The reset circuit is to use it to restore the circuit to the initial state. Just like the reset button of the calculator, it can return to the original state and recalculate. Unlike the reset button of the calculator, the method of resetting the circuit is different. One is to perform the reset operation immediately when the circuit is energized; the second is to be manually operated when necessary; the third is to be automatically performed according to the needs of the program or circuit operation.
1 RC reset circuit
1.1 Low level reset
The active low reset circuit is as follows:
The diode is used to quickly release the voltage across the capacitor when the power is off, preparing for the next power-on reset.
When the power is on, the capacitor is regarded as a short circuit, and the RESET level is 0. As the amount of time increases, the capacitor slowly charges and becomes a high level. So when power is on, there will be a process from low level to high level, that is, it will reset when power on.
Assuming that the initial voltage across the capacitor is U0 (usually set to 0V), the voltage across the capacitor at time T is UT. The 3.3V voltage is set to VCC.
The relationship between the current I flowing through the capacitor and the voltage change at both ends of the capacitor: I=C*dUt/dt
You can get: I*dt=C*dUt
Integrate the two sides separately to get: I*T=∫(0-1)C*dUt
That is, I*T=C*Ut−C*U0 (where U0=0V)
The formula can be obtained from VCC=UR+UT : VCC=R1*(C*UT/T)+UT
Assuming that the reset is completed when the capacitor is charged to 0.9*VCC, T=9*RC can be obtained at this time, and T is the required reset time.
1.2 High level reset
The active high reset circuit is as follows:
Assuming that the initial voltage across the capacitor is U0 (usually set to 0V), the voltage across the capacitor at time T is UT.
The charging current of the capacitor is:
In the same way, the value of the current flowing through the resistor at time T can be obtained as I=C1*VCC/T. The voltage across the resistor can be determined: UR=R1*(C1*UT/T)
So again: VCC=UR+UC1
At time T, the capacitor is charged to UT. If UR≥0.9VCC, the high-level reset is valid, and UT=0.1VCC can be obtained.
So it can be: 0.9VCC=R1*(C1*0.1*VCC/T), so it can be: T=(1/9)*R1*C1.
The above RC reset circuit is a relatively primitive reset circuit, and its reset signal waveform is not very standard rectangular wave, especially when it is used for power-down reset and is sometimes unreliable. Therefore, it has been basically eliminated.
Nowadays, special reset devices are generally used to realize the reset function, which not only ensures that the reset signal waveform is a standard rectangular wave, but also ensures that there is sufficient pulse width.
Commonly used power-on reset circuits (power-down reset circuits) include MAX809 (low-level reset circuit) and MAX810 (high-level reset circuit) and many compatible models, with manual reset function MAX811 (low-level reset circuit) And MAX812 (high level reset circuit) and compatible models, as well as MAX813L and compatible models with both high and low reset signal output and watchdog (program monitoring).
2 MAX809
MAX809/MAX810 is a single-function microprocessor reset chip used to monitor the power supply voltage of microcontrollers and other logic systems. It can provide a reset signal to the microcontroller under power-up, power-down and power-saving conditions. When the power supply voltage is lower than the preset threshold voltage, the device will send a reset signal until the power supply voltage returns to higher than the threshold voltage within a period of time.
MAX809 has an active low reset output. The MAX810 has a high-level active reset output. The typical value is 17μA. The low supply current makes the MAX809/MAX810 ideal for portable, battery-powered devices.
▲Typical application block diagram
▲MAX809 series complementary active low output diagram
According to the data manual description as shown below:
It can be seen from the above figure that the reset signal will be triggered within 10us when the voltage is lower than the threshold voltage. After the voltage has risen above the threshold voltage, the reset signal will be maintained for at least 140ms.
The following figure shows the relationship between the transient duration of maximum glitch suppression and the maximum negative offset (overload).
▲The relationship between the maximum transient duration of glitch suppression and the maximum negative offset (overload) at 25°C
Any combination of the duration under the curve and the overload will not generate a reset signal.
The combination above the curve is detected as power failure or power failure. Usually, the transient voltage is lower than the reset threshold 100 mV and lasts for 5 µs or less without generating a reset pulse. The transient immunity can be improved by adding a capacitor near the VCC pin of MAX809.
Regarding the threshold voltage and low-level or high-level reset, different models have been used to distinguish them, and you can choose according to your needs in the actual project.
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