Microprocessors are usually designed to operate at a nominal voltage of, for example, about 5 volts. They may operate satisfactorily at voltages of less than 5 volts, as long as the supply voltage exceeds a threshold voltage on the order of 4.5 volts. In particular, below the threshold voltage compatibility problems occur between the microprocessor and the peripheral devices connected to it. Peripheral devices are equipment such as serial/parallel interfaces, printers, display units or the A/D, D/A (Analog/Digital, Digital/Analog) converters which are associated with microprocessors.
When the power supply voltage returns to its nominal value after dropping below the threshold voltage, the microprocessor must carry out an internal reconfiguration which corresponds to placing it into an initialization state before placing it back into communication with its peripheral equipment. This return to an initialization state is also called "reset".
A "reset" type power supply voltage monitoring device outputs information indicating that the power supply voltage has changed and gone below or above the threshold voltage. This information may be supplied to a monitoring system external to the microprocessor, or to the microprocessor itself. Such a reset type power supply monitoring circuit may be used in equipment or machines using one or several microprocessors, for example, such as microcomputers.
FIG. 1 schematically shows a known type of power supply voltage monitoring device. The device is based primarily upon a comparator 10. A resistance bridge 12, connected between a power supply terminal 14 at the power supply voltage V.sub.CC, and a ground terminal 16, applies a voltage denoted V.sub.A proportional to the power supply voltage V.sub.CC to a first input (inverting) 18 of the comparator. A second (non-inverting) input 20 of the comparator 10 is connected to a reference voltage generator 22 shown schematically.
Note that the reference voltage generator, also connected between the power supply terminal 14 and the ground terminal 16 is capable of outputting a constant voltage with value, denoted V.sub.GAP, on the order of 1.2 volts, and is practically unaffected by ambient temperature variations.
A comparator output 24 is connected to the gate of a MOS (Metal Oxide Semiconductor) type output transistor 26. The output transistor 26 is connected in series with a load resistor 28 between the power supply terminals 14 and the ground terminal 16. The load resistor 28 connects the transistor drain to the power supply terminal 14 at the voltage V.sub.CC.
The output terminal of the monitoring device located between the output transistor drain and the load resistor is identified by reference 30. This output terminal, the voltage of which is denoted V.sub.reset is in a high state (V.sub.reset .apprxeq.V.sub.CC) when the power supply voltage V.sub.CC exceeds a voltage threshold denoted V.sub.th and changes to a low state (V.sub.reset .apprxeq.0V) whenever the power supply voltage V.sub.CC becomes less than the threshold voltage V.sub.th.
The threshold voltage may be adjusted by modifying the value of the reference voltage output by the reference voltage generator 22 and/or by adjusting the values of the resistors forming the resistance bridge 12 to modify the voltage V.sub.A. Preferably, the threshold voltage V.sub.th is adjusted so that it is approximately equal to the operating threshold voltage of a microprocessor with which the monitoring device is used.
The static operation of the device is explained with reference to FIGS. 2 to 4. FIG. 2 is a reference graph showing the power supply voltage expressed in volts on the abscissa and on the ordinate. The voltage increases very slowly from a value of 0 volts to a value of V.sub.CC =5 volts.
FIG. 3 is a graph expressing the output voltage V.sub.reset of the output terminal 30 (the ordinate) as a function of the power supply voltage(the abscissa). Finally, FIG. 4 is a graph showing the value of the voltage V.sub.A of the resistance bridge and the voltage V.sub.GAP of the reference voltage generator, as a function of the power supply voltage V.sub.CC.
FIG. 3 shows that the output voltage V.sub.reset on the output terminal increases linearly with the power supply voltage and is approximately equal to the power supply voltage up to V.sub.CC =V.sub.reset .apprxeq.about 0.9 V.
When the power supply voltage is less than about 0.9, the output transistor switching threshold is not reached. The transistor is thus blocked and the output voltage "follows" the power supply voltage. This phenomenon, marked by the letter A in FIG. 3, is not harmful to the extent that an output voltage of less than 1 volt is considered to be a low state of the monitoring device. Furthermore, FIG. 4 shows that the voltage V.sub.GAP of the reference voltage generator reaches a stable value on the order of 1.2 volts starting from a power supply voltage V.sub.CC greater than or equal to about 1.5 volts.
