LPC11U68 - internal temperature sensor

[johanntaferl]

Hello all,

since I have no luck in another thread (https://community.nxp.com/message/764698), I want to ask my question in a different way.

I do use an LPC11U68 on an Xpresso V2 Rev C

The data sheet and manuals say that there is an 12bit ADC.

The internal temperature sensor is connected to channel 0 when powered up.

The sensor measures the temperature inverse to the ADC value.

The temperature range is from -40 to 105°C.

I get a raw ADC value of about 650.

So I calculate:

const constexpr std::int32_t maxRawValue = 4095; //12bit ADC
const constexpr std::int32_t temperatureRange = 145;
const constexpr std::int32_t temperatureOffset = 40;

const std::int32_t rawValue = getRawValue();
const std::int32_t invertedRawValue = maxRawValue - rawValue;
const std::int32_t result = (invertedRawValue * temperatureRange / maxRawValue) - temperatureOffset;‍‍‍‍‍‍‍‍‍‍‍‍‍‍‍‍‍‍‍‍‍‍‍‍‍‍‍‍‍‍‍‍‍‍‍

My result is than -17.

When I use some coolant spray, the rawValue will not change much.

Does anybody have some suggestions what I could do wrong?

Thanks a lot!

PS: I get correct ADC values when I turn off the internal temperature sensor and do connect a potentiometer.

[rocketdawg2]

there is an example

LPCOpen Software for LPC11XX|NXP 

[johanntaferl]

Hello David,

I found the example "periph_temp", but it only prints the raw 12bit hex values from the ADC. I try to figure out how to translate these values to degree Celsius.

Did I miss another example, or the conversion?

Thanks,

Johann

[kerryzhou · Accepted solution]

Hi Johanntaferl,

   What the temperature formula you are using?

  From the lpc11u68 datasheet, we can get that:

So, the temperature formula should be:

Vtemp= -2.36T+606;

=>T=(Vtemp-606)/(-2.36),

Take care, Vtemp should be mV.

Besides, in the code, please note the user manual mentioned points:

For an accurate measurement of the temperature sensor by the ADC, the ADC must be configured in single-channel burst mode. The last value of a nine-conversion (or more)
burst provides an accurate result.
After the Temperature Sensor is powered, it requires some time to stabilize and output a voltage that correctly represents the device temperature. A much shorter settling time
applies after switching the A/D converter to use the sensor output. Software can deal with both of these factors by repeatedly converting and reading the Temperature Sensor output
via the A/D converter until a consistent result is obtained.

Please also do the calibration of the temperature sensor in the code.

Please check these points, and test it again.

Wish it helps you.

Have a great day,
Kerry

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