Temperature Indicator Operation

The Temperature Indicator circuit, inside many PIC® Microcontroller Unit (MCU) Devices, outputs a proportional voltage relative to the silicon temperature. The proportional voltage output is achieved by measuring the forward voltage drop across multiple silicon junctions inside the device. A voltage across two (low range) or four (high range) silicon junctions can be selected.

The temperature sense circuit is integrated with the Fixed Voltage Reference (FVR) module. The circuit is enabled by setting the Temperature Sensor Enable bit (TSEN) of the Fixed Voltage Reference Control (FVRCON) register. When disabled, the circuit draws no current.

tempcirc.png

The ideal voltage of a single junction temperature can be calculated per the equation below.

(1)
\begin{equation} Vt = 0.659 – (Temperature Degrees C + 40) * (0.00132) \end{equation}

The equations below describe the output characteristics of the temperature indicator where Vt is the voltage across a single silicon junction.

High Range:

(2)
\begin{equation} Vtemp = VDD - 4Vt \end{equation}

Low Range:

(3)
\begin{equation} Vtemp = VDD - 2Vt \end{equation}

The high range, selected by setting the TSRNG bit of the FVRCON register, provides a wider output voltage. This provides more resolution over the temperature range, but may be less consistent from part to part. This range requires a higher bias voltage to operate and thus a higher VDD is needed.

The low range is selected by clearing the TSRNG bit of the FVRCON register. The low range generates a lower voltage drop and thus a lower bias voltage is needed to operate the circuit. The low range is provided for low voltage operation.

Minimum Operating VDD vs. Minimum Sensing Temperature

When the temperature circuit is operated in low range, the device may be operated at any operating voltage that is within specifications. When the temperature circuit is operated in high range, the device operating voltage, VDD, must be high enough to ensure that the temperature circuit is correctly biased.

The table below shows the recommended minimum VDD vs. Range setting for a PIC16F1825 device. The device data sheet should be reference for the exact range.

temprange.png

The final temperature value is presented as an ADC value per the equation below.

(4)
\begin{equation} ADCResult = (Vtemp/VDD) * (2^n-1) \end{equation}

where n is the resolution of the ADC; 8 or 10 bits.

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