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Power on reset and related monitoring functions

Abstract: This paper discusses the power on reset function and the selection strategy of threshold voltage for single power supply or dual power supply processors. In addition, it also discusses the functions of manual reset, power failure and low voltage detection. In this paper, the reasons for avoiding the use of discrete por and processor internal por are described, and the sequential power supply, voltage tracking and sequential reset are explained

One of the tasks of power on reset (POR) is to ensure that the processor starts running from a known address when the power is just turned on. For this reason, the por logic output locks the processor in the reset state when the processor power is just turned on. The second task of por is to prevent the processor from running from a known address until the following three things are completed: the system power supply has been stabilized at an appropriate level; The clock of the processor has been established; And the internal registers have been loaded correctly. Por completes the second task by means of on-chip timer, which continues to keep the processor in the reset state within a predetermined time interval. This timer is triggered after the power supply of the processor reaches the specified voltage threshold. After the set time is completed, the timer terminates and causes the por output to become invalid. The processor leaves the reset state and starts running (Figure 1). The data book of the processor will give the required timer delay interval. By the way, this timer is the difference between por and general voltage monitor. The latter can also monitor voltage with a certain voltage threshold, but it does not have timing function

Figure 1 The por keeps the processor in the reset state until the power supply voltage exceeds the por threshold and a specified time delay has elapsed

por's good anti-interference ability is also necessary when monitoring the processor, which is also the difference between it and the voltage monitor. When there is a small and fast interference on the power supply, the por should not issue a reset, because this interference will not affect the work of the processor. However, relatively long small interference and short or long large interference will cause problems to the processor. Therefore, the best way is to use a POR, which can monitor the amplitude and duration of the interference entering the power supply voltage at the same time, and decide whether to send a reset. The ultimate goal is to truly reflect the behavior of the processor itself, and only issue a reset when necessary, but it should not be reset when the processor is working normally. Figure 2 is a curve from the max6381/max6382 data book, which describes the interference amplitude/interval on the power supply voltage that can trigger reset. This curve shows that max6381/max6382 will trigger reset only after monitoring that the power supply voltage is lower than the specified threshold of 100mV for at least 10ms

Figure 2 Whether por generates reset is related to the amplitude and duration of interference

once the power supply voltage returns above the threshold, the por timer will cancel the reset signal only after a predetermined interval

some processors provide a bi-directional reset pin through which not only a reset signal can be received, but also a reset can be sent. It seems that a POR with open drain output can meet this condition. However, there are other problems, because the processor must determine whether it is itself or the reset sent by an external device. It is necessary to use a POR specially configured for this condition (see max6314 data manual)

determine the por threshold voltage - how to determine the correct por threshold level and the requirements for the accuracy of this level for single power processors are often not correctly understood. In order to make the designer have a clearer understanding of the details of this task, we take a processor as an example to illustrate this problem, assuming that the processor ensures the correct operation of 3.3V 0.3V power supply - more specifically, from 3.00v to 3.60v. When selecting the voltage threshold, the designer should follow one of the following two strategies

one of the strategies is to ensure that the 3.3V power supply has sufficient accuracy. For this purpose, you can choose a POR whose threshold plus tolerance is completely within the range of 0.3V. In this case, the por threshold is located between the low end of the power range (3%) and the low end of the allowable voltage range of the processor (Fig. 3a). Based on this strategy, the por will not issue a reset when the power supply voltage is within tolerance. However, when the power supply voltage drops below the tolerance and remains within the range within which the processor guarantees correct operation, the por will send a reset signal. This ensures that a reset is issued before the processor is misoperated (because the voltage drops below the guaranteed operating range)

Figure 3 When the power supply voltage is lower than the specified voltage range and higher than the bottom line of the allowable voltage range of the processor, in order to ensure the reset of the processor, the por threshold can be selected according to figure 3a. However, selecting a POR whose threshold voltage is lower than the allowable range of the processor (Fig. 3b) will not trigger reset as long as the power supply voltage is within this range, and a coarser tolerance power supply is allowed

according to this strategy, one of the appropriate por choices is a model in max6381, which has a threshold range of 3.00v to 3.15v over the entire temperature range (Figure 3a). With this por, once the power supply falls below its specified voltage range, the processor will reset, and at this time, the power supply has not fallen below the specified voltage range of the processor. In addition, since the upper limit of the threshold range is 3.15v, reset will not occur when the power supply is within its allowable range. However, when the power supply is connected to the processor, the voltage on the processor may drop below 3.15v due to the voltage drop on the connector and circuit board wiring. In this case, although the power supply voltage is still within the specified range, reset may still occur. At this time, it is necessary to choose a power supply with smaller tolerance, a POR threshold with smaller tolerance, or both

