Realization of Single Chip Microcomputer Operating Frequency Switching

First, we did such an experiment, as shown in Figure 1. We used an external oscillator circuit to obtain two high and low oscillation sources (4:1) through frequency division, and connected them to the external oscillation input of the microcontroller through two controllable gates. P1.0 is used to select the input oscillation frequency, that is, during the execution of the program, the input frequency is directly changed by the language (of course, when changing the frequency, the serial port and timer cannot be used). After the experiment, it was found that although the microcontroller can still work normally, the interference generated during the frequency change process caused some registers in the microcontroller and the data in the RAM to change, and the working state after the change was not very stable.

From the above experiments, it can be seen that it is not advisable to directly change the working frequency of the microcontroller in the normal working mode. In fact, the microcontroller has two other working modes: waiting working mode and power-off working mode (all microcontrollers made by CHMOS process generally have them). After the microcontroller enters the power-off working mode, it must be reset by hard reset, which is obviously not advisable. In the waiting working mode, the clock signal sent to the CPU is blocked, and the CPU enters the waiting state. The internal state of the CPU, such as the stack pointer (SP), program counter (PC), program status word (PSW), accumulator and other registers, is completely preserved, and the waiting working mode can be awakened by individual interrupts, which is exactly what we need. So we add a delay circuit (which can be made with a counter) to the original circuit diagram and connect it to the frequency switching control line, so that the controllable gate can act after entering the waiting working mode. In addition, P1.1 is used to generate individual control signals, which are used as interrupt sources through the delay circuit to wake up the microcontroller. Experiments show that this method can indeed stably realize the switching of the microcontroller's operating frequency, and the additional circuit is also very convenient to implement.

In actual applications, it is found that the I/O port lines of the microcontroller are often not enough. If two I/O ports are used here, it is actually a waste, and it is more troublesome to implement with one. In fact, after the microcontroller enters the waiting working mode, ALE and PSEN are both high levels. ALE continuously outputs pulses with a frequency of fosc/6 when the microcontroller is in normal working state. Only when the CPU accesses the off-chip data memory will a pulse be lost. The frequency control signal and interrupt signal can be generated by the change of the ALE signal. As shown in the figure, the counter counts the input pulses to generate them, and the ALE signal is used as the selection signal of counter 1, so that the control signal and interrupt signal are stopped due to continuous refresh during normal operation. After entering the waiting working mode, counter 1 starts to count formally due to long-term selection, and first generates a control signal to switch the frequency. After the delay of counter 2, the interrupt is used to wake up the microcontroller to enter the normal working state and continue to execute at equal distances.

Experiments show that this method can indeed run reliably, without the need for I/O ports and many control commands. As long as a control command is used to enter the waiting working mode, it will automatically generate a signal to change the frequency and an interrupt signal to wake up the microcontroller. Just add a simple interrupt handler to switch between the two working frequencies at will, fast when it is fast, slow when it is slow, and truly as you wish.