MPU-9150 FIFO Mode

[maurosmartins]

Hello all,

for a current project that I'm working I need to interface to a MPU-9150 9 axis IMU.

I've been reading the documentation and there are two possible ways to read data, directly from the registers or using a FIFO.

This is where my doubts are, in the datasheet "FIFO_R_W" (address 116) allows reading from the FIFO, but this is an I2C device, so if I read the first byte I'll read address 116 and the second byte read should be address 117, but address 117 is a "WHO AM I" register (constant).

I saw in this code https://github.com/kriswiner/MPU-9150/blob/master/STM32F401/MPU9150.h line 446 that they just perform a normal read to addr 116 asking for multiple bytes.

My question is, is this a special register? won't the MPU-9150 auto increment the internall I2C address that I'm reading?

Thanks in advanced,
best regards, Mauro.

[dannyf]

You are overthinking it: enable the fifo and read it and see if the pointer increments.

[maurosmartins]

Hello dannyf and thank you for your reply,

you are probably right, I'll test it as soon as I can,

the reason for me to post here was hopping some user had already worked with this.

Thanks anyway,

best regards, Mauro.

[mikerj]

The datasheet implies that the FIFO register does not auto-increment if repeated reads are made (it says the FIFO register supports "burst" reads)  though I agree this is not very clear.

[maurosmartins]

Hello mikerj,

that is also my thoughts, thank you for your reply!

Best regards, Mauro.

[andyturk]

Just read the number of bytes you want starting from that register.

Code: [Select]

bool MPU9150::read_fifo_data(uint16_t &count, uint8_t *buffer) {
    if (!read(FIFO_COUNT_H, count)) return false;
    count = byteswap16(count);

    assert(count <= 1024);

    return bus.read(address, FIFO_R_W, buffer, count, 100);
}