this design method is more sensitive to interference or noise on the power supply, because the power supply voltage may be very close to the por threshold (depending on the position where the por threshold and the power supply voltage are within their tolerance range respectively). Therefore, this method is suitable for systems with small interference and noise and small power tolerance

some designers will adopt the second different strategy when choosing the por threshold. They use por whose threshold is lower than the guaranteed working voltage of the processor (3.00v in this example). This allows the processor to operate at any voltage within the allowable range without encountering a reset. It also allows for looser power tolerances. These designers easily assume that during power on, the power supply will continuously rise above the por threshold and stabilize at the voltage within the specified range (3.20 in this example) and improve the surface finish and hardness V to 3.40v. And it is expected that these will happen long before the por timer is far from full. Many times, designers use the power ok signal provided by some power supplies to determine whether the power supply works within the specified range

these designers did not consider the influence of electrical undervoltage. In case of power undervoltage, the processor may operate under a power supply lower than its minimum guaranteed working voltage, but it is still above the por threshold for the time being (if it is lower than it, the por will send a reset). When operating within such a power supply voltage range, the processor may operate incorrectly

different from the threshold selected within the allowable power supply voltage range of the processor, the second method is more suitable for those systems that may have large interference and noise. Because the por threshold and power supply voltage are far apart. As mentioned earlier, this approach also allows for wider power tolerances. Max6381 models with a threshold range of 2.85V to 3.0V over the entire temperature range can be used in this design because the threshold is lower than the bottom line of the processor's allowable voltage range (Figure 3B). At this time, a power supply with a wider tolerance than that in Figure 3 can also be used

sometimes, the designer will set the rated voltage of the power supply near the bottom line of the allowable range of the processor in order to reduce power consumption. This is effective because the power consumption is proportional to the square of the supply voltage. Assuming that the allowable voltage range of the processor is 3.0V to 3.6V, a 3.15v 2% power supply is desirable if there is no significant voltage drop on the connector and wire on the path connecting the power supply to the processor. If the noise level is low enough not to cause false triggering, max6381 por with threshold voltage in the range of 2.85V to 3.0V is a suitable choice

determine the threshold voltage of por - dual power supply processor

in addition to the 3.3V power supply, if the processor needs another power supply (such as a 1.8V nuclear power supply), this design may need por that can monitor two circuits of voltage. This type of por will not be reset until both power supplies exceed the two corresponding thresholds of the por and the specified delay period has passed. Pors that can monitor two, three and four voltages at the same time can be found

the same selection method is applicable to the monitoring of multiple power supplies or single power supplies. For the case of dual power supplies (such as 3.3V and 1.8V), the designer can choose that both thresholds of por are higher or lower than the minimum guaranteed working voltage of the processor. Similarly, the designer can also make the threshold of monitoring 3.3V i/o power supply lower than the guaranteed working voltage, while the other threshold used for 1.8V nuclear power supply is above the guaranteed working voltage. Many designers prefer the latter strategy, because many times the processor core is more sensitive to the problems caused by low power supply voltage than i/o

the core power supply voltage always decreases with the passage of time, so it is necessary to reduce the por threshold voltage. The devices in max6736 series can provide a threshold as low as 788mv without external resistance, and can also be as low as 488mv with external resistance. This threshold voltage is sufficient to monitor the most advanced core power supply

for low-cost systems, many circuit designers choose to monitor only 3.3V power supply, if 1.8V power supply is obtained from it. They believe that if the 3.3V power supply reaches the normal voltage, the 1.8V power supply will also. For systems that require high reliability, designers usually choose to monitor two power supplies

sometimes, when the power supply voltage is still within the tolerance, it is also useful to trigger a reset manually. This function is not only used for debugging and final testing, but also useful when the processor is locked -- it enables the processor to restart without turning off the power. This feature is particularly useful for products where the processor never loses power. It is also commonly used in on/off switches that do not turn off the power of the processor but wake up/suspend the processor

although the logic signal from the i/o line, watchdog timer or power failure output are often used to trigger manual reset, button switches are often used to trigger manual reset. When pressed, this type of switch will usually rebound, and the percentage of open and close inflection points to identify compliance conditions can be stabilized by many times. Therefore, most manual reset inputs contain de jitter circuits, which do not respond to the ringing caused by the button switch

discrete por and processor built-in por

it is a dangerous practice to use a discrete por composed of resistors and capacitors (FIG. 4A). This slow rise and fall time of por output will cause problems for many processors -- especially those processors that do not include Schmidt trigger in reset input and have bidirectional reset pin. Adding a Schmidt trigger is effective for the former case, but it also brings cost and space

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