This isnt quite a spark specific question (so i also asked at arduino.cc)
But since my plan was to use a spark, im thinking, why not ask here too!
So I know that onewire devices use a bus to allow for multiple sensors on a single pin… But I’m trying to do the reverse. I have a single Onewire sensor (ds18b20) and I want two controllers to have access to read the temp.
I am trying to hack a polyscience sous vide. It already has the sensor inside, and their own proprietary microcontroller listening to the sensor. I was hoping to split the vcc, gnd and signal wires and have one set go to the existing mcu and the second going to my SPARK where I could simply listen to the temp…and post it to the net. Tweet it. upload it to xively. Create an IFTTT recipe. Make it send a fax. Damn i love connected devices…
Any thoughts?
If you are monitoring the bus, there are two challenges:
a) since your device is not the master and not initiating communications on the bus, you will need to determine when each read and write slot starts. Doing this with a bit-banged digital pin with software, or relying on exti interrupts will be difficult, but you could use the USART hardware instead. See Maxim APP 214, (http://www.maximintegrated.com/app-notes/index.mvp/id/214).
b) It will be difficult to distinguish between WRITE1 and READ1 slots without some additional knowledge of when the onewire master is reading or writing. Reverse engineering will be required to find how the other onewire master sequences his reads and writes to interact with the sensor.
Alternatively if the existing onewire master leaves enough free time on the bus, you may be able to sneak in a scratchpad read at the right time and read the temperature register without interfering with the other master.