SPI BUS and slave devices

[pucca]

Hi guys,
I have a SPI bus with several slave devices, usb controller,
tranceiver and sensors. There is an independent slave select line for each slave.
If I am not wrong most devices must have tri-state outputs that become high impedance ("disconnected") when the device is not selected.
I have a sensor that has a SPI serial data output pin, and a SPI serial data input pin. These pins are not mentioned as tristated in the datasheet. Should I add external resistors or a diode to the spitxd line?
The sensor is the following:

http://www.freescale.com/webapp/sps/site/prod_summary.jsp?code=MMA7455L&nodeId=01126911184209

Thanks!

 

[benta]

I would place something like a Fairchild NC7SZ125 (three state buffer) in series with the SDO pin.

Benta.

 

[itsmoked]

Yeah, I would too. SPI is like the Wild West. If a part has no CS then you bes provide your own via something like benta suggests.

Keith Cress
kcress -

http://www.flaminsystems.com

 

[pucca]

The sensor has a CS pin. Do I stll have to add a buffer or a diode to the SPITXD line? When the CS of the sensor is not enabled why should I add the buffer to the spitxd line?It can't sent something as long the cs in not enabled...correct?

 

[benta]

Huh? First you ask the question, then you don't like the answer?
The datasheet says the SDO pin is an _output_, not a _three-state-putut_

Benta.

 

[pucca]

Sorry but I got confused from itsmoked answer..I added the buffer...

 

[geekEE]

How about if you contact Freescale to verify whether that pin is truly output-only? Or if you have a device in-hand, just try it and see?

Of course if this is a small quantity, then it may be better to just add the tri-state buffer and not worry about it.

Glenn

 

[melone]

I have never seen a SPI solution (Freescale included) that drives the SDO when the CS is de-asserted. As pointed out previously, it would never work without external circuitry (which would add latency to the signal).

 

[itsmoked]

pucca; Skip the extra device. That SDO pin is absolutely Tri-stated when the part is not chip selected. Freescale is correcting the data sheet.

Keith Cress
kcress -

http://www.flaminsystems.com

 

[pucca]

I have already added a bat54j schottky diode. So if there is a problem I guess that it will show..I'll have the board in about one week