MISO line on BRKT-STBC-AGM01

[mohit306]

I am using BRKT-STBC-AGM01 breakout board for FXAS21002C 3-axis gyroscope and FXOS8700C 6-axis integrated e-compass. I must use it in SPI mode. Looks like the MOSI and CLK are common for both the sensors. Any reason why the MISO lines are seperate? Can I short them and just use CS to differentiate?

I just want to make sure that I don't get in to any trouble by using common line for MISO for the two sensors.

[TomasVaverka]

Hi Mohit,

The reason is described in the FXOS8700CQ datasheet, page 17:

"FXOS8700CQ does not tri-state the MISO pin when the CS_B pin is deasserted (logic high), which can lead to a bus conflict if multiple slave devices are present on the bus."

So the FXOS8700CQ pulls MISO down to GND, instead of tri-stating, which would block the FXAS21002C from sending data to the master.

Best regards,

Tomas

[mohit306]

That makes it clear. Do you think if I use an OR gate, I can connect both MISO signals to the same micro pin??

if yes, I will strongly recommend having an OR gated MISO output option on the EVM. 

[TomasVaverka]

Hi Mohit,

Yes, an OR gate is a possible workaround as well as a tri-state buffer with the MISO line and the CS_B line as input to the buffer.

Best regards,

Tomas