ARM-based design of thermistor thermometers

As semiconductor technology continues to evolve, as a new type of thermistor temperature sensing device widely applied. He has a small size, high sensitivity, light weight, thermal inertia, long life and low cost advantages.

Thermistor thermometer traditional hardware they use a common microcontroller (MCS-51 series) + A / D converter and the LED display modules, discrete components and more power consumption, design complexity and difficult to debug; software also to use more long tedious assembly language to implement, design and low efficiency, poor portability, performance is difficult to guarantee.

Currently, the application of embedded system has entered a high, low-end parallel development stage, marked by 32-bit micro-controller development. ARM (Advanced RISC Machines) for embedded systems used widely as a 32-bit microprocessor core, with small, low power, highly integrated, convenient and portable hardware debugging the operating system and so on. For smart devices to portable, intelligence, and computer integration of the direction of development of the necessary conditions.

As the rapid development of electronic technology, electronic components continue to improve cost performance. This 32-bit ARM7 TDMI-S microprocessor core LPC2142 as the control center comes with its internal A / D converter and SPI interface to control the LED display driver MC14489 for real-time temperature display.

2, thermistor temperature conversion principle

Thermistor is a temperature sensor, he formed by the imitation ceramic semiconductor. Thermistor (NTC) resistor is different from the ordinary, he has a negative resistance temperature characteristic, that is, when the temperature rises, its resistance decreases. Figure 1 shows the thermistor characteristic curve. Thermistor resistance-temperature characteristic curve is an exponential curve, nonlinear, so the use of linear process can be carried out. Linear treatment could improve the thermal resistance characteristic curve, but more complicated. To this end, in the less demanding general applications, often made in a certain temperature range a linear relationship between temperature and resistance of assumptions to simplify the calculation. Using the thermistor to sense temperature, a thermistor is connected to a constant current, resistance can be measured at both ends of a voltage, then the formula the following formula can be obtained by temperature:

T is the measured temperature; T0 thermistor characteristics with the temperature parameter; K with the coefficient of thermal resistance characteristics; VT is the voltage across the thermistor.

ARM-based design of thermistor thermometers

Under this formula, if the measured voltage across the thermistor, and then to know parameters T0 and K, you can calculate the thermistor temperature, the temperature is measured, so put the variation of resistance with temperature into a voltage with temperature relationship. Resistance thermometer digital design major work, is the voltage across the thermistor through the A / D converter into a digital number sent to the microcontroller, then the software calculated the temperature, and then to display, print and other processing.

3 hardware design

The rapid development of electronic technology today, some powerful components falling prices make it cost-effective is improving, more and more extensive applications. This is a 32-bit LPC2142 ARM microprocessor core instead of the traditional 805l MCU core for control, for A / D conversion and the temperature real-time display. Figure 2 shows the schematic structure of the entire system.

ARM-based design of thermistor thermometers

Thermistor NTC resistor in series on a common R, still harder to +5 V power supply, check the voltage across RT, and into the LPC2142 microcontroller, the AINl (P0.28 pin) channel A / D conversion. Conversion start mode and the conversion channel selection can be controlled by setting the ADC registers ADC0DR to achieve. Conversion results through a synchronous, full duplex serial SPI interface to output to the LED display driver MCl4489 for real-time display of temperature.

About 3.1 ARM LPC2142 microcontrollers

ARM 7 TDMI-S core is a general-purpose 32-bit microprocessor core, using Feng. Neumann architecture, high performance and low power consumption. ARM structure is based on reduced instruction set computer (RISC) principles of design, instruction set and related decode mechanism than the complex instruction set computer is much simpler. . ARM 7 TDMI-S processor pipeline technology, processing and storage systems for all parts can work continuously. In this way, using a small, cheap processor core can easily achieve high throughput and real-time interrupt response.

LPC2142 is based on a real-time emulation and embedded trace 3Z/16-bit ARM7TDMI-s CPU micro-controller with embedded high-speed 64 kB FLASH memory and 16 kB of on-chip SRAM. 128-bit wide memory interface and a unique accelerator interface enables 32-bit code can run at the highest clock frequency, there is strict control on the code size of applications can use the 16 Thumb 模式 reduces code by more than 30%, while its performance loss very little.

