Schematic diagram of microcontroller clock circuit

Schematic diagram of microcontroller clock circuit

The following uses an actual clock circuit to illustrate the software programming method of the timer. The clock is the most common timing tool that displays hours, minutes, and seconds as units. It is a typical application representative. The
minimum timing unit of the clock is seconds, but if a single-chip 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 timing unit of seconds. It is easy to achieve 8 counts using software methods.
Timer 1 is used in the circuit, working in working mode 1, and the timer performs 125ms timing. Use the interrupt method to

The number of overflows accumulates, and when it reaches 8 times, 1 second is obtained.
To realize the carry of minutes and hours in the accumulation of a clock, multiple bases are used. The carry of seconds, minutes and hours is in decimal, while the carry of seconds to minutes and the carry of minutes to hours are in sexagesimal. There is a twelve-hour system or a twenty-four-hour system every day, which are duodecimal and twentieth-century respectively. From seconds to minutes and from minutes to hours can be realized by software accumulation and numerical comparison methods.
In the internal RAM of the single-chip microcomputer, a display buffer needs to be set. The displayed hour, minute and second values ​​are taken from the display buffer. Four units are set in the RAM as display buffers, namely 7AH, 7BH and 7CH. In order to facilitate the description of the circuit and principle, the second value is not displayed here. The carry of seconds is realized by flashing the minute value. In this way, there are four LEDs to display the hour and minute values ​​respectively. At the same time, the clock needs to be calibrated. In the program, a display code table needs to be set. The value to be displayed sends the real code value for display to the LED through the table lookup instruction. In the circuit, the micro switches of P3.4 and P3.5 of the single-chip microcomputer AT89C2051 are connected 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 it continuously to realize the calibration of the clock.
A buzzer is also set in the circuit for simple time reporting. For example, it can be set to get up at 7:30 in the morning and get up at 1:30 in the afternoon. Each time it rings for 1 minute, 1 second, and stops for 2 seconds, instead of ringing continuously. This program uses the 12-hour system. For this reason, the corresponding flags should be set in the program to facilitate the recognition of the main program. There are also several related flags in the timing program, which mainly control the direction of the program flow. The program has made detailed comments, which will not be repeated here (see Appendix).

In Figure 5, the hardware circuit still uses the low-cost AT89C2051 microcontroller as the microprocessor unit. This chip is compatible with the C51 instruction system. Programs written on the C51 can be easily transplanted to this chip without any modification.

As the field bit drive output of 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 is used for bit drive, and the display is displayed in a dynamic scanning mode. 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, the 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 use LQ5101BS ordinary light-emitting diodes, and the driving transistor 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. To facilitate the experiment, the single-chip microcomputer AT89C2051 can use a DIP20P socket. After the program is compiled and debugged, it can be burned into the AT89C2051. It is worth mentioning that the AT89C2051 is a Falsh program memory, and the program can be erased and written repeatedly, which is very convenient for experiments.