ITTO20080528A1 - WATCHDOG CIRCUITS PARTICULARLY FOR USE IN A HOUSEHOLD APPLIANCE - Google Patents

Description

â € œWATCHDOG CIRCUIT PARTICULARLY FOR USE IN A HOME APPLIANCEâ €

DESCRIPTION

The present invention refers to a watchdog circuit according to the preamble of claim 1.

As is known, watchdog circuits are used to monitor the operating status of a microcontroller or, more generally, of a microprocessor and allow it to be restarted in the event of a malfunction.

During normal operation, the microprocessor cyclically transmits to the watchdog a test signal with a predetermined frequency higher than a threshold frequency.

The watchdog circuit is basically a timer that restarts each time it receives a test signal; if this does not arrive within a predetermined time (corresponding to the inverse of the threshold frequency), then the watchdog generates a reset signal which restarts the microprocessor.

The best known and most sold watchdogs on the market are typically integrated circuits made using logic gates.

However, these solutions are very expensive and very sensitive to electromagnetic noises; therefore they are not particularly suitable for applications inside household appliances, such as a washing machine, equipped with components that produce disturbances capable of interfering with the operation of integrated digital circuits.

It is also known that cheaper solutions can be realized using discrete circuits, as for example described in US patent 5,704,038.

The watchdog known from this patent substantially provides a monitoring module and a reset module.

The monitoring module basically consists of a transistor which discharges a capacitor when it receives a test signal from the monitored microprocessor. If, due to a malfunction, the latter does not generate the test signal, the capacitor charges until the reset module is activated, which then generates a pulse on the reset input of the microprocessor.

This solution has the disadvantage that any microprocessor malfunctions, such as to generate a signal with a very high frequency at the watchdog input, would not be perceived as such and therefore no reset pulse would be generated.

The writer has identified that this problem is particularly felt in household appliances, where a possible malfunction of the microcontroller that manages the loads can generate high frequency test signals.

The aim of the present invention is therefore to improve known watchdog solutions, in particular for applications in the field of household appliances.

This object is achieved by means of a watchdog circuit, and a relative household appliance, incorporating the characteristics of the attached claims, which form an integral part of the present description.

The idea behind the present invention consists in a watchdog circuit equipped with a band pass filter at the input.

The use of such a filter ensures that any high frequency test signals generated by the monitored microprocessor due to its malfunctions are cut.

In this way, the watchdog modules downstream of the filter do not notice these high frequency signals, and behave exactly as if no test signal had arrived.

The use of the input filter, therefore, causes the reset signal to be generated if the microprocessor does not supply the watchdog with a test signal with the desired frequency.

Advantageously, the filter is selected with a pass band between 10 and 200Hz; this band has been found to be particularly suitable when it is necessary to monitor a microcontroller of a household appliance.

Advantageously, the input filter is obtained by means of a capacitor whose charge time is much shorter than the discharge time; in this way, the low frequency signals are unable to load it in good time, while the high frequency ones are unable to load it up to a threshold such as to be read as a high logic value signal by the circuits downstream of the filter.

Further objects and advantages of the present invention will become clearer from the following description of a preferred embodiment of the present invention, described purely by way of non-limiting example, with reference to the attached drawings, in which:

fig. 1 shows an application of a watchdog circuit according to the present invention;

fig. 2 shows the watchdog circuit of figure 1;

fig. 3 shows a second possible application of a watchdog circuit according to the present invention;

Throughout the present invention, identical or equivalent means will be designated by identical reference numerals.

With reference to Figure 1, a block diagram of the watchdog circuit 1 connected to a microcontroller 2, for example a microcontroller which controls the operation of a household appliance, for example a washing machine, is shown.

The watchdog circuit 1 comprises an input band-pass filter 10 which receives the signals supplied by the microcontroller through the output 21, filters them, and transmits them to a monitoring module 20 which has the task of monitoring the operating status of the microcontroller 2.

The monitoring module 20 is essentially a timer which counts the time elapsed between two signals received at the input; if this time exceeds a predetermined value, then the monitoring module 20 activates the reset module 30 which generates a reset signal.

The microcontroller 2, to signal its correct operation, transmits through the output 21 a test signal (for example a square wave) having a predetermined frequency.

In case of malfunction, the microcontroller no longer generates the test signal, or it generates one with a different frequency.

If the test signal is correct, then the band pass filter 10 lets it pass unaltered and the monitoring module 20, receiving it, restarts the count of the time waiting to receive a new test signal.

If the microcontroller 2 does not work correctly and generates a test signal having a frequency not included within the passband of the filter 10, then the test signal thus generated does not pass through the filter and the monitoring module 20, after a predetermined time, activates the reset module 30.

The output of the reset module 30 is connected to a reset input 22 of the microcontroller 2; in this way, when the reset module 30 generates a reset signal, this is received on the input 22 of the microcontroller 2 which restarts.

The wiring diagram of the watchdog circuit 1 is shown in figure 2, where the filter 10, the monitoring module 20, and the reset module 30 are indicated in the dashed line. The input filter 10 comprises a first capacitor C17 placed in series at input 100 where the test signal transmitted by microcontroller 2 arrives through output 21.

