Reset circuit of 51 single chip microcomputer

1. Basic reset circuit      The basic function of the reset circuit is to provide a reset signal when the system is powered on, and cancel the reset signal after the system power supply is stable. For the sake of reliability, the reset signal must be canceled after a certain delay after the power supply is stable, in order to prevent the jitter caused by the power switch or the power plug during the separation and combination process from affecting the reset. The RC reset circuit shown in Figure 1 can achieve the above basic functions, and Figure 3 is its input-output characteristics. However, it cannot solve the problems of power burrs (point A) and slow power drop (battery voltage is insufficient). Moreover, adjusting the RC constant to change the delay will make the driving ability worse. The circuit on the left is a high-level reset valid, and the right is a low-level Sm is a manual reset switch Ch can avoid the interference of high-frequency harmonics on the circuit. Figure 1 RC reset circuit The reset circuit shown in Figure 2 adds a diode, which makes the capacitor discharge quickly when the power supply voltage drops instantly. A certain width of power burrs can also reset the system reliably. The lower half of the input-output characteristic diagram of the reset circuit shown in Figure 3 is its characteristics, which can be compared with the upper half to add the effect of the discharge circuit. Figure 2 The RC reset circuit with a discharge loop uses a comparison circuit, which can not only solve the problem of system instability caused by power glitches, but also can reliably reset the system when the power supply drops slowly. Figure 4 is an example. When VCC x (R1/(R1+R2)) = 0.7V, Q1 is cut off to reset the system. The amplification effect of Q1 can also improve the load characteristics of the circuit, but the transition threshold voltage Vt is affected by VCC, which is a prominent disadvantage of this circuit. The use of a voltage regulator diode can make Vt basically unaffected by VCC. See Figure 5. When VCC is lower than Vt (Vz+0.7V), the circuit resets the system. Figure 3 Input-output characteristics of the RC reset circuit Figure 4 Reset circuit with voltage monitoring function Figure 5 Stable threshold voltage Figure 6 Practical reset monitoring circuit