Opto isolating an SPI bus

Go To Last Post
10 posts / 0 new
Author
Message
#1
  • 1
  • 2
  • 3
  • 4
  • 5
Total votes: 0

Maybe this should be posted in AVR 8-bit forum, but it has to do with electronics too so I posted here.

Will I have any problems opto-isolating a bi directional SPI communication from an AVR MCU to another device? Let's say I'm running the AVR at 1 MHz (internal clock with DIV 8 fuse set) and then setting SPI scale register to scale to fosc/128. I would be using normal transistor output opto isolators (multiple channels obviously) since the SPI would be running in the low KHz range. Would I encounter any problems with this? The device I'm isolating it from must be electrically isolated from the MCU but still have serial communication.

  • 1
  • 2
  • 3
  • 4
  • 5
Total votes: 0

The devil is in the details, but conceptually, it is not unreasonable to opto couple spi. Something like a Sharp PC814 should work at low frequencies, for higher, something like the Sharp PC900V. There's also special isolator devices from the likes of Analog Devices that may suit.

Sometimes its easier to have the AVR on the isolated side and use the uart to communicate via opto and save yourself two maybe three opto isolators.

  • 1
  • 2
  • 3
  • 4
  • 5
Total votes: 0

Are there any potential problems I may encounter though?

  • 1
  • 2
  • 3
  • 4
  • 5
Total votes: 0

There's plenty of potential problems you may encounter - life is risky! Seriously, the concept is sound. There's plenty of examples on the web to choose from to decrease your risk. I've done on a couple of my products - I used PC900V optos. However, there are newer solutions available now that may be better than using optos. Again, see Analog Devices.

When you can present a circuit, we can better discuss the finer points.

  • 1
  • 2
  • 3
  • 4
  • 5
Total votes: 0

IIRC HP has series of fast optocouplers that can be used exactly in such a situations. Appnote shall contain all the information.
Could refer to SPI programmers that have optocouplers for insulation (IIRC AVRDragon based).

  • 1
  • 2
  • 3
  • 4
  • 5
Total votes: 0

Kas, HP got out of the semiconductor business years ago. Avago does the HP products these days.

  • 1
  • 2
  • 3
  • 4
  • 5
Total votes: 0

Kartman - thanks for the correction! Will take into account; the couplers I got are sourced from salvaged PCB's thus the reference.

  • 1
  • 2
  • 3
  • 4
  • 5
Total votes: 0

We use H11L1M. Those are very nice digital optocouplers and supposed to work up till 1MHz. We tested them in UART at 115200 baud and working excellent.

  • 1
  • 2
  • 3
  • 4
  • 5
Total votes: 0

you can also use a Silabs SI8422xD AND SI8423xD at the design (or it's variations), maintain very low currents,5KV isolation, and save space on the pcb. Analog Devices and TI also have a similar isolator using a different technology.

  • 1
  • 2
  • 3
  • 4
  • 5
Total votes: 0

Something like this series:

http://www.analog.com/en/interfa...

Single chip 2 inputs 2 outputs, 90Mbps, 3.5mA operation at 10Mbit, 25KV transient protection, about 1 dollar in wholesale.