1. Introduction
The factors, which affect MCU operation stability, can be divided into 2 groups: external factors and internal factors.
1.1 External factor
1. RF interference: It is transferred within electromagnetic field, which causes the disturbance inducted by inner conductors (wires or pins). To reduce this interference, you can use an electromagnetic screen or properly arrange lines layout/components layout.
2. Interference caused by power supply wires or inner power source: It is coupled via power supply wires and power components, or transferred directly. To minimize this interference, you can use power filter.
1.2 Internal factor
1. Oscillator source stability: The stability is mainly determined by the oscillation start-up time, frequency and duty cycle. Different types of oscillators, temperatures and voltages may also affect the stability. As for the start-up time, it is determined by circuit parameters.
2. Reset circuit reliability
2. Reliability design of reset circuit
2.1 Basic reset circuit
The basic function of reset circuit is to provide a reset signal when power on, and release the reset signal after the power supply become stable. For ensuring that the system works reliably, the signal release should be delayed for a certain time until the power supply is stable. In this way, it can prevent reset failing when plug in/out or power on/off.
Please see the figure 1, the reset circuit has the basic functions as above mentioned. The figure 3 shows
the input/output characteristics of this circuit (upper part). But the circuit cannot solve the problems that the slow drop of supply voltage (battery energy deficiency) and unwanted glitches. In addition, adjusting the constant of RC to modify delay time will weaken the driving ability.
See the figure 1, the circuit in left side is for high-level reset, while the right one is for low-level reset. Sm is manual reset switch. Ch can filter the interference caused by high-frequency harmonic.
The figure 2 shows an improved reset circuit, which is added a diode to enable the capacitor to discharge quickly when supply voltage drops instantly. A glitch with certain width on power side can also make the system reset reliably. Figure 3 shows the input/output characteristics of this circuit (bottom part). Compared with the upper part, the discharge effect of bottom part has been improved。
Using comparator circuits can not only solve the problem of system instability caused by glitches, but also assert reset when the supply voltage drops slowly.
Please see figure 4, it shows an example that using comparator circuit. When VCC x (R1/(R1+R2) ) = 0.7V, Q1 will be cut-off and the system will be reset. The amplification of Q1 can also improve the load characteristic of circuit. But the disadvantage of this circuit is that the threshold of trigger voltage (Vt) would be affected by VCC. To eliminate the affection by VCC, we should add a Zener diode. Please see figure 5, if VCC is lower than Vt (Vz+0.7v), system will be reset.
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