Only need MCU, LCD, 32kHz to shake brilliantly and a resistor can form a basic count-down timer, can realize it exceeds the working time of 10 years continuously with an ordinary button battery to and only need, . For example, this kind of count-down timer can be used in the restaurant and offer the service time assurance, attendants can start the count-down timer while pressing the next button, the little controller begins automatic count-down, the time size can presume, the count-down course will reveal on the display screen in advance. If attendants have not provided service to guests yet before timing to finish, then the display will be flickered and revealed information in overtime, point out and has not reached serving to guarantee. Two great critical designs can minimize the consumption, and lengthen the service life of battery as much as possible, first, choose the battery conscientiously, second, fully utilize the low consumption mode of MCU.

The reason why we choose CR2032 button type lithium battery, mainly because its occupation of land is relatively small, can support portable application, discharge curve to be flatter, can drive LCD directly, does not need to compensate, and it is low to leak electrodes and can realize longer working time. The typical CR2032 specified capacity is about 200mA, working time can last several hours. In order to realize working continuously the design object in 10 years, the average system electric current can’t exceed 2.28uA to consume, for the intersection of battery and the intersection of energy and value divided by working life that employ the intersection of calculation and way that electric current consume, u.i.:

We choose MCU because it awaits the opportune moment electric current is extremely low, it is only 0.8uA, and include the crystal oscillator, integrated LCD driver and stop driving and waking the timer up. The extra system electric current that figure LCD display screen of 3.5 inches has increased 1uA is consumed. The intersection of count-down and timer during the whole work always await the opportune moment the electric current is consumed as follows:

Generally, MCU works and awaits the opportune moment under the mode, the crystal frequency of the clock is being touched off for one second to break down for the timer of 32kHz, the work is mainly circulated Mainloop to make MCU return ,Mainly circulating and adopting is can be in order to reduce the direct BCD subtraction of the software expenses in order to realize the progressive decrease of software count-down register notably. We add 99h of the decimal system to the count-down register of the software, in fact the number deducted is 1. The direct BCD subtraction is not only effective, but also can make count-down reveal on LCD directly, do not need to consume the binary scale BCD subtraction operation of the electric current again. The software reveals the value in the count-down register on LCD next. Finally, the value in the count-down register will have already expired for the time slot when determined to be programmed in advance compared with zero’s, information that if will reveal overtime at the specified date. Circulate, require and CPU high-speed oscillator work under the circumstances that 250uA electric current is consumed at the block mainly. However, because we have already counted circulating to reduce to under 100 while writing the software, that is to say, it is equivalent to 100us under 1MHzCPU frequency of acquiescence, so during so short working time, mainly circulate the increased electric current and is consumed and can be ignored, the computing technology is as follows:

The total electric current of the digital count-down timer is consumed in order to await the opportune moment with mainly circulating the sum that the electric current consumes:

Because the average electric current consumes and nearly reaches 1.8uA, this count-down timer can realize easily the electric current consumes the design object less than 2.28uA, thus can work for more than 10 years continuously. Since the battery life-span can be up to 10 years, then we can adopt the long-efficiency battery while designing the count-down timer, thus has simplified its structure and reduced the unit cost at the same time. Because a large number of MCU functions and pins have not been used yet, we can add extra characteristic with him. It is quite small to implement the firmware that the counter needs, can not take up 250 byteses only on flashing memory of 8k byte of MCU.