18B20 in parasitic mode

[ocwo92]

I want to use an 18B20 temperature sensor with a 08M. Two questions:

1. How is the 18B20 addressed? (Or isn't this necessary? My only experience with 18B20 devices is that I've wired my house with multiple 18B20 sensors using the 1-wire USB adapter available from Hobby Boards; and with several 18B20 sensors in parallel, they need to be addressed properly.)

2. 18B20 devices can work in parasitic mode where they draw current from the data pin. Does the 08M support this mode so I won't have to add a 5-volt supply to the 18B20's V+ pin?

 

[ocwo92]

...I've wired my house with multiple 18B20 sensors...

For those that are interested in that project, it's documented here. It doesn't involve PICAXE devices, however, so we should probably keep any questions about the project off this board unless they're about the setup outside of the Linux machine.

 

[eclectic]

For your first question, see
READTEMP in Manual 2


Please see this thread:
http://www.picaxeforum.co.uk/showthread.php?t=5028&highlight=DS18B20+paras*

for your second question.

Then, try an advanced search using the terms:
DS18B20 paras*

e

 

[ocwo92]

For your first question, see
READTEMP in Manual 2

Thanks. I already looked there, though, and it isn't clear to me whether the 18B20 needs to be addressed first, or whether readtemp produces some kind of "respond regardless of your ROM code" command to the devices attached on the 1-wire interface.

Please see this thread:
http://www.picaxeforum.co.uk/showthread.php?t=5028&highlight=DS18B20+paras*

for your second question.

From what I'm gathering from this and various other posts, the PICAXE processors don't support parasitic mode. I suppose that support for parasitic-mode 18B20 devices would be a good suggestion for future processors.

 

[BeanieBots]

ReadTemp can only support one device per I/O line and does not support parasitic mode.
However, devices which support the one-wire commands CAN address individual sensors on a single line and also support parasitic mode. However, you will need to know what commands to send. (unlike ReadTemp which does it all for you).

Rmeldo has posted how it's done. Have a search.

 

[westaust55]

recognising that you are trying to use the READTEMP as available on the 08M (as opposed to the OWIN/OWOUT commands on the X1 and X2 parts),

My past findings were that when used in parasitic mode the pull-up resistor with a short cable (say less than 2 metres) needs to be around 1.5kOhm rather than the 4.7 kOhms as used with separately powered 1-wire circuits.

Although maybe a bit heavy going, you may wish to have a read of the 1-Wire tutorial at post 43 here:
http://www.picaxeforum.co.uk/showthread.php?t=15306&page=5

Have a look in particular at page 18