Reliability Design of Single Chip Microcomputer Reset Circuit and Classic Practical Reset Circuit

The factors that affect the stability of the operation of the single-chip system can be roughly divided into two parts: external factors and internal factors:  
1. External 
  radio frequency interference, which is transmitted in the form of electromagnetic field in the space. The conductor (lead or part pin) inside the machine induces corresponding interference, which can be attenuated by electromagnetic shielding and reasonable wiring/device layout;  
  interference generated by the power line or the power supply, which is coupled or directly conducted by the power line or the components inside the power supply, can be attenuated by power supply filtering, isolation and other measures.
2. 
  The stability of the internal oscillation source is mainly determined by the start-up time, frequency stability and duty cycle stability. The start-up time can be set by the circuit parameters. The stability is affected by the oscillator type, temperature and voltage parameters. The reliability of the reset circuit.  

2. Reliability design of the reset circuit 

1. Basic reset circuit 
     
The basic function of the reset circuit is: provide a reset signal when the system is powered on, and cancel the reset signal until the system power supply is stable. For reliability, the reset signal must be canceled after a certain delay after the power supply is stable to prevent the
reset from being affected by the jitter caused by the power switch or the power plug during the separation and combination process. The RC reset circuit shown in Figure 1 can achieve the above basic functions, and Figure 3 shows its input-output characteristics. However, it cannot solve
the problems of power burrs (point A) and slow power decline (battery voltage is insufficient). Moreover, adjusting the RC constant to change the delay will make the driving ability worse. The circuit on the left is high-level reset valid. The circuit on the right is low-level.
Sm is a manual reset switch. Ch can avoid the interference of high-frequency harmonics on the circuit.  

 
Figure 1 RC reset 

    circuit The   reset circuit shown in Figure 2 adds a diode, which allows the capacitor to discharge quickly when the power supply voltage drops instantly. A power glitch of a certain width can also reset the system reliably. The lower half of the input-output characteristic diagram of the reset circuit shown in Figure 3 is its characteristics, which can be compared with the upper half to show the effect of adding a discharge circuit.

 
Figure 2 RC reset circuit with added discharge loop 

Using
a comparison circuit can not only solve the problem of system instability caused by power glitch, but also can reliably reset the system when the power supply drops slowly. Figure 4 is an example. When VCC x (R1/(R1+R2) ) =
0.7V, Q1 is cut off to reset the system. The amplification effect of Q1 can also improve the load characteristics of the circuit, but the transition threshold voltage Vt is affected by VCC
, which is a prominent disadvantage of this circuit. Using a voltage regulator diode can make Vt basically unaffected by VCC. See Figure 5. When VCC is lower than Vt (Vz+0.7V), the circuit resets the system.  

 
Figure 3 RC reset circuit input-output characteristics  Figure 4 Reset  circuit  5 Stable threshold voltage Figure 6 Practical reset monitoring circuit  On this basis, add delay capacitor and discharge diode to form a reset circuit with excellent performance, as shown in Figure 6. Adjusting C1 can adjust the delay time, and adjusting R1 can adjust the load characteristics, as shown in Figure 7. The upper part is the characteristics of the circuit in Figure 5, and the lower part corresponds to Figure 6. Figure 7 Input-output characteristics of reset circuit with voltage monitoring function  2. Power supply monitoring circuit  The above reset circuit with voltage monitoring is also called power supply monitoring circuit. The monitoring circuit must have the following functions:     power-on reset, to ensure that the system can be started correctly when powered on;   power-off reset, when the power fails or the voltage drops below a certain voltage value, reset the system;   there are similar integrated products on the market, such as MAX809 and MAX810 produced by PHILIPS Semiconductor. Such products are small in size, low in power consumption, and have selectable threshold voltages. It can ensure that the system can be reliably reset under different abnormal conditions to prevent the system from losing control. Rm and Sm in Figure 8 implement manual reset. If this function is not needed, the Reset terminal (or /Reset) terminal can be directly connected to the RST terminal (or /RST terminal) of the microcontroller to simplify the peripheral circuit as much as possible. You can also choose the MAX708 with manual reset function from Philips Semiconductor.  
 

 

 



 

 
Figure 8 Integrated reset monitoring circuit 

  
In addition, MAX708 can also monitor the second power supply signal to provide the processor with a voltage drop warning function. With this function, the system can perform certain safety operations, save parameters, send
alarm signals, or switch backup batteries when the power drops before resetting. Figure 9 Application example of electric meter Using MAX708 electric meter, the current electricity can be saved to E2PROM before power glitch or power outage.
Combined with the backup algorithm for saving multiple electricity, it can effectively solve the problem of electricity loss in E2PROM that troubles engineers. When using this circuit, the appropriate warning voltage point must be selected to ensure that the maintenance time
(tB) of the VCC voltage from the warning voltage to the reset voltage must be long enough when the power supply is powered by the energy storage of the power supply. The write cycle of E2PROM is about 10-20ms.
Generally, tB>200ms can ensure stable data writing. Warning voltage adjustment method When VDC is equal to the warning voltage, adjust R1 and R2 to make the voltage of PFI 1.25V.
At this time, you can detect /PFO to confirm whether the internal voltage comparator is working. When adjusting, you must pay attention to the fact that this comparator is a window comparator. Figure 10 is the program flow chart of this application  

