PIC 8-bit microcontroller power supply and clock

A single-chip microcomputer is a very large-scale integrated circuit, which contains tens of thousands of transistors or field-effect transistors. Therefore, in order for the single-chip microcomputer to operate normally, it is necessary to provide energy for it, that is, to supply power to the transistors or field-effect transistors in the chip so that they can work in the corresponding state.
The PIC16F84 single-chip microcomputer requires a 5V power supply (the actual working voltage is 4.0V to 6.0V). Therefore, the simplest way is to use three 1.5V batteries in series to supply power, or to use rectification and voltage stabilization, as shown in Figure 1. Figure 1-a shows three batteries in series, which can obtain a voltage of about 4.5V; Figure 1-b shows four batteries in series, and a silicon diode is used to reduce the voltage, and the actual output voltage is about 5.4V. Figure 1-c shows that after rectification (the rectifier is not shown in the figure), the AC power of the mains is converted into a DC voltage of 7V to 20V, and then stabilized by the integrated voltage regulator 7805 to obtain a stable +5V voltage. Figure 1-d is similar to Figure 1-c, but instead of an integrated voltage regulator, a cheap voltage regulator diode is used to stabilize the voltage. PIC16F84 consumes only 1mA of power (low clock consumes less power). If PIC is to drive light-emitting diodes (LEDs) or other high-current devices, the microcontroller power supply must also power such devices. Therefore, the circuit in Figure 1-d uses a Zener diode to stabilize the voltage. It cannot drive devices such as LEDs. This should be noted.
In Figure 1, all four power supply circuits require a capacitor of 0.1μF to be connected between pin {14} (V+) of the PIC16F84 microcontroller and the ground. Moreover, this capacitor should be installed close to the microcontroller to filter out the ripple of the power supply and prevent the PIC and adjacent components from being disturbed by noise. It should be emphasized here that this capacitor must be installed no matter how "pure" the power supply is.
Note: In addition to pin {14} (V+ or VDD) of the PIC16F84 microcontroller being directly connected to the power supply, pin 4 (MCLR - reset input pin) is usually also connected to the power supply V+ through a 10kΩ resistor. MCLR is active at low level. If it is grounded, the PIC will be reset and the RAM will be cleared. If the power-on is slow, the PIC may be in an uncertain state. At this time, a normally open reset button should be installed between MCLR and ground.
Like any microprocessor, the operation of PIC16F84 is rhythmic, so a beat generator - clock is needed to control the running speed of the CPU and step through various operations. The maximum clock speed of 16F84-04P becomes 4MHz. Reducing the clock frequency can save energy and slow down the execution speed of PIC. When the clock frequency is 30kHz, it only consumes 0.1mA.
The most commonly used clock circuit of PIC microcontroller is shown in Figure 2. Figure 2-a is a direct input of external clock, that is, the clock signal is generated by an external oscillator circuit. Figure 2-b uses the internal clock circuit of PIC, and then connects the components that determine the clock frequency from the outside, namely resistor R and capacitor C. The figure shows three resistors with different resistance values. When used with a 100pF capacitor, they can generate three different clock frequencies of 1.5MHz, 600kHz or 100kHz. Figure 2-c still uses the PIC microcontroller's internal oscillator circuit, but is connected to an external quartz crystal, thus generating a more accurate and stable clock signal. However, quartz crystals are usually more expensive than resistors and capacitors. If crystals commonly used in televisions (such as 3.58MHz crystals) are used, the price is still relatively low.