Analysis of Reset System Scheme of MSP430 Full Range Microcontroller

0 Preface

TI's mixed-signal processor MSP430 series single-chip microcomputer has the characteristics of powerful processing capacity, high degree of peripheral device integration, low power consumption, complete product series, and a full range of industrial grades. As one of the current MCU mainstream products, it is used in electronic applications. Widely used in the field, it is favored by more and more electronic designers. The failure of the system due to reset circuit design problems or the incorrect state of the system after power-on is a problem encountered by many circuit designers of MSP430 microcontrollers in designing, debugging, and applications, although the probability of this occurring is very low. However, for applications with high reliability requirements, this phenomenon still needs to attract enough attention from electronic designers.

For this reason, this article has carried on the detailed and in-depth analysis to the reset system and reset mechanism of the MSP430 full range of single-chip microcomputers, and specifically proposed a specific peripheral reset circuit design scheme and related electronic components detailed introduction, for the peer reference and communication .

1 MSP430 reset mechanism 1.1 MSP430 reset circuit The MSP430 reset circuit includes a power-on reset (POR) and power-on-clear signal (PUC). POR is the device reset signal, which is usually triggered when the following three events occur: a. Power on; b. The RST/NMI pin appears low in reset mode; c. Voltage monitoring device (Brownout) triggers.

When the supply voltage VCC rises slowly, the POR monitor keeps the POR signal active until VCC exceeds the VPOR level. When the supply voltage VCC rises rapidly, the POR delay (POR DELAY) provides a long enough effective POR signal to ensure that the MSP430 is sufficient. Time for initialization.

1.2 Brownout Circuit The Brownout circuit is an under voltage reset circuit. It replaces the POR detection and POR delay circuits. The brownout circuit can detect a lower supply voltage during power-up or power-down and can reset the chip by triggering the POR signal during power-up or power-down. Brownout circuit reset timing. When VCC exceeds the starting voltage VCC (star-t), the POR signal is valid, and the POR signal remains valid until the VCC voltage value is higher than the threshold voltage V (B_IT+) and a time delay t(BOR) elapses; the hysteresis voltage Vhys (B_IT- The Brownout circuit will generate the next POR signal to ensure that the supply voltage must be less than V (B_IT-).

　 1.3 Analysis of reset failure process If there is no Brownout reset circuit, if the power supply of the MSP430 is periodic, when the power is turned on again, the supply voltage VCC must be lower than Vmin during the falling period of the previous cycle or when it is disturbed. To ensure the occurrence of the POR signal, if VCC cannot be lower than Vmin, no POR signal will be generated. At this time, even if a low-level signal appears at RST/NMI, a valid POR signal will not be generated and the system cannot be reset normally.

Brownout reset circuit exists in some MSP430 series chips, such as MSP430F13X and MSP430F14X series, there is no Brownout circuit. For a chip application system without a Brownout circuit, during operation, due to interference, power grid fluctuations, misoperations, etc., a brief voltage drop causes power supply recovery when the voltage does not meet the POR occurrence condition (below Vmin), and the reset terminal is low. The level reset signal cannot restart the system to reset the work again. At this time, the system crashes. Although this phenomenon is not frequent, it is fatal for a remote automatic control system that cannot be manually reset at any time.

Carefully review the product's technical manual before proceeding with circuit design, and design different reset circuits for different application conditions of each series of chips.

2 reset circuit design method 2.1 increase the reset threshold to increase the MSP430's reset threshold Vpor ", so that Vpor" in the MSP430 normal operating voltage range, and close to the MCU's normal operation of the minimum threshold Vcc min, at this time can guarantee the supply voltage is at Vcc When near the min and the MCU can still work normally, send a reset signal to the MSP430 at the threshold. The value of Vpor at this time should be between the lower limit of power supply (Vp min) and the lower limit of normal operating voltage of MSP430 (Vcc min). As shown in Figure 3.

MSP430F149 is one of the most widely used MSP430 chips. Its operating voltage range is between Vcc min=1.8V and Vcc max=3.6V. When low-dropout linear regulator TP-S76033 is selected, its power supply voltage Vp max= 3.34V, Vp min = 3.23V, the reset threshold range should be between 3.23V and 1.8V. In order to improve the system's reliable working conditions, Vpor chose to be as low as possible. Select the reset chip MAX809S with a reset threshold of 2.89V (Vpor min)

This method is applicable when the power supply system has a small tolerance and the supply voltage is highly accurate.

2.2 external reset signal to extend the time <br> <br> due MSP430 internal POR only provide a reset on power-up, for a brief encounter to fluctuations in supply voltage drops Vpor less but still higher than the case of Vcc-min and rapid recovery of the The second voltage drop shown), the above method can not trigger an effective reset, at this time, you can take the method of extending the reset signal duration or increasing the reset period. Allowing the reset signal to maintain a sufficiently long period after the voltage value is restored can still cause the chip to reset normally. MAX809S can maintain a valid reset signal of 140ms at most, which is much larger than the sum of MSP430F149's effective reset signal request time (25μs) and POR signal sustain time (250μs). When the system encounters the voltage fluctuation within 120ms, the system can ensure the correct reset. .

By using the MAX803 reset chip with a watchdog timer, it is also possible to control the transmission interval of the reset signal for transmission of up to 1s, which is suitable for use in a voltage fluctuation situation with a longer period.

　 2.3 Monitor the power supply The tolerance range of the power supply system is large (when the range of medium Vp is greater than or close to the range of VCC), the accuracy of the pressure value is low, or when the power grid is under long-term operation in the under-voltage state, Lowering the reset voltage threshold will cause the system to frequently reset under normal operating conditions. At this time, the power supply voltage can be monitored. When the above voltage fluctuation is detected, the monitoring chip sends an abnormal voltage signal to the MSP430. The MSP430 responds to the signal and interrupts the running program, enters the power-failure protection subroutine, and sets the reset status register to avoid the next time. The POR reset cannot be started at power-on due to a bad register status.

The MAX6342 is a reset chip with an internal power-fail comparator. The comparator feeds unstable voltages to the regulator and generates a power supply for the processor and monitoring circuitry because unstable voltages fall before the regulator output voltage. A power failure signal (PFO) is generated after an unstable or lower voltage is detected, and the MSP430 enters the power-down process before being reset.

3 summarizes <br> <br> for different (Brownout circuit) internal reset mechanism MSP430 MCU sub-series, face a different system working environment, by improving the design reset circuit reset threshold, reset time and extend the source voltage monitoring methods, can Effectively avoid the accidental reset failure problems encountered in practical applications, improve system reliability. The practice shows that the above method can effectively reduce the occurrence of occasional reset failure.