The voltage V.sub.A output by the resistance bridge 12 is proportional to the power supply voltage V.sub.CC. When the voltage V.sub.A is less than the reference voltage V.sub.GAP, the comparator output is in a high state and the output transistor is conducting. Thus, the output voltage V.sub.reset is approximately equal to the voltage of the ground terminal 16, in this case 0 volts. Then starting from a power supply voltage V.sub.CC of 4.5 volts, which in this case is the threshold voltage, the voltage V.sub.A becomes greater than the reference voltage V.sub.GAP.
The comparator output 24 thus changes to a low state and blocks the output transistor which thus behaves practically like an open switch. The output voltage then changes rapidly to a high state, such that V.sub.reset =V.sub.CC. This phenomenon is shown by the letter B in FIG. 3.
We will now describe FIGS. 5, 6 and 7 which illustrate the dynamic behavior of the device in FIG. 1. FIG. 5 shows the value of the power supply voltage V.sub.CC on the ordinate, as a function of the time shown on the abscissa and expressed in units of 10.sup.-4 sec. Time is counted starting from when the device is switched on. It can be seen that the power supply voltage increases from 0 volts to a voltage close to 5 volts during a period of 100 .mu.sec, and then remains at this value.
FIG. 6 shows the behavior of the output voltage V.sub.reset, as a function of time. The voltage is shown on the ordinate and is expressed in volts. The time shown on the abscissa starts when power is switched on and is expressed in 10.sup.-4 sec. Initially, and until values of the power supply voltage reach the order of 1 volt, the output voltage follows the power supply voltage. This phenomenon, shown by the letter A, is explained above and is not discussed again.
Starting from 190 .mu.sec, the output voltage V.sub.reset changes to the high state (about 5 volts) (reference letter B). Thus there is a delay of about 90 .mu.sec between the power supply voltage and the response of the device going into the high state. This delay is not harmful to correct operation of the power supply voltage monitoring device, since it delays the supplied information very little.
However another phenomenon, marked with letter C in FIG. 6, is a particular nuisance. When the power supply voltage V.sub.CC has reached its nominal value of 5 volts, the device output voltage changes to the high state for a brief period, 10 .mu.sec, and then returns to its low state, before finally switching to the high state (at B).
This transient phenomenon is a nuisance especially to the extent that it introduces a doubt about the true state of the power supply voltage. The explanation of this phenomenon is given with reference to FIG. 7.
FIG. 7 is a graph which shows the voltage V.sub.A of the resistance bridge on the ordinate, and the voltage V.sub.GAP of the reference voltage generator as a function of time, on the abscissa. The scales are identical to the scales in the previous Figures.
It is found that the voltage V.sub.A output by the resistance bridge varies proportionally with the power supply voltage. However, the voltage V.sub.GAP increases with a delay after the power supply voltage. This delay is shown as reference D. Then, after the delay, the voltage V.sub.GAP increases rapidly and exceeds the voltage V.sub.A. This phenomenon, marked by reference E, represents an over-oscillation related to a transient condition when the constant voltage generator starts operating. The over-oscillation lasts for a few tens of microseconds.
By comparing FIGS. 6 and 7, the following operation described as a function of time is observed starting from the instant zero when power is switched on. Until a time of 100 .mu.sec, the power supply voltage is too low and the comparator 10 (FIG. 1) does not work. The output 30 is in a low state. Starting from a time of 105 .mu.sec, the comparator 10 operates and it is observed that V.sub.A &gt;V.sub.GAP. Its output changes to a low state and the output terminal 30 of the device changes to a high state V.sub.reset .apprxeq.V.sub.CC.
Then during the over-oscillation (115 .mu.sec), the voltage V.sub.GAP increases until it becomes greater than V.sub.A. The output of the comparator 10 changes to a high state and the device output terminal 30 changes to a low state V.sub.reset .apprxeq.0V. This low state of the output terminal does not represent the state of the power supply voltage which has reached its nominal value.
Finally, after a time of 190 .mu.sec, the over-oscillation is finished and the voltage V.sub.A is once again (slightly) greater than V.sub.GAP. The output terminal 30 is then in a high state. It appears that the over-oscillation of the constant voltage generator under dynamic conditions, and its operating delay, are essentially the cause of incorrect behavior of the power supply voltage monitoring device.