Digital countdown timer never needs battery replacement

Using just an MCU, LCD, 32kHz crystal and a single resistor a basic countdown timer can be built that achieves better than 10 years of continuous operation from a common coin cell battery shown in Figure 1.



An application for such a countdown timer could be service guarantee within a certain time at a restaurant, for example. The server starts the countdown timer by pressing a single button that resets MCU automatically starting at a preprogrammed interval visible on the LCD. Should the countdown timer interval expire, a time-out message will flash indicating the service guarantee was not met.

The two design keys used in the application minimize power consumption and maximize battery life, and are the careful choice for the battery and diligent usage of the MCU’s low-power modes.

A CR2032 lithium coin-cell battery is selected for the battery specifically for its small form factor supporting portable applications, flat discharge curve allowing direct LCD drive with no compensation and extremely low-leakage for extended operation. The rated capacity of a typical CR2032 is ~200mA hours of energy.

To achieve the design goal of 10 years of continuous operation, the average system current consumption must not exceed 2.28uA as calculated by dividing the battery energy value by the operational life of the application as follows:



The MCU is selected because of its low standby current of 0.8uA that includes a crystal oscillator, integrated LCD driver and interrupt driven wake timer. The 31/2 digit LCD display adds an additional 1uA of system current consumption. The total standby current consumption for all active countdown timer components is:



The MCU operates normally in standby with a one second interrupt triggered from a timer clocked from the 32kHz crystal. The timer interrupt returns the MCU to an active mainloop thatdecrements in software a countdown register using direct BCD subtraction that significantly reduces software overheard. By adding decimally 99h to software countdown register the number 1 is actually subtracted. Direct BCD subtraction is not only effect, but allows the countdown to be directly displayed on the LCD without a current wasting binary-BCD subtraction routine.

Software next displays the value in the countdown register on the LCD. Finally, the value in the countdown register is compared to zero to determine if the preprogrammed interval has expired and if so f lashes the timeout message.

The mainloop requires the CPU and on-chip high-speed oscillator to be active with a current consumption of 250uA. The software has been written to reduce cycle count to less 100— equivalent to 100us at the default 1MHz CPU frequency. With such a short active period the additive current consumption of the mainloop is virtually negligible and calculated as follows:



Total current consumption for the digital countdown timer is the sum of the standby and mainloop:



At ~1.8uA average current consumption, the countdown timer easily achieved the design goal of staying below 2.28uA thus allowing over 10 years of continuous operation. With 10 years of battery life the countdown timer could be built with a permanent battery simplifying construction and reducing the cost of the unit. Plenty of MCU functions and pins are unused and can be used to add additional features. The firmware required to implement the counter is compact requiring less than 250 bytes of the 4k bytes available Flash memory on the MCU.


Author information
Mark E. Buccini is the marketing director of the Advanced Embedded Controls Division, Texas Instruments.