PIC18F26K80-E/SP Microchip Technology Inc., PIC18F26K80-E/SP Datasheet - Page 255

• Manufacturer Part Number: PIC18F26K80-E/SP
• Description: 28 SPDIP .300IN TUBE, ECAN, 64KB FLASH, 4KB RAM, 16 MIPS, 12-BIT ADC, CTMU
• Manufacturer: Microchip Technology Inc.

Specifications of PIC18F26K80-E/SP

• A/d Inputs: 8-Channel, 12-Bit
• Comparators: 2
• Cpu Speed: 16 MIPS
• Eeprom Memory: 256 Bytes
• Input Output: 24
• Interface: I2C/SPI/UART/USART
• Memory Type: Flash
• Number Of Bits: 8
• Package Type: 28-pin SPDIP
• Programmable Memory: 64K Bytes
• Ram Size: 3.6K Bytes
• Speed: 64 MHz
• Temperature Range: –40 to 125 °C
• Timers: 2-8-bit, 3-16-bit
• Voltage, Range: 1.8-5.5 V
• Lead Free Status / Rohs Status: RoHS Compliant part

18.7
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 ADC Channel 29 to further
simplify design and cost.
18.7.1
We can show that the forward voltage ( V
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
T is the absolute junction temperature in kelvin, q is the
electron charge (1.6 x 10
applied to the diode and I
saturation current, which varies between devices.
Since k and q are physical constants, and I
for the device, this only leaves T and I
variables. If I
tion that V
term of the equation will always be negative, the temper-
ature will be negatively proportional to V
words, as temperature increases, V
By using the CTMU’s current source to provide a
constant I
temperature by measuring the V
18.7.2
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
multiplexer is shared by the CTMU and the ADC.
EXAMPLE 18-5:
 2011 Microchip Technology Inc.
// Initialize CTMU
CTMUICON = 0x03;
CTMUCONHbits.CTMUEN = 1;
CTMUCONLbits.EDG1STAT = 1;
// Initialize ADC
ADCON0 = 0xE5;
ADCON1 = 0x00;
ADCON2 = 0xBE;
ADCON0bits.GO = 1;
while(ADCON0bits.G0);
Temp = ADRES;
Note:
Measuring Temperature
with the CTMU
F
F
The temperature diode is not calibrated or standardized; the user must calibrate the diode to their application.
will vary as a function of T . As the natural log
BASIC PRINCIPAL
, it becomes possible to calculate the
IMPLEMENTATION
F
is held constant, it follows from the equa-
V
F
=
(Figure
kT
ROUTINE FOR TEMPERATURE MEASUREMENT USING INTERNAL DIODE
q
-19
S
1n 1 –
is the diode’s characteristic
C), I
18-2). The A/D channel
(
F
F
is the forward current
across the diode.
I
I
F
S
F
)
decreases.
F
as independent
S
is a constant
F
F
) of a P-N
. In other
-23
// Enable ADC and connect to Internal diode
//Right Justified
// Start conversion
// Read ADC results (inversely proportional to temperature)
J K
Preliminary
-1
),
PIC18F66K80 FAMILY
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
measured by the ADC. A code snippet is shown in
Example
FIGURE 18-4:
Simplified Block Diagram
Equivalent Circuit
18-5.
F
. The resulting V
I
F
PIC
Current Source
MUX
®
V
F
CTMU TEMPERATURE
MEASUREMENT CIRCUIT
Microcontroller
A/D Converter
CTMU
V
F
F
CTMU
A/D
across the diode is
DS39977C-page 255
ADC