LPC2142-house with a 10-bit successive approximation A / D converter, whose main characteristics are:

(1) 6 pin multiplexed input pin;

(2) power-down mode;

(3) Range OV ~ Vref usually 3 V, not more than VDDA voltage);

(4) Each converter includes a programmable divider, the clock can be adjusted to a successive approximation conversion required 4.5 MHz (maximum). This 10-bit conversion time is greater than or equal to 4.55μs;

(5) one or more of the input burst conversion mode;

(6) may opt for the direct start, input or Timer Match signal to trigger conversion;

LPC2142-house also has a hardware SPI (Serial Peripheral Interface) interfaces. He is a synchronous, full duplex serial interface, the maximum data bit rate of the clock rate of 1 / 8, can be configured as master or slave.

3.2 LED display driver management chip MC14489

MCl4489 produced by the United States MOTOROLA serial interface LED display driver ICs. The main CPU and system input only three between the I / 0 the port associated, for receiving serial data to be displayed. Output either directly drive seven-segment LED display, can also drive light.

MCl4489 integrates a data receiver / decoder / scan output / drive shows all the necessary circuitry, requires only one external current setting resistor can be displayed on the LED brightness control. Each MC14489 chip can display any of the following to display: 5 LED digital plus decimal point display; four and a half the number plus a decimal point with a symbol display; 25 LED display; five and a half figures. The chip includes a seven-segment decoder can output format hexadecimal numbers 0 to 9,16 letters A ~ F and 15 letters and symbols.

With a MC14489 chip to form a 5 LED display examples. Can see from the chart, with the MC14489 constitute a display circuit not used any of the current limiting resistor increases, also without additional inverting or driver circuit, the circuit design is very simple.

MC14489 chip with special design techniques to the power pin in the case of large current work still has the lowest peak and a smaller EMI (electromagnetic interference interactions).

4 Software Design

Thermistor temperature conversion from the previous principle briefly known: thermal resistance characteristic curve is an exponential curve, nonlinearity large, and because non-linear processing more complex design is not very high in this case can done to simplify the deal.

4.1 Program flow chart

Limitations, this article gives only the flow chart of program design. The whole process flow chart shown in Figure 3.

ARM-based design of thermistor thermometers

4.2 Temperature calculation program

In the formula T = T0-KVT, the coefficient K is a very small number. To facilitate the calculation, after taking to expand 256 times as the product of K value and VT, or 256 × K × VT. After multiplication of the product only to take away from the high 8 low 8 bits, you can offset the value of an expanded 256-fold of K, to get the right results.

In addition, from Figure 1, the thermistor resistance of a temperature characteristic that, in the +10 ~ 150 ℃ temperature range, the relationship between resistance and temperature good linearity. This temperature range is usually put as an effective temperature range. When the temperature is above this range, all with digital tube display F as a symbol.

As the effective temperature range does not exceed 150 ℃, so the digital temperature display with 3 tubes, its display format: AD XXX which, XXX for the temperature, Figure 2 LED1 and LED2 display only characters A and D, behind the three digital control LED3, LED4 and LED5 display temperature.

5 Conclusion

With SPI serial interface and MCl4489 management chip to form the intelligent instrument system display driver circuit can be significantly improved cost performance. In this paper, the precision required is not very high case, the characteristics of thermistor made simplistic linear processing, and use the design of this circuit on +10 ~ 150 ℃ temperature range were measured, achieved good results. Throughout the design process issues that need attention include the following:

(1) LPC2142 microcontrollers have a separate analog power supply pins VDDA, USSA, in order to reduce noise and error probability, analog and digital power supply should be used to isolate a 10μH inductor.

(2) A / D converter reference voltage Vref of choice to meet the needs of accuracy. If you would like to improve the A / D conversion precision, the general reference source chips are used to provide a reference voltage. TL431 is a good thermal stability, low noise three-terminal adjustable shunt reference source (temperature coefficient of 30 × 10-6 / ℃). This is the reference source used to provide reference voltage chip.

(3) The LPC2142 microcontroller of the system to use as a SPI master, so the P0.7 pin SSEL to take a 10 kΩ pull-up resistor.

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