 
Figure 9 Typical application of MAX708

 
Figure 10. Flowchart of E2PROM data protection program in electric meter application 

3. Multifunctional power supply monitoring circuit 
  In addition to power-on reset and power-off reset, many monitoring circuits integrate the functions required by the system, such as:   
  power supply measurement and control, providing early warning indication or interrupt request signal when the power supply voltage is abnormal, so as to facilitate the system to implement abnormal processing;  
  data protection, when the power supply or system is working abnormally, necessary protection is performed on the data, such as write protection, data backup or switching backup battery;  
  watchdog timer, when the system program "runs away" or "deadlocks", the system is reset;  
  other functions, such as temperature measurement and control, short circuit test, etc.  

    We call it a multifunctional power supply monitoring circuit. The following are two multifunctional monitoring circuits that are particularly suitable for widespread use in industrial control, security, and financial industries:
   
Catalyst's CAT1161 is a 16K-bit E2PROM (I2C interface) that integrates watchdog, voltage monitoring, and reset circuits
. It is not only highly integrated and low power (zero power consumption is truly achieved when the E2PROM part is static), but also the watchdog is realized by changing the SDA level, saving system I/O
resources. Its threshold voltage can be modified by the programmer, and the modification range covers most applications. When the power drops below the threshold voltage, the hardware prohibits access to the E2PROM to ensure data security. When  
  
using it, please note that the RST and /RST pins are I/O pins. When CAT1161 detects that any voltage on the two pins is abnormal, it will generate a reset signal. The
pull-down resistor R2 and the pull-up resistor R1 connected to the RST /RST pins must be connected at the same time, otherwise the CAT1161 will continue to generate resets! Similarly, when the manual reset function is not required, the two components Rm and Sm can be saved.

 
Figure 11. Built-in WDT RESET /RESET E PROM monitoring device interface circuit 

      
PHILIPS SA56600-42 is designed to protect the SRAM data in the microcomputer system when the power supply voltage is reduced or the power is off. When the power supply voltage drops to the typical value of
4.2V, the output CS becomes a logic low level, and CE is also pulled low, thereby disabling
the operation of the SRAM. At the same time, a low-level effective reset signal is generated for the system to use. If the power supply voltage continues to drop and reaches the typical value of
3.3V or lower, the SA56600-42 switches the system operation from the main power supply to the backup lithium battery power supply. When the main power supply returns to normal (the voltage rises to 3.3V or higher),
the power supply of the SRAM will be switched from the backup lithium battery back to the main power supply. When the main power supply rises to a value greater than the typical value of 4.2V, the output CS becomes a logic high level, which makes CE become a high level, enabling
the operation of the SRAM. The reset signal continues until the system returns to normal operation. When the system power supply voltage is insufficient or suddenly powered off, this device can reliably protect the system data in the SRAM.   Figure 12. Typical application of the monitoring device SA56600-42 with built-in SRAM data protection circuit
 

4. ARM MCU reset circuit design 
    Whether in mobile phones, high-end handheld instruments or embedded systems, 32
-bit MCU ARM occupies an increasing share, and ARM has become the de facto industrial standard for high-end products. Due to ARM's high speed, low power consumption and low operating voltage, its noise tolerance is low.
This is a challenge to the limits of digital circuits, and it also puts forward higher requirements for power supply ripple, transient response performance, clock source stability, power supply monitoring reliability and many other aspects. ARM monitoring technology is complex and very important
.
    The monitoring circuit implemented by discrete components is greatly affected by external factors such as temperature, humidity, pressure, etc., and the impact on different components is inconsistent.
Large board area, too many and too long pins are prone to introduce radio frequency interference, and high power consumption is also unacceptable for many applications. Integrated circuits can solve such problems very well. At present, there are also many microprocessors with integrated monitoring circuits. Due to
manufacturing cost and process technology reasons, most of these monitoring circuits are implemented using low-voltage CMOS technology, which is still a gap compared to the performance of dedicated monitoring circuits manufactured using high-voltage and high-linearity bipolar processes.
Conclusion: Using ARM without dedicated monitoring circuit may lead to more losses than gains. Experience also tells us that using dedicated monitoring circuit can avoid many strange problems. ARM application engineers, remember to avoid detours!   Figure 13. ARM reset  circuit 13 is a practical and reliable ARM reset circuit. The operating voltage of ARM core is low. R1 can ensure that the voltage is lower than the working power supply of MAX708 and can still be reset reliably. The TRST signal is for JTAG interface. Using HC125, multiple reset sources can be used to reset ARM, such as resetting ARM through PC serial port or JTAG interface.