MPU9250 gives garbage when spiExchange is used but not when spiSend

[KalRal]

Hi,

I am using MPU9250 to become familiar with Accelerometers and Gyroscope and eventually apply that knowledge by implementing Sensor fusion. I am using SPI to communicate with that chip.

Strangely I noticed that when

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spiExchange

is used to get the data from the chip, it returns garbage but if I use

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spiSend

and

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spiReceive

I am getting correct data.

If I send and receive only one byte

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 spiExchange 

also works. It gives garbage when I use

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spiExchange

to receive multiple bytes.

Here is a code that works with

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spiExcange

. It just reads the

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WHO_AM_I

register of the board.

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spiAcquireBus(&SPID2);
spiStart(&SPID2, &spiConfig);
spiSelect(&SPID2);
spiExchange(&SPID2, 2, &txbuf, &rxbuf);
chprintf(chp, "Sending data over SPI %x\n", txbuf[0]);
chprintf(chp, "Second byte : %x\n", txbuf[1]);
chprintf(chp, "Received data over SPI %x\n", rxbuf[1]);
spiUnselect(&SPID2);
spiReleaseBus(&SPID2);
chThdSleepMilliseconds(10);

The above code works as I am just reading one byte, but If i want to get Accelerometer data, then I should read 6 bytes and it only works If I use

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spiSend

.

Has anybody worked with this chip(MPU9250) and will I get any advantages if I switch to

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spiExchange

?
If further code is needed, I can share it.
Please let me know whether I need to spend time on this or I can let it go by thinking that it might be MPU9250s fault?

[Giovanni]

Hi,

spiExchange() transmits and receives at same time, perhaps it is not what you want to do, just an hypothesis.

Giovanni

[KalRal]

Maybe you are right. I even tried it with an Arduino and comparing both using a Logic analyzer. The chi(MPU9250) replies when there is a timegap of 6.4microseconds between the question(register I want to read) and its answer(contents of the register). But I am no expert in SPI and I am just starting off with all this stuff. There is no mention about simultaneous transmit and receive in MPU9250 data sheet. Thanks for your time.

[alex31]

Hello,

I have no experience of using MPU9250 with SPI, but i use ICM20600 with SPI and theses chip from invensense are fairly similar.

With the ICM20600, i have no problem using spiExchange, with a spi clock @ 6.25Mhz

What STM MPU are you using ?
What Polarity and Phase are you using ? (post your SPIConfig structure)
How do you declare your arrays txbuf and rxbuf when you try to get the 6 bytes of accelerometer data ?

Alexandre

[mikeprotts]

I've had issues when I didn't setup the registers correctly - migrating from ChibiOS 3 to 18, and with different boards having different configuration requirements, so my first suggestion is to check the configuration again. It's possible you've ended up with a wrong data size setting.

The second problem I had was with spiExchange where the slave behaviour was changed by the value in the TX buffer. In my case, the slave stopped sending data when it encountered any bit set in the TX buffer, so all the low bits ended up as the same value as this doesn't show as an error at SPI transfer level. Simple solution was to zero the TX buffer before use.

Mike