Communication SPI between 2 18f4550

D_Portgas · Joined: 18 Jan 2018 Posts: 1

Hello, I'm trying to make a continuous communication with 2 18F4550. I make one based in another program but i failed when i compiled it.

The main thing that one of my pics it's connected to a dip switch and when i pull up in a combination send to other pic the value and then this other pic returns and answer.
I'm making probes with leds, but it doesn't work.
Can someone help me?

Ttelmah · Joined: 11 Mar 2010 Posts: 19537

If you just want to send a number, use I2C.

SPI is the better interface if you need very high speed, but is more complex to get right. Problem is that there is nothing to guarantee initial synchronisation, unless you add a select line. Also remember that it is taking place both ways at once. So when you send a byte you receive 'back' at the same time what was in the slave's buffer register. To then send a reply back, you have to then load the reply, before the master then clocks this back, with suitable timings at each stage.

Now I2C, has in the hardware a synchronisation system, based upon the start and stop, and also using open collector lines, so the slave can delay the clock release. Makes it much simpler to implement....

temtronic · Posted: Thu Jan 18, 2018 1:10 pm

Unless it's a requirement to use SPI, use serial, aka RS-232. CCS supplies examples, even single wire communication betwen PICs.
For the short distance that SPI implies, using 'RS-232' will definitely do the job !

Jay

Ttelmah · Joined: 11 Mar 2010 Posts: 19537

Downside is he won't have hardware receive on these pins. The same pins though will have hardware I2C.
Hence my suggestion.

temtronic · Posted: Thu Jan 18, 2018 4:42 pm

He should hopefully be able to use the hardware UART(RC6 RC7). we really need more information as to what is available and required.
So far he's got 3 or 4 options for the project !

PCM programmer · Joined: 06 Sep 2003 Posts: 21708

Here are two SPI slave examples. There are many more in the forum
if you search for: SPI slave (and set it to search for all terms).
http://www.ccsinfo.com/forum/viewtopic.php?t=26888
http://www.ccsinfo.com/forum/viewtopic.php?t=39145

Also look in your CCS examples directory for this program:
ex_spi_slave.c