×
INTELLIGENT WORK FORUMS
FOR ENGINEERING PROFESSIONALS

Log In

Come Join Us!

Are you an
Engineering professional?
Join Eng-Tips Forums!
  • Talk With Other Members
  • Be Notified Of Responses
    To Your Posts
  • Keyword Search
  • One-Click Access To Your
    Favorite Forums
  • Automated Signatures
    On Your Posts
  • Best Of All, It's Free!
  • Students Click Here

*Eng-Tips's functionality depends on members receiving e-mail. By joining you are opting in to receive e-mail.

Posting Guidelines

Promoting, selling, recruiting, coursework and thesis posting is forbidden.

Students Click Here

Jobs

SPI BUS and slave devices

SPI BUS and slave devices

SPI BUS and slave devices

(OP)
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!

RE: SPI BUS and slave devices

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

Benta.

RE: SPI BUS and slave devices

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

RE: SPI BUS and slave devices

(OP)
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?

RE: SPI BUS and slave devices

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.

RE: SPI BUS and slave devices

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

RE: SPI BUS and slave devices

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

RE: SPI BUS and slave devices

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).

RE: SPI BUS and slave devices

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

RE: SPI BUS and slave devices

(OP)
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 :)

Red Flag This Post

Please let us know here why this post is inappropriate. Reasons such as off-topic, duplicates, flames, illegal, vulgar, or students posting their homework.

Red Flag Submitted

Thank you for helping keep Eng-Tips Forums free from inappropriate posts.
The Eng-Tips staff will check this out and take appropriate action.

Reply To This Thread

Posting in the Eng-Tips forums is a member-only feature.

Click Here to join Eng-Tips and talk with other members!


Resources