LM75A: Temp Error Issue

David Zhao

Part Number: LM75A

My customer is adopting LM75A solution and schematic is as attached. Register are all default value. Now there is an issue when doing temperature box experiment. Setting temperature box change rate 25C/hour, so about 0.5C/min. When temperature box shows -30C, however the readback value is -66.5C. I2C waveform is as attached. Register 0X00 is0XBDFF, temp is -66.5C. Could you help comment possible reasons or what i can test to get the reason? Thanks!

over 6 years ago

0 Aaron Heng over 6 years ago

TI__Mastermind 31600 points

Hi David,

Let me look into this, and I will get back to you.

Aaron

0 Aaron Heng over 6 years ago in reply to Aaron Heng

TI__Mastermind 31600 points

Hi David,

I reviewed your schematic, and don't see anything wrong except the START condition. It look like your firmware brings the SCL and SDA lines to a LOW each time the communciation protocol is ended. Let fix this issue first. Could you please bring SCL and SDA signals to a HIGH so that the device can see the START condition?

Aaron

0 Aaron Heng over 6 years ago in reply to Aaron Heng

TI__Mastermind 31600 points

Hi David,

Just checking in with you if you have a chance to fix the issue by bringing the SCL and SDA signals to a HIGH before execute the command protocol.

Aaron

0 Vera Mao over 6 years ago in reply to Aaron Heng

TI__Expert 5065 points

HI Aaron,
thank you for supporting!
checked with customer, they did not capture the whole picture of the timing, actually they did pull SCL&SDATA to high at start bit, and after that, they will pull it to low for 200us and then send the address bit. so is it ok?
and BTW, did BU make the experiment on such condition? temp change 25degree/hour, and test the reported temp during the temp change, will the read out be precise? tks a lot!

0 Aaron Heng over 6 years ago in reply to Vera Mao

TI__Mastermind 31600 points

Hi Vera,

Thank you for providing the update. Could you please ask the customer to capture a scope photo when they read a negative temperature? Please do not use the phone's camera instead save the data file from the oscilloscope. This will allow me to view on my pc in details.

Aaron