FXOS8700 SPI Woes

[warren1]

I am attempting to use the FXOS8700 in SPI mode due to the faster read / write times associated with SPI (higher possible bus speed). I have set up the SPI bus to run with the following parameters which are consistent with the FXOS8700 data sheet:

CLK: 1 MHz

Order: MSB First

CPOL: Rising/Falling

CPHA: Sample/Setup

I have verified the above settings on an oscilloscope so I know that the bus is set up correctly. 

In order to get the FXOS into SPI mode, I tri-state the MISO pin before toggling the reset pin, and waiting the required time for the auto detection circuit to put the device into SPI mode. I then attempt to read the WHO_AM_I register and get the value 0x1e instead of 0xc7 which is what I would expect. Below is the code I am using to reset the device and read the WHO_AM_I register:

#define FXOS_MISO   26
#define FXOS_CS     11
#define FXOS_RST    3
void FXOSSPI()
{
    // Tri-state MISO
    pinMode( FXOS_MISO, TRI_STATE );
    delay( 10 );

    // Reset the FXOS
    digitalWrite( FXOS_RST, HIGH );
    digitalWrite( FXOS_RST, LOW );
    delay( 1 );

    // Set up the bus
    SPISettings _settings = SPISettings( 1000000, MSBFIRST, SPI_MODE0 );
    SPI1.beginTransaction( _settings );

    // Read WHO_AM_I
    uint8_t tx[3] = { ( 0x0D >> 1 ) & 0xFF, ( 0x0D << 7 ) & 0xFF , 0xFF };
    digitalWrite( FXOS_CS, LOW );

    for( uint8_t i = 0; i < 3; i++)
        buff[i] = SPI1.transfer( tx[i] );

    digitalWrite( FXOS_CS, HIGH );
    SPI1.end();
}

After I run that function I check the contents of buff using a break point and ICD, the contents of buff are:

buff[0] = 0xFF

buff[1] = 0xFF

buff[2] = 0x1E

Obviously, bytes 0, and 1 are DC because we are writing the R/W bits and the register address, but byte 2 should be 0xC7.  Not really sure what is going on here, I have looked at this many times under a scope, verified SPI register settings via ICD, slowed down the clock rate, etc. Still get the same results. Anybody else getting this problem? The only thing I can think of now is that my FXOS is damaged or something.

[warren1]

Hi Tomas,

I can't believe I missed that! Just goes to show the value of having a fresh set of eyes on your code. Thank you, everything is working as planned now.

- Warren

[TomasVaverka]

You are very welcome, Warren.

Very glad to hear it is working now!

Best regards,

Tomas

[TomasVaverka · Accepted solution]

Hi Warren,

I think I found where the problem is. You are not addressing the WHO_AM_I register (0x0D) correctly.

As shown in the datasheet:

Byte 0: R/W, ADDR[6], ADDR[5], ADDR[4], ADDR[3], ADDR[2], ADDR[1], ADDR[0]

Byte 1: ADDR[7], X, X, X, X, X, X, X

In your case tx[0] = ( 0x0D >> 1 ) & 0xFF while the correct way is tx[0] = 0x0D & 0x7F. Similarly tx[1] = ( 0x0D << 7 ) & 0xFF while the correct form is tx[1] = 0x0D & 0x80.

Here is my complete read a register function:

/******************************************************************************
* Read a byte from the FXOS8700CQ
* 
* Byte 0: 0,ADDR[6],ADDR[5],ADDR[4],ADDR[3],ADDR[2],ADDR[1],ADDR[0]
* Byte 1: ADDR[7],0,0,0,0,0,0,0
* Byte 2: 0,0,0,0,0,0,0,0
******************************************************************************/ 
unsigned char FXOS8700CQ_ReadRegister(unsigned char RegisterAddress)
{
 unsigned char i;
 
 CS_Enable;
 
 while (!(SPI0_S & SPI_S_SPTEF_MASK));
 SPI0_D = 0x7F & RegisterAddress;
 while (!(SPI0_S & SPI_S_SPRF_MASK));
 i = SPI0_D; 
 
 while (!(SPI0_S & SPI_S_SPTEF_MASK));
 SPI0_D = 0x80 & RegisterAddress;
 while (!(SPI0_S & SPI_S_SPRF_MASK));
 i = SPI0_D;
 
 while (!(SPI0_S & SPI_S_SPTEF_MASK));
 SPI0_D = 0x00; // Dummy byte
 while (!(SPI0_S & SPI_S_SPRF_MASK));
 i = SPI0_D;
CS_Disable;
 
 return i;
}‍‍‍‍‍‍‍‍‍‍‍‍‍‍‍‍‍‍‍‍‍‍‍‍‍‍‍‍‍‍‍

I hope it helps.

Best regards,

Tomas