Is this a circuit that can prevent the MCU from hanging?

西里古1992

Is this a circuit that can prevent the MCU from hanging? [Copy link]

 

My colleague said that this is a circuit to prevent the MCU from hanging up. Fan_Ctrl is the signal from the microcontroller, C is the DC blocking capacitor, Fan_Ctrler is the signal sent to the fan, and D is the reflux function. This is a brief introduction from my colleague. The purpose is that even if the MCU fails to work, as long as there is a continuous signal at the input end, Fan_Ctrler can continue to provide fan work. I don't quite understand what the working principle is?

maychang

Is this a circuit that can prevent the MCU from hanging? This circuit is wrong. The transistor has no base current. There is no need to say anything else.

dcexpert

I guess it means that even if the program crashes, as long as the hardware timer is still working, the PWM drive signal can be sent.

西里古1992

dcexpert posted on 2019-3-27 15:21 I guess it means that even if the program crashes, as long as the hardware timer is still working, the PWM drive signal can be issued.

Almost, is this a hardware timer structure?

chunyang

What is "MCU hang"? If the MCU does not work or works abnormally due to program runaway or other faults, it cannot be called "hang". If the MCU does not work or works abnormally, the IO controlled by the software may not continue to output pulse signals, so there is nothing to prevent. The correct way is to add a watchdog to ensure the normal operation of the MCU.

dcexpert

Sirigu1992 posted on 2019-3-27 15:31 Almost, is this a hardware timer structure?

This is a simple DC isolation driver

西里古1992

maychang published on 2019-3-27 14:32 Is this a circuit that can prevent the MCU from hanging? This circuit is wrong. The transistor has no base current. There is no need to say anything else.

Teacher, I have built a simulation circuit here. According to the simulation waveform analysis, the PWM output is normal. When the high level is high, the base voltage is about 0.6V, and the collector output voltage is low; when the low level is low, the base voltage is -4.37V, and the collector outputs a high level of 12V. When the MCU continues to output at a high level, the base voltage is only close to 0V. I don’t quite understand why one transistor is turned on and the other is not turned on at the same high level.

西里古1992

dcexpert published on 2019-3-27 21:27 This is a simple DC isolation driver

I built a simulation circuit here. According to the simulation waveform analysis, the PWM output is normal. When the high level is high, the base voltage is about 0.6V, and the collector output voltage is low; when the low level is low, the base voltage is -4.37V, and the collector outputs 12V high level. When the MCU continues to output high level, the base voltage is only close to 0V. I don’t understand why one transistor is turned on and the other is not turned on when the same high level is reached.

maychang

Sirigu1992 posted on 2019-3-29 09:07 Teacher, I have built a simulation circuit here. According to the simulation waveform analysis, PWM output is normal. When the level is high, the base voltage is about 0.6V, and the collector output voltage is...

I don't quite understand why one transistor is turned on and the other is not turned on when the level is high. This is the result of the base not being biased (or as the 6th floor said, the base is DC-isolated). You simulated, the capacitor capacity is quite large (10uF), and the initial condition of the capacitor is that the voltage across the two ends is zero (no charge is stored in the capacitor). When the signal starts to input a high level, the capacitor is charged, and at this moment there is a forward current in the emitter junction of the transistor. When the input is low level, the capacitor has nowhere to discharge, so the voltage across the capacitor will increase a little. After several cycles, the capacitor will be fully charged, and the voltage across the capacitor will be close to the peak value of the input signal, and the transistor can no longer conduct when the input signal is high level. This is exactly the same as the operation of the diode half-wave rectifier circuit (the transistor emitter junction is the rectifier diode).

maychang

Sirigu1992 posted on 2019-3-29 09:07 Teacher, I built a simulation circuit here. According to the simulation waveform analysis, PWM output is normal. When the base voltage is high, it is about 0.6V, and the collector output voltage is...

Your simulation was only observed for a short time, so you can see the part of the waveform where the transistor can conduct at the beginning of the signal input. If you observe for a longer time, you will see the part where the capacitor is fully charged and the transistor is no longer conducting.

maychang

Sirigu1992 posted on 2019-3-29 09:07 Teacher, I have built a simulation circuit here. According to the simulation waveform analysis, PWM output is normal. When the base voltage is high, it is about 0.6V, and the collector output voltage is...

If the input is continuously high, the capacitor will be filled quickly. The transistor has no base current and will no longer conduct. If the input is continuously high, it will only conduct for a short time when the signal starts (the input signal changes from zero to high).

maychang

Sirigu1992 posted on 2019-3-29 09:07 Teacher, I have built a simulation circuit here. According to the simulation waveform analysis, PWM output is normal. When the base voltage is high, it is about 0.6V, and the collector output voltage is...

The solution is to connect a diode in reverse parallel with the emitter junction. After the diode is connected in parallel, the input signal changes from low level to high level, and the transistor emitter junction has current (capacitor charging current). When the input signal changes from high level to low level, the capacitor discharges through the diode. In this way, the capacitor will not be filled with the input signal, and the transistor can be turned on every time a high level comes.