Only needs MCU, LCD, the 32kHz crystal oscillator as well as a resistor then constitutes a basic countdown timer, moreover only needs an ordinary button battery then to realize surpasses 10 year operating time continuously, as shown in Figure 1. For example, this kind of countdown timer may use in the restaurant providing the servicing time to guarantee that the service personnels press down a button then to start the countdown timer, the micro controller starts the automatic countdown, the time length may establish in advance, the countdown process will demonstrate on the display monitor. If service personnels before the time ended not yet provides the service to the visitor, then the monitor will flash the demonstration overtime information, prompted has not achieved the service guarantee. Two big crucial design may minimize the power loss, and lengthens the battery service life as far as possible; first, chooses the battery earnestly; second, uses MCU fully the low power loss pattern.
The reason that we choose the CR2032 button type lithium battery, is mainly because it occupies a land area is young, may support the portable application, the discharging curve is even, may direct drive LCD, not need to compensate, moreover leaks extremely lowly, can realize the longer operating time. The typical CR2032 rated capacity approximately is 200mA, the operating time sustainable several hours. In order to achieve the continuous working 10 year project objective, the average system current consumption do not surpass 2.28uA, the current consumption computing mode for the battery energy content dividing application working life, as follows shows: 
We choose MCU are because it waits for an opportunity the electric current to be extremely low, is only 0.8uA, and also contained the crystal oscillator, to integrate the LCD driver as well as the interrupt actuation awakens the timer. 3.5 inch digital LCD display monitor increased the 1uA extra system current consumption. The entire duration of work’s countdown timer always waits for an opportunity the current consumption to be as follows:
Usually in the situation, the MCU work in waits for an opportunity under the pattern, the clock crystal frequency is the 32kHz timer triggers 1 second interrupt, causes MCU to return to the labor to take responsibility circulates (Mainloop), the main loop uses may obviously reduce the software overhead the direct BCD subtraction to realize software countdown register’s decreasing progressively. We increase decimal base’s 99h to the software countdown register, in fact subtracts the number is 1. The direct BCD subtraction is not only effective, moreover may also cause the countdown direct demonstration on LCD, but does not need to carry on the consumption electric current again the binary BCD subtraction operation. The software then demonstrates in the countdown register’s value on LCD. Finally, in the countdown register’s value will compare with zero, decided that pre-programming time section whether already to expire, if will expire that will demonstrate the overtime information. The main loop requests on CPU and the piece the high speed oscillator works under the 250uA current consumption. However, because we when will compile the software the cycle count will already reduce to below 100, i.e., frequency will be equal in default 1MHz under the CPU to 100us, therefore in such short operating time, the main loop will increase the current consumption may ignore, the computational method will be as follows: 
The digital countdown timer’s joint current consumption to wait for an opportunity sum of with the main loop current consumption: 
Because the mean current consumption approximately achieves 1.8uA, this section of countdown timer may realize the current consumption to be lower than 2.28uA easily the project objective, can thus above the continuous working 10 years. Since the battery life may amount to for 10 years, then we when designs the countdown timer may use the persistent effect battery, thus simplified its structure, simultaneously also reduced the unit cost. Because the massive MCU function and the pin not yet obtain the use, therefore our also available its increase extra characteristic. The implementation counter needs the firmware is quite small, only dodges in the MCU 8k byte saves takes less than 250 bytes.
