DIP optical (or otherwise) isolator for SPI Communications  - Page 1
Forum section: electronics / projects

author: ftransform
author_rank: Frequent Contributor
timestamp: March 19, 2013, 02:06:36 am
content: I would like to use some kind of isolation for SPI communicationI need 4 channels in order to communicate with a MCUThe SPI is set to 9600 baud and my MCU is running at 10MHz.I am having trouble selecting a DIP opto isolator for my device. I found a logic level opto isolator on digikey with 4 channels but it costs 80$ for some reason.There are cheap 2 channel ones such as here,http://www.digikey.com/scripts/dksearch/dksus.dll?FV=fff40039%2Cfff8033d%2Cb40038%2C1140050&mnonly=0&newproducts=0&ColumnSort=-1000011&page=1&stock=1&pbfree=0&rohs=0&quantity=0&ptm=0&fid=0so I suppose that I can use two, but I am unsure how to select the baud rate and rise time that I need.There are also these opto isolators (not under the logic section)http://www.digikey.com/scripts/dksearch/dksus.dll?FV=fff40039%2Cfff8033f%2Cb4002e&vendor=0&mnonly=0&newproducts=0&ptm=0&fid=0&quantity=0&PV69=80&stock=1which are cheap, and have 4 channels, making a "neater" single chip solution to my problem, but I don't know how to determine their suitability for transmitting the SPI signal.Can anyone help me out?

author: David_AVD
author_rank: Super Contributor
timestamp: March 19, 2013, 02:10:38 am
content: Maybe this is close to what you want?http://www.digikey.com/scripts/dksearch/dksus.dll?vendor=0&keywords=ADUM5402ARWZ&stock=1EDIT: I realise you asked for DIP package, but far more choices in SMD.

author: Harvs
author_rank: Super Contributor
timestamp: March 19, 2013, 02:25:43 am
content: I use these:http://www.digikey.com.au/product-detail/en/SI8431AB-D-IS/336-1758-5-ND/2170665Cheap as chips. But you obviously will need an adapter board if you want it in a through hole package.At those sorts of data rates (9600bps) you'll have no problem. As you get much higher you need to watch the propagation delays through the isolator on the return path. Easiest way to visulise it is to grab an SPI signal diagram and shift all the signals by the propagation delay each time they go through the isolator. You'll find that the clock gets delayed on the forward path, which then the slave will output the data bit which will get delayed again back through the isolator. So that closes the time where the master outputs the clock then has to have valid return data to sample.But as I said, at 9600bps you'll have no problems with a modern isolator.

author: Harvs
author_rank: Super Contributor
timestamp: March 19, 2013, 02:28:50 am
content: Woops that was the three channel version, bu they also have 4-channel and 2-channel version (so8 package.) You just need to look up the part number for the configuration of channels you want (i.e. how many channels you want in each direction.)

author: ftransform
author_rank: Frequent Contributor
timestamp: March 19, 2013, 03:07:02 am
content: Eh, actually I realized that having 4 on one die is not good ( I want 2 input and 2 output) so I will just use one of the devices under the logic level opto isolation and put one chip upside down .Or maybe I will just buy the SOIC package you listed....... dip is dying I suppose...

author: David_AVD
author_rank: Super Contributor
timestamp: March 19, 2013, 03:52:44 am
content: Yep, so many parts are only available in SMD versions now and the trend is not about to reverse.

author: c4757p
author_rank: Super Contributor
timestamp: March 19, 2013, 03:59:04 am
content: I've pretty much completely converted to SMD for PCBs by now, but damn, I'd like to have through-hole versions of some of these newer parts for testing and prototyping. So easy to just walk over to the bench and throw something together in a breadboard.

author: ftransform
author_rank: Frequent Contributor
timestamp: March 19, 2013, 04:12:06 am
content: I like dip because I feel that I can avoid the use of vias because I can solder the top of the chip if I want a topside connection or the bottom of the chip if I want a bottom side connection.I am resistant to change but in the case of the opto-isolator I can be pretty confident that it will work with no testing. I will have to put a dummy wire to connect it to ground though..

author: c4757p
author_rank: Super Contributor
timestamp: March 19, 2013, 04:19:25 am
content: I find SMD much, much more convenient to assemble onto a PCB. I can solder down a surface mount chip with any number of pins at all and any pitch with some flux and a quick swipe of the iron. I've seen people do similarly with DIP but I can't do it, I end up making a big old mess. I don't really think you're avoiding vias with DIPs - every pin is a via...

author: M. András
author_rank: Super Contributor
timestamp: March 19, 2013, 09:25:52 am
content: those silicon lab devices have a range of 100mbps models with6 channel 3-3 or other choices yeah its smd but a wide body chip

author: KeithBrown
author_rank: Contributor
timestamp: March 19, 2013, 03:16:36 pm
content: Quote from: ftransform on March 19, 2013, 02:06:36 amI assume by that statement that you are using a processor with a built-in SPI block and also with a pre-scaler that is also meant to be used with RS-232-timed serial comm? At any rate there are a coiple of aspects to consider:1) You are correct to be concerned with the rise time since variations in the rise time between devices may cause the clock edge to be in a bad place with respect to the data. I have not looked at the devices you are considering, but you might best consider this parameter.2) A typical opto circuit will have a fast falling edge and a slow rising edge so this could shorten clock pulses with similar effects.Not sure which consideration is most important to you design.Also I'm confused why you say you have 2 channels in one direction and 2 in the other: full SPI would have three from the processor to the device (MOSI, SCK, SS) and one back (MISO).