Schematic diagram of clock circuit composed of AT89C2051

We use an actual clock circuit to illustrate the software programming method of the timer. The clock is our most common timing tool that displays hours, minutes, and seconds. It is a typical application representative.

The minimum unit of the clock is seconds, but if the microcontroller timer is used for timing, if a 6.0MHz crystal oscillator is used, even if it works in working mode 1, the maximum timing time can only reach 131ms, so we can take each timing time as 125ms, so that the timer overflows 8 times (125ms╳8=1000ms) to get the minimum unit of seconds. It is easy to achieve 8 counts using software methods.

We use timer 1, working in working mode 1, and the timer performs 125ms timing. The interrupt method is used to accumulate the number of overflow times. When the overflow times are reached 8 times, 1 second of timing is obtained.

The time accumulation of a clock requires multiple bases to realize the carry of minutes and hours. The carry of seconds, minutes and hours is in decimal, while the carry of seconds to minutes and minutes to hours is in sexagesimal. There is a twelve-hour system or a twenty-four-hour system every day, which are in duodecimal and twentieth-century respectively. From seconds to minutes and from minutes to hours can be realized through software accumulation and numerical comparison methods.

In the internal RAM of the single-chip microcomputer, it is necessary to set up a display buffer. The displayed hour, minute, and second values ​​are taken out from the display buffer. Four units are set up in the RAM as display buffers, namely 7AH, 7BH, and 7CH. In order to facilitate the description of the circuit and principle, we do not display the second value here. We realize the carry of the second by flashing the minute value. In this way, we have a total of four LEDs to display the hour and minute values ​​respectively. At the same time, the clock needs to be calibrated. In the program, it is also necessary to set up a display code table. The value to be displayed is sent to the LED through the table lookup instruction to send the real code value for display. We use the two I/O ports PP3.4 and P3.5 of the single-chip microcomputer AT89C2051 to connect micro switches to realize the correction of hours and minutes. Each time the hour or minute value is pressed, 1 is added. The value is accumulated by pressing continuously to realize the calibration of the clock.

We also set a buzzer in the circuit for simple time reporting. For example, we can set a wake-up time of 7:30 in the morning and another wake-up time of 1:30 in the afternoon. Each time it rings for 1 minute, 1 second, and stops for 2 seconds, instead of continuous ringing. We use the 12-hour system in this program. For this reason, we need to set the corresponding flags in the program to facilitate the recognition of the main program. There are also several related flags in the timing program, which are mainly used to control the direction of the program flow. We have made detailed comments in the program, so I will not repeat them here.

For the hardware circuit, we still use the low-cost AT89C2051 microcontroller as the microprocessor unit. This chip is compatible with the C51 instruction system. Programs written on C51 can be easily transplanted to this chip without any modification. We use P0 port as the field bit drive output of the LED. The "carry" of the second uses the fractional value flashing prompt, which is on for 0.5 seconds and off for 0.5 seconds. P3.1-P3.3 are used for bit drive, and the dynamic scanning method is used for display. The display time of each LED can be between 10-25ms. The scanning frequency cannot be too high, otherwise the display time of each LED is too short and the brightness is too low, which is not easy to watch. It is better to make the naked eye not feel the LED flickering. For the sake of intuitiveness, our drive output does not use integrated circuits, but uses discrete components-transistors, but the working principle is the same.

This circuit structure determines that the LED uses a common anode digital tube, which can be LQ5101BS ordinary light-emitting diodes, and the driving transistors can use the easily available 2SA1015 and 2SC1815 models. Of course, low-power transistors such as S9012, S9013, S9014, 2N5401, 2N5555 can also be used. There are no special requirements for other devices. For the convenience of experiments, the single-chip microcomputer AT89C2051 can use a DIP20P socket. After the program is compiled and debugged without error, it can be burned into the AT89C2051. It is worth mentioning that AT89C2051 is a Flash program memory, and the program can be repeatedly erased and written, which is very convenient for experiments.