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2010-2012 Microchip Technology Inc.

DS39977F-page 249

PIC18F66K80 FAMILY

18.7

Measuring Temperature

with the CTMU

The constant current source provided by the CTMU

module can be used for low-cost temperature

measurement by exploiting a basic property of com-

mon and inexpensive diodes. An on-chip temperature

sense diode is provided on A/D Channel 29 to further

simplify design and cost.

18.7.1

BASIC PRINCIPAL

We can show that the forward voltage (VF) of a P-N

junction, such as a diode, is an extension of the

equation for the junction’s thermal voltage:

where k is the Boltzmann constant (1.38 x 10-23 J K-1),

T is the absolute junction temperature in kelvin, q is the

electron charge (1.6 x 10-19 C), IF is the forward current

applied to the diode and IS is the diode’s characteristic

saturation current, which varies between devices.

Since k and q are physical constants, and IS is a constant

for the device, this only leaves T and IF as independent

variables. If IF is held constant, it follows from the equa-

tion that VF will vary as a function of T. As the natural log

term of the equation will always be negative, the temper-

ature will be negatively proportional to VF. In other

words, as temperature increases, VF decreases.

By using the CTMU’s current source to provide a

constant IF, it becomes possible to calculate the

temperature by measuring the VF across the diode.

18.7.2

IMPLEMENTATION

To implement this theory, all that is needed is to con-

nect a regular junction diode to one of the microcon-

troller’s A/D pins (Figure 18-2). The A/D channel

multiplexer is shared by the CTMU and the A/D.

To perform a measurement, the multiplexer is config-

ured to select the pin connected to the diode. The

CTMU current source is then turned on and an A/D

conversion is performed on the channel. As shown in

the equivalent circuit diagram, the diode is driven by

the CTMU at IF. The resulting VF across the diode is

measured by the A/D. A code snippet is shown in

Example 18-5.

FIGURE 18-4:

CTMU TEMPERATURE

MEASUREMENT CIRCUIT

EXAMPLE 18-5:

ROUTINE FOR TEMPERATURE MEASUREMENT USING INTERNAL DIODE

VF =

1n 1 – IF

IS )

(

PIC Microcontroller

A/D Converter

CTMU

Current Source

MUX

A/D

CTMU

A/D

Equivalent Circuit

Simplified Block Diagram

// Initialize CTMU

CTMUICON = 0x03;

CTMUCONHbits.CTMUEN = 1;

CTMUCONLbits.EDG1STAT = 1;

// Initialize ADC

ADCON0 = 0x75;

// Enable ADC and connect to Internal diode

ADCON1 = 0x00;

ADCON2 = 0xBE;

//Right Justified

ADCON0bits.GO = 1;

// Start conversion

while(ADCON0bits.G0);

Temp = ADRES;

// Read ADC results (inversely proportional to temperature)

Note:

The temperature diode is not calibrated or standardized; the user must calibrate the diode to their application.

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