Ultra-low power electronic thermometer solution designed by MSP430F microcontroller

Single chip microcomputer STM32L151CCU6

Crystal oscillator

Quartz Oscillator 3225 20M OSC

The ultra-low-power electronic thermometer designed in this paper can measure and display the temperature of the measured point through the temperature sensor, and can be extended and controlled. The thermometer has an electronic clock, the detection range is l0 ° C ~ 30 ° C, the detection resolution is 1 ° C, using LCD liquid crystal display, the static power consumption of the whole machine is 0.5 μA. The system design idea also has certain reference value for the research and development of other types of ultra-low power micro-portable intelligent detection instruments.

1 component selection

The temperature sensor of this system can be selected with a thermistor. In the measurement range of 10~30°C, the resistance of the device varies greatly with temperature, the circuit is simple, the power consumption is low, the installation size is small, and the price is also low, but the thermistor is accurate, repeatable and reliable. The sex is relatively poor, so this sensor is not suitable for detecting temperature signals below 1 °C, especially for higher resolution requirements.

The display portion can be a pen-type LCD liquid crystal display. In particular, the black-and-white pen-segment liquid crystal display has extremely low power consumption, moderate appearance, low price, and the driver chip can be selectively selected. To this end, the design uses a versatile LCD driver HT1621 with mature technology, low power consumption, stable performance and low price.

As the core component of the entire system, the choice of microcontroller is crucial. By comparing a number of single-chip chips, TI's MSP430 series controllers were finally selected. The controllers have extremely low power consumption, high performance and low cost.

2 MSP430F microcontroller features

The MSP430F series is an ultra-low-power FLASH controller produced by TI Corporation of the United States. The device is known as the "green" controller (GREEN Mcu), and its technical characteristics represent the development direction of the microcontroller. On-chip memory of the MSP430 This device unit is a very low-power unit that consumes only one-fifth the power of other flash microcontrollers. Compared with other controllers, the MSP430F can reduce board space and reduce system cost.

The MSP430F family of devices integrates ultra low power flash, high performance analog circuitry and a 16-bit reduced instruction set (RISC) CPU with short instruction cycles, most of which can be completed in one instruction cycle. The device's operating current is extremely small, and ultra-low power consumption, the current in the off state is only 0.1μA, the standby current is 0.8μA, the normal mode (250μA/1MIPS@3V), the port leakage current is less than 50 nA, Zero-power brown-out reset (BOR) is available. In addition, the chip is a low-voltage device that requires only 1.8 to 3.6V to supply power, thereby effectively reducing system power consumption. Due to its ultra-low power digitally controlled oscillator technology, frequency regulation and crystalless operation are possible. Its 6μs fast start-up time extends standby time and makes startup faster, while also reducing battery power consumption. The MSP430 series chips are rich in on-chip resources, and the I/O ports are powerful and flexible. All I/O bits can be individually configured. Each port line corresponds to multiple registers such as input, output, direction and function selection. One. Therefore, its temperature analog control can be controlled with low voltage with isolation.

3 ultra low power electronic thermometer hardware design

Figure 1 shows the hardware block diagram of this ultra-low power electronic thermometer. The design of other unit circuits is given below.

3.1 temperature acquisition conversion circuit

Using the MSP430 to measure the resistance, you can use the slope technique instead of the A/D converter circuit, which is easy to handle. For this technique, the A/D conversion of the slope can be done using the comparator and clock on the MSP430 family of chips.

The specific temperature measurement of the system is to apply the capacitor charge and discharge to convert the measured resistance value into time, and then use the capture comparison register inside the MSP430 to accurately capture the time, thereby measuring the resistance value of the thermistor to obtain the temperature value indirectly. Its temperature detection circuit structure is shown in Figure 2.

In the figure, Rref is the reference resistor for calibration and Rsens is the resistance to be measured.

When the system works, the MSP430 is first connected to the Rref port, then the high level Vcc is output and the capacitor is periodically charged through the standard resistor. After the timing is up, the port is reset, the capacitor is discharged, and the discharge process continues until the voltage drop across the capacitor. Until the charging port is at the upper limit of the "0" level, the cut-off time is accurately captured by the capture inside the Timer_a through the capture entry CA0. This period of discharge can be marked as Tref. Then, the same operation is applied to P2.1 to obtain the time Tsens at which the capacitor is discharged through the resistance to be measured. Finally, Tref and Tsens are compared, and the measured resistance value is calculated by the following formula:

Rsens=RrefTmeas/Tref