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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 (V_F) of a P-N
junction, such as a diode, is an extension of the
equation for the junction’s thermal voltage:
( ) V_F =
kT
q
1n
1–
I_F
I_S
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), I_F is the forward current
applied to the diode and I_S is the diode’s characteristic
saturation current, which varies between devices.
Since k and q are physical constants, and I_S is a constant
for the device, this only leaves T and I_F as independent
variables. If I_F is held constant, it follows from the equa-
tion that V_F 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 V_F. In other
words, as temperature increases, V_F decreases.
By using the CTMU’s current source to provide a
constant I_F, it becomes possible to calculate the
temperature by measuring the V_F 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 I_F. The resulting V_F 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
Simplified Block Diagram
PIC^® Microcontroller
Current Source ^CTMU
A/D Converter
MUX
A/D
V_F
Equivalent Circuit
I_F
CTMU
A/D
V_F
EXAMPLE 18-5: ROUTINE FOR TEMPERATURE MEASUREMENT USING INTERNAL DIODE
// Initialize CTMU
CTMUICON = 0x03;
CTMUCONHbits.CTMUEN = 1;
CTMUCONLbits.EDG1STAT = 1;
// Initialize ADC
ADCON0 = 0x75;
ADCON1 = 0x00;
ADCON2 = 0xBE;
// Enable ADC and connect to Internal diode
//Right Justified
ADCON0bits.GO = 1;
while(ADCON0bits.G0);
Temp = ADRES;
// Start conversion
// 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.
 2010-2012 Microchip Technology Inc.
DS39977F-page 249