DMP reads is useless. I understand your worry about aliasing, but 200hz is still 20x faster than the crossover frequency of even my fastest feedback controller. A properly designed compensator (eg not PID!) will have gain roll-off well before 200hz. Best, James

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On Fri, Jan 13, 2017 at 8:11 PM, Jason Kridner ***@***.***> wrote: @mirkix <https://github.com/mirkix> @StrawsonDesign <https://github.com/StrawsonDesign> shall I try to ping Invensense on the DMP documentation issue? — You are receiving this because you were mentioned. Reply to this email directly, view it on GitHub <#41 (comment)>, or mute the thread <https://github.com/notifications/unsubscribe-auth/AF9HoG3HfUVE3PXFggK265YpEBZ8QUGBks5rR8xNgaJpZM4LVFkp> .

[jadonk]

@mirkix Can you confirm you are comfortable with @StrawsonDesign's answer?

[mirkix]

@jadonk 👍

[jadonk]

Issue migrated to beagleboard#43