C8051F970-A-GM Datasheet, PDF (113/454 Pages) Silicon Laboratories – Low Power Capacitive Sensing MCU with up to 32 kB of Flash

C8051F97x
17.7.2. Internal Voltage Reference
For applications requiring the maximum number of port I/O pins, or very short VREF turn-on time, the 1.65 V high-
speed reference will be the best internal reference option to choose. The high-speed internal reference is selected
by setting REFSL to 11b. When selected, the high-speed internal reference will be automatically enabled/disabled
on an as-needed basis by ADC0.
For applications with a non-varying power supply voltage, using the power supply as the voltage reference can
provide ADC0 with added dynamic range at the cost of reduced power supply noise rejection. To use the 1.8 to
3.6 V power supply voltage (VDD) or the 1.8 V regulated digital supply voltage as the reference source, REFSL
should be set to 01b or 10b, respectively.
17.8. Temperature Sensor
An on-chip temperature sensor is included, which can be directly accessed via the ADC multiplexer in single-ended
configuration. To use the ADC to measure the temperature sensor, the ADC mux channel should select the
temperature sensor. The temperature sensor transfer function is shown in Figure 17.8. The output voltage (V_TEMP)
is the positive ADC input when the ADC multiplexer is set correctly. The TEMPE bit in register REF0CN enables/
disables the temperature sensor. While disabled, the temperature sensor defaults to a high impedance state and
any ADC measurements performed on the sensor will result in meaningless data. Refer to the electrical
specification tables for the slope and offset parameters of the temperature sensor.
V_TEMP = (Slope x Temp_C) + Offset
Temp_C = (V_TEMP - Offset) / Slope
Slope (V / deg C)
Offset (V at 0 Celsius)
Temperature
Figure 17.8. Temperature Sensor Transfer Function
17.8.1. Calibration
The uncalibrated temperature sensor output is extremely linear and suitable for relative temperature
measurements. For absolute temperature measurements, offset and/or gain calibration is recommended. Typically
a 1-point (offset) calibration includes the following steps:
1. Control/measure the ambient temperature (this temperature must be known).
2. Power the device, and delay for a few seconds to allow for self-heating.
3. Perform an ADC conversion with the temperature sensor selected as the ADC input.
4. Calculate the offset characteristics, and store this value in non-volatile memory for use with subsequent
temperature sensor measurements.
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