Uart + 1-wire same cable

Gabriel · Joined: 03 Aug 2009 Posts: 1067 Location: Panama

Hi, has anyone had any success sharing 1-wire data with a UART?
I have some remote mounted ds18b20 sensors and the customer requested a screen AT the sensor location.

Wiring this setup again is something i REALLY want to avoid... ive spent over 16 hours laying cable just for the sensors...its only about 40 feet, but a terribly complicated path.

The data line has a strong pullup driven by a FET at the master... as per AN-148. Its a simple setup but it works great.

3.3V, GND and 1-wire data line is all i got.
AWG22

Any ideas?
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CCS PCM 5.078 & CCS PCH 5.093

PCM programmer · Joined: 06 Sep 2003 Posts: 21708

See this CCS FAQ article:
"What is an easy way for two or more PIC MCUs to communicate?"
http://www.ccsinfo.com/faq.php?page=connect_pics
You have the two files it mentions. They are in your
CCS examples folder.

Gabriel · Joined: 03 Aug 2009 Posts: 1067 Location: Panama

I meant 1 wire as in "Maxim/Dallas 1-wire" and using that same wire for regular serial.
_________________
CCS PCM 5.078 & CCS PCH 5.093

newguy · Joined: 24 Jun 2004 Posts: 1909

You'd have to experiment. The only 1 wire device I've used is a DS1820 (or 18B20?) temperature sensor. From memory, you have to start the transaction by first asserting the 1-wire line low for a relatively long time. If you rig things so that your serial communication's baud rate is set so that the line is never low long enough to trigger the 1-wire device, you may be able to pull it off. However you'd need to make sure that whatever you choose operates under power supply extremes and anticipated temperature extremes.

Edit: I suppose you could also go to the other extreme. If you slow the baud rate down so much that the 1-wire's "answers" look like jitter, then you should be able to communicate by asserting the line low (as is appropriate given what you want to transmit), hold it low for your bit time, then release the line (high-Z), and let the temperature sensor think it's doing something useful while it holds the line low for a few microseconds before releasing it. You'd need to enable the pullup on that pin, if the PIC has pullups. If not, add an external pullup and ensure you enable the open drain on that pin (which would also be required).

temtronic · Posted: Mon Sep 09, 2019 8:02 pm

Rethink the overall hardware !
Since you're going to have 'screen' as well as the temp sensor, add a PIC at the remote location..then single wire comm is easy.
What 'screen' will you use ? I'm wondering if the DS data will affect the screen?

Adding a $1 PIC still seems the right way to go though. Especially when the client says 'now I need 2 pushbuttons and a buzzer and ........'

PCM programmer · Joined: 06 Sep 2003 Posts: 21708

PrinceNai · Joined: 31 Oct 2016 Posts: 480 Location: Montenegro

I'd also go Mr. Temtronic's way. Add another PIC at sensor location and use those three wires just for UART communication.

smee · Joined: 16 Jan 2014 Posts: 24

You could make use of a ds2404s and make a remote screen, and then use the Dallas one wire bus to display on the screen.
Thats if they still make it. Maxim killed off many parts.
Otherwise as long as you don't pull the bus low for more than 480uS you can use serial like you wanted to.

Or issue a reset, and a match rom and then 8 zeros, to deselect all devices on the bus, then it's yours to do as you wish, provided you don't issue a reset pulse ( >480us ).