Application of μp monitoring chip in single chip microcomputer

Single chip microcomputer STM32L151CCU6

0201 package SLP0603 ESD electrostatic diode 5V one-way ESD

Supply SMD LED Light Emitting Diode SMD LED LED Beads 0603, 080

1206RGB (single)

MCUs are mostly used in industrial automation systems. Industrial automation systems are not allowed to "crash", otherwise accidents or major losses may occur. In the running process of the single-chip microcomputer, due to interference from the power supply and the surrounding environment, the program often runs "misplaced" and enters an infinite loop. At this time, the microcontroller must be reset by the off-chip hardware circuit, and then restarted to enter the normal state. This type of hardware circuit is often referred to as a "watchdog" circuit. In the early years, there were no such special circuits. Most of them were timers composed of some gate circuits or 555 circuits to perform this function. Not only the circuit was complicated, but also the performance was not satisfactory. In recent years, such dedicated series of chips have been produced, and they also have a variety of functions, including a watchdog, power threshold, voltage monitoring, and reliability reset. Small in size and cost-effective, these chips are collectively referred to as μp monitoring chips.

The above table shows the performance of some μp monitoring chips, and the corresponding package form is shown in the figure below. They are all 8-pin dual-column type, with in-line and patch-type packages, the same pin and the same performance.

The power threshold in the table indicates that when the power supply voltage is lower than this value, the monitoring chip will cause the microcontroller to reset and stop working. When the power supply voltage rises above the threshold voltage, the microcontroller can be restarted to enter the normal working state. Because the power supply voltage of the single chip microcomputer and the peripheral chip is too low, the system will be unstable, and this function of the chip provides an additional layer of protection for the normal operation of the system. The reset pulse width means that the reset pulse generated by the MCU under any circumstances must have a certain width (about 200ms) reset pulse to ensure reliable reset of the MCU, which is difficult to achieve in general circuits. The reset state refers to whether the reset pulse appears at a low level or a high level to accommodate different microcontrollers. The watchdog time refers to the maximum time of the microcontroller's dead loop. After this time, the chip will reset the microcontroller and restart it to enter the normal state. Voltage monitoring means that when the monitored voltage drops to a certain value (adjustable), it will cause the PFO of the chip to output a low level and cause the CPU to generate an external interrupt. The microcontroller executes a specified interrupt routine for some Data and variable implementation protection.

Application example of μp monitoring chip

The figure above shows an example of the application of the bucket p monitor chip to the MCS-51 series. Because the CPU adopts 89C51 in the 51 series, the reset pulse requires a high level. As can be seen from the table, the IMP813L monitoring chip should be selected (if the other two are used, an inverter must be added at the 7th pin of the chip). Since the watchdog time is 1.6s, it is necessary to change the WDI pin (pin 6) of the chip to a level within 1.6s (from high to low or from low to high, implemented by the P3.4 pin of 89C51) , commonly known as "feeding the dog" once. When the program runs indiscriminately, the watchdog timer overflows due to the inability to "feed the dog" on time. The reverse of WDO becomes low level, and the MR is also changed by diode D1 (which can also be omitted by D1). Low level, the watchdog timer is cleared, and the reset output RST outputs a positive pulse (200ms) to reliably reset the 89C51 and restart to enter normal operation. It can be seen that when programming in 89C51, be sure to "feed the dog" once in 1.6s. Press button A to reset manually. The power threshold of the IMP813L is 4.65V. When VCC is lower than this voltage, RST outputs a high level, and 89C51 stops working until the voltage rises above 4.65V. RST still keeps 200ms high and then falls, so that the MCU can be reset reliably and enter normal working state. The PFI is the voltage monitoring input with a monitored value of 1.25V. Above this value, the inverse of the output pin PFO is high; when it is lower than this value, it becomes low level, triggers the 89C51 external interrupt INTO, executes the interrupt subroutine, and stores some important data and variable values ​​into An off-chip RAM chip with a spare battery (not shown). The input voltage of PFI is divided by R1 and R2. When the power supply is normal, the DC voltage of the input terminal of the voltage regulator 7805 is about +10V. When the power is off, the voltage of the chip drops rapidly. If we ask to start protection when it drops to 7.5V, we can calculate Rl: R2 = (7.5-1.25); 1.25 = 5:1. If R2 takes 2kΩ, then Rl takes 10kΩ. Although the power supply voltage drops at a fast speed, the single-chip microcomputer runs faster, so in a short period of time, the microcontroller can still perform certain tasks.

The figure above shows that the μP monitor chip is applied to the MCS-96 series, and the CPU uses 80C196KB. Because the reset pulse RST of this series chip is low, the IMP705 or IMP706 chip should be selected. The MCS-96 series chip itself comes with a watchdog, but the watchdog time is very short (10~20ms), usually not used. But sometimes it is used at the same time as the outside watchdog for the sake of insurance. The working principle and process are the same as the 51 series, and will not be described here.