DS18B20, 16F88 problem

[Darrel Taylor]

I'm trying to figure out how you got that result, and I can't find anyway to make it come up with 0Fh.

Backtracking a bit, originally you said that you were getting a reading of 74ºF.
With this formula temperature=((((temperature>>4)+50)*9)/5)-58 temperature would have to be greater that 65500 to get 74ºF.

((((65535>>4)+50)*9)/5)-58 = 7403
Then dividing by 100 before displaying on the LCD will give you 74

That's why I think you're recieving all 1's (no response) from the DS18B20.

If you removed the /100, you should get 7403 instead of 0Fh. So This is really wierd. What changes did you make?

A few things to check...
What is the actual data being received? 65535?

How many loops are executed waiting for the conversion in Wait_Temp:? My guess here is that NO loops are performed because it receives a 1, apparently indicating that the conversion is complete, even though it's not working at all.

While the program is running, ground out the data pin. Does the reading change?

The OWIN/OUT portions of the program are fine, and should work. So there's got to be something going on with the hardware.

[DynamoBen]

How many loops are executed waiting for the conversion in Wait_Temp:? My guess here is that NO loops are performed because it receives a 1, apparently indicating that the conversion is complete, even though it's not working at all.

While the program is running, ground out the data pin. Does the reading change?

I put some serial debug messages in...as expected it never actually loops at the wait. If I ground DQ out PIC processor reboots.

I have gone thru that section of the prototype and found nothing wrong with the wiring. Very Odd.

[DynamoBen]

Okay I'm stupid. The data sheet shows what I thought was a top image of the TO92 package...its actually a BOTTOM image. I have +5 and Gnd flipped. I'm allowed one mistake every once in a while.

Now that the hardware problem is corrected I'm getting a reading of zero again. I wouldn't think swapping 5V and Gnd would kill the sensor, would it?

[Bruce]

I have fried Dallas 1-wire parts before inserting them backwards for only a few seconds.

Try something super simple like reading & displaying the raw temp value returned. If that doesn't work, it's probably toast.

[Darrel Taylor]

In engineering, you're allowed to make lots of mistakes, as long as the next EC (engineering change) solves the problem.

[DynamoBen]

If I ground the DQ pin I get all 1's. When I attach DQ to the pic I get a Raw reading of 170. The problem is its still at 170 after several minutes in the freezer. I'm thinking its dead.

[bcd]

I know this is an old post - but I though I might add what I discovered as it might help someone else in this position.

I had a similar situation using Bruce Reynolds demo code. After a load of time spent trying a whole lot of stuff including swapping sensors I found this -

When the DS18B20 is running in Parasitic Power mode (ie VCC input is tied to GND) then it cannot pull the DQ line high to notify the conversion is in progress.

I reconnected the Vcc on my DS18B20 to +5 and now it runs like a charm.

Hope this helps someone else !

Bill.