Reliability Design of Single Chip Microcomputer Power Supply

The factors that affect the stability of the microcontroller 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 spatial electromagnetic fields to the conductors (leads or component pins) inside the machine to induce corresponding interference, which can be attenuated by electromagnetic shielding and reasonable wiring/device layout; interference 
  
generated by the power cord or power supply, which is coupled or directly conducted through the power cord or 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 adjusted by circuit parameters. The stability is affected by parameters such as oscillator type, temperature and voltage. The reliability of the reset circuit. 

2. Reliability Design of Reset Circuit

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

Figure 1 RC reset circuit

The reset circuit shown in Figure 2 adds a diode to discharge the capacitor quickly when the power supply voltage drops instantly, and 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 additional discharge loop

The use of a comparison circuit can not only solve the problem of system instability caused by power glitches, but also 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 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 the 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 with voltage monitoring function
 

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Figure 5 Stable threshold voltage
 

Figure 6 Practical reset monitoring circuit

On this basis, adding a delay capacitor and a discharge diode constitutes 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 a reset circuit with voltage monitoring function

2. Power monitoring circuit

The reset circuit with voltage monitoring mentioned above is also called the 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, to reset the system when the power fails or the voltage drops below a certain voltage value; 
  
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 optional threshold voltages. They can ensure that the system can be reliably reset under different abnormal conditions to prevent the system from getting out of control. Rm and Sm in Figure 8 realize manual reset. When 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 to the maximum extent. You can also choose the MAX708 product 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 to before reset. Figure 9 Application example of electric meter Using MAX708 electric meter can save the current electricity to E2PROM before power glitch or power outage, and then cooperate with the backup algorithm to save multiple electricity, which can effectively solve the problem of electricity loss in E2PROM that engineers are worried about. When using this circuit, the appropriate warning voltage point must be selected to ensure that the maintenance time (tB) of 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, /PFO can be detected to confirm whether the internal voltage comparator is working. When adjusting, it must be noted that this comparator is a window comparator. Figure 10 is a flowchart of the application 

Figure 9 Typical application of MAX708

Figure 10. Flowchart of E2PROM data protection procedure 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 data protection is performed, such as write protection, data backup or switching backup battery; [page]
  
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 of 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 data of SRAM in microcomputer system when the power supply voltage is reduced or the power is cut off. When the power supply voltage drops to the normal value of 4.2V, the output CS becomes a logic low level, and CE is also pulled low, thereby prohibiting the operation of 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 normal value of 3.3V or lower, 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 SRAM will be switched from the backup lithium battery back to the main power supply. When the main power supply rises to 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 SRAM. The reset signal continues until the system resumes normal operation. When the system power supply voltage is insufficient or the power is suddenly cut off, this device can reliably protect the system data in SRAM.
 

Figure 12. Typical application of the SA56600-42 monitoring device 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, which is a challenge to the limit of digital circuits, and 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. The large board area, too many and too long pins are prone to introduce radio frequency interference, and the 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 costs 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 circuits may lead to more losses than gains. Experience also tells us that using dedicated monitoring circuits can avoid many strange problems. ARM application engineers, remember to avoid detours! 
 

Figure 13. ARM reset circuit implemented with PHILIPS MAX708

Figure 13 is a practical and reliable ARM reset circuit. The operating voltage of the ARM core is relatively low. R1 can ensure that the voltage is lower than the operating power supply of MAX708 and can still be reset reliably. The TRST signal is used for the JTAG interface. Using HC125, multiple reset sources can be used to reset ARM, such as resetting ARM through the PC serial port or JTAG interface.