LM71 reads garbage sometimes.

[CuriousOne]

Hello.

I'm facing a strong problem with LM71. Sometimes it reads garbage. Changed IC, changed PIC, changed power supply - same problem. If I cycle power sometimes one time, sometimes 3 times, etc. it then works. The code is as follows:



READTEMP: 'READ TEMPERATURE
low thcs
pause 5
shiftin thdta,thclk,msbpre,[result\16]
pause 5
high thcs
THC=result>>2/28
THC=THC-3
RETURN


tried to play with pause times - no difference.

What I'm doing wrong?

[Demon]

Totally off the top of my head, are the variables the proper size for that math?

Robert

[CuriousOne]

THC and result variables are WORD type.

[mackrackit]

Do you have the pins on the 71 pulled high?

[CuriousOne]

No, I should?

[CuriousOne]

Tried 4.7k pull-ups on all pins, does not helps.

[aratti]

From cs low to the out of data the maximum time delay is 70 ns. you are using 5 ms. play with that delay using pauseus.
See data sheet timing table.
Cheers

Al.

[Archangel]

Hello.

I'm facing a strong problem with LM71. Sometimes it reads garbage. Changed IC, changed PIC, changed power supply - same problem. If I cycle power sometimes one time, sometimes 3 times, etc. it then works. The code is as follows:



READTEMP: 'READ TEMPERATURE
low thcs
pause 5
shiftin thdta,thclk,msbpre,[result\16]
pause 5
high thcs
THC=result>>2/28
THC=THC-3
RETURN


tried to play with pause times - no difference.

What I'm doing wrong?
Maybe enable Power up timer in the configs

[CuriousOne]

Delay changes nothing, since I tried different values of PAUSE. The problem is startup, if once started OK, then it works OK. Also, I've tried to initialize it later, say after 5 seconds after power on - no luck.

[Archangel]

Pause has nothing to do with power up , sometimes mcu does not get nice crisp power on and it doesn't like slow lazy power on like when capacitors are on it's input, power up delay holds in reset until voltage is up to snuff and then allows it to boot.
One other thought : you have changed everything except the breadboard, breadboards do go bad.

[CuriousOne]

Well, I'm not using breadboard at all. I've tried to make different PCBs, switched power supplies, etc. no help.

[Charlie]

You will need to post your whole code , but here are a couple clues... The device needs a write on the rising edge, then data will be available on the falling edge. While CS is held low, send a command with a PRE then receive data with a POST. Also, are you sure you get valid data ever? The datasheet looks like you should be using LSB first, not MSB.

[CuriousOne]

The whole code, related to temperature measurement is posted above. Regarding the valid data, I had to add division by 28, to get reasonable output. I've used melting ice and boiling water to determine that coefficient.