“In the microprocessor system, in order to ensure the stable and reliable operation of the microprocessor system, it is necessary to provide a power monitoring circuit for the microprocessor system. SGM803 is such a chip. It can generate a reset low-level output of no less than 140ms when the microprocessor is powered on, powered off and the voltage is lower than a certain value of the supply voltage, ensuring that the microprocessor is running in a known state and avoiding the execution of error codes. The chip is packaged in SOT-23, which greatly reduces the complexity of the system circuit and the number of components compared to circuits composed of discrete components or general-purpose chips, and significantly improves the reliability and accuracy of the system.
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In the microprocessor system, in order to ensure the stable and reliable operation of the microprocessor system, it is necessary to provide a power monitoring circuit for the microprocessor system. SGM803 is such a chip. It can generate a reset low-level output of no less than 140ms when the microprocessor is powered on, powered off and the voltage is lower than a certain value of the supply voltage, ensuring that the microprocessor is running in a known state and avoiding the execution of error codes. The chip is packaged in SOT-23, which greatly reduces the complexity of the system circuit and the number of components compared to circuits composed of discrete components or general-purpose chips, and significantly improves the reliability and accuracy of the system.
Internal structure and pin function
The internal structure of the SGM803 chip is shown in Figure 1. The circuit includes a voltage comparator, a low-power voltage reference source, a voltage divider, an output delay circuit and an output drive circuit.
Figure 1 Internal structure of SGM803
Parameters and timing diagram
1 Reset threshold voltage
The reset threshold voltage VTH is the most important parameter of SGM803. When the power supply voltage drops to the reset threshold power supply voltage, the reset terminal of the chip gives a reset signal and outputs a low level, so that the monitored system will reset in time when the power supply voltage drops, playing an effective monitoring role.
2 Reset sequence
When the power supply voltage drops below the threshold voltage, the reset signal of SGM803 is low, and the low level state of the reset signal remains valid for at least 140ms after the power supply voltage rises above the threshold.
As shown in Figure 2, there is a reset delay of about 140ms when the reset level changes from low to high. This delay can ensure that the controlled system can work normally when the power supply voltage is restored to above the threshold.
Figure 2 Reset timing
3 VCC transient response
SGM803 can protect the microcontroller from power-down errors. It has a filtering function for short-term sudden drops in the power supply voltage, that is, the power supply voltage will not generate a reset signal if the power supply voltage is lower than the reset threshold in a short period of time. However, as the magnitude of the power supply voltage dip increases (becomes lower than the reset threshold), the pulse width that does not generate a valid reset signal will decrease. Normally, when the power supply voltage is lower than the reset threshold by 100mV for less than 10μs, a valid reset output will not be generated. Figure 3 shows the maximum pulse width of a VCC transient without reset. As the amplitude of the VCC transient becomes smaller and smaller than the threshold (the value of VTH-VCC continues to increase), the maximum pulse width also continues to decrease. In order to better use SGM803, connect a 0.1μF ceramic bypass capacitor as close as possible to the VCC and GND pins to provide a more accurate reset threshold voltage and improve the anti-interference ability of the system voltage monitoring circuit.
Figure 3 Maximum pulse width application circuit for VCC transient without reset
Since SGM803 is an open-drain output, when in use, add a pull-up resistor to the power supply VCC and reset terminal. The size of the resistor is generally 100kΩ. The reset terminal can also be connected to the power supply through a pull-up resistor alone, instead of being connected to VCC. The voltage of this power supply can be any value between 0 ~ 5.5V, so there will be a reset current flowing to the VCC (Leakage Current) , The leakage current of SGM803 is very small, the maximum is only 1μA.
In order to ensure that the state of the reset pin of SGM803 is known when VCC is lower than 1.0V, it is recommended to connect a pull-down resistor of about 100kΩ between the reset pin and GND (see Figure 4).
Figure 4 Effective reset when VCC is lower than 1.0V
Because SGM803 provides an open-drain reset output, SGM803 can be connected to the μP/μC bidirectional reset pin by connecting a 4.7kΩ resistor in series between the SGM803’s reset output and μP/μC bidirectional reset pin. Such as MC68HC05 series microcontroller, its reset pin is a bidirectional port, apply a sufficiently wide low-level pulse voltage on its reset pin to reset MC68HC05. After MC68HC05 resets, it can control this port to become low level through software at the same time, in order to make other external devices in the system reset, the concrete circuit is shown as in Fig. 5.
Figure 5 Connection of bidirectional reset pin
In general applications, the open-drain output of SGM803 is usually pulled up to the monitored power supply voltage, that is, the power supply terminal VCC of SGM803. In some applications, it is also necessary to pull up the open-drain output of SGM803 to another power supply to achieve the purpose of level conversion, as shown in Figure 6. It should be noted that the open-drain output of SGM803 no longer pulls down the current when the power supply voltage is lower than 1.15V. In addition, due to the presence of the pull-up current, as the power supply voltage decreases, the reset output voltage of the SGM803 will increase. This phenomenon is caused by the monitored voltage, the value of the pull-up resistor, and the voltage connected to the pull-up resistor. decided.
Figure 6 Multi-power supply system
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