The capacitor C17, in this configuration, therefore has the task of cutting DC voltage signals and considerably attenuating very low frequency signals.

The filter 10 then comprises a second capacitor C30 connected on one side to the ground GND, and on the other to the capacitor C17 through a resistive network consisting of two branches in parallel.

A first of these two branches has an equivalent resistance greater than the equivalent resistance of the second.

In the example of fig. 2, the first branch of this resistive network is constituted by the resistance R128 of 10Kâ „¦.

The second branch, on the other hand, is constituted by the 1Kâ „¦ resistance R126 and by the diode D22 having the cathode connected to the capacitor C17.

The filter 10 further comprises a diode D21 connected between the capacitor C17 and the ground, with the cathode facing the capacitor C17. This diode has a protective role against input overvoltages.

The monitoring module 20 comprises the npn type BJT transistor (Q20) in common emitter configuration, the capacitor C11, the resistor R75 and the resistor R23.

The transistor Q20 is connected to the capacitor C30 of the filter 10 through the divider R127 and R72, in such a way that a test signal present at the input 100, and not filtered by the filter 10, is presented to the node N1 on the basis of Q20.

Resistor R23 connects the collector of Q20 to capacitor C11, and has the task of limiting the current flowing through Q20 to preserve its integrity during operation.

The capacitor C11, at the end not connected to R23, is connected to ground.

The resistor R75, on the other hand, connects the capacitor C11, in the node N2 to which R23 also belongs, to the supply voltage through the base resistor of the pnp transistor of Q22 and the resistor R74 in parallel with the latter.

In addition to Q22 and R74, the reset module 30 comprises the transistor Q1 and the transistor Q19.

Q1 is a pnp transistor in a common emitter configuration and reports on the collector the signal present on the collector of Q22.

Q22 is then connected with the base to the collector of Q1 and with the emitter to the supply voltage.

The divider R79 and R80 allows to fix the polarization potential of the base of Q1.

In series with the collector of Q1 there is a resistor R76 which has the task of limiting the current that passes through the transistor itself during its operation.

R76 then connects the collector of Q1 to node N3, where the capacitor C19 and the divider consisting of the resistances R78 and R77 assert.

C19 is connected between node N3 and ground and has the task of delaying the rise of the base voltage of Q19, and therefore imposing a delay in the generation of the reset signal, present at the collector of Q19.

The divider R77 and R78 has the task of fixing the bias voltage of the base of Q19.

The node N4, to which the collector of Q19 refers, constitutes the output (OUT) of the reset module 30.

In the non-limiting example of figure 2, between the output of the reset module 30 (node N4) and the input of the monitoring module (node N1) there is a retraction branch suitable for supplying the monitoring 20 a signal such as to deactivate the reset module.

The feedback branch comprises the diode D19 and the resistor R81 connected in series; D19 being connected to the node N4 through the cathode, and R81 being connected to the node N5 where the resistors R73, R70 and the capacitor C16 are connected.

C16 and R70 connect the node N5 to the supply voltage VCC, while the resistor R73 connects the base of the bipolar transistor Q21 of the pnp type to the node N5. The latter is in a common emitter configuration with the emitter connected to the supply voltage.

The feedback branch is therefore constituted by the diode D19 and by an inverter module which, connected to the anode of D19 and to the input N1 of the monitoring module, allows to invert the polarity of a voltage signal present at the anode of D19. .

C16 and R70 introduce a delay between the moment in which the signal present at node N5 is found inverted to the collector of Q21 which is connected to node N1 through resistor R71. Operationally, when a test signal of sufficient amplitude is presented to node N1 to turn on Q20, this goes into conduction and discharges the capacitor C11.

The discharge of C11 causes Q22 to turn on and Q1 and Q19 to turn off.

The output of the reset module N4 therefore assumes a high logic value, while the feedback remains off due to the diode D19. The output N4, being the reset input 22 of the microcontroller 2 typically connected to Vcc by means of a pull-up resistor (not represented in figure 2), therefore remains at the high logic value.

When the signal present at node N1 assumes the low logic value, Q20 is cut off and C11 starts charging through the resistance R75.

If a new test signal with a high logic value is not presented to node N1 in good time, the capacitor C11 charges up to a predetermined voltage value such as to turn off Q22.

Turning off Q22 causes Q1 and Q19 to conduct and lowers the N4 output of the reset module 30.

At this point, the feedback through R81 and Q21 has the effect of raising the potential of node N1 again after a time interval set by C16 and R70.

Q20 then goes into conduction and, as explained above, the N4 output goes back high.

The effect of the retraction, therefore, is to generate a pulse with a low logic value on output N4. This pulse constitutes the reset signal which allows the restart of the microcontroller 2.

The input filter 10 has the purpose of preventing anomalous test signals from appearing at node N1 by turning on Q20 and thus avoiding the generation of the reset signal.

For this purpose, the presence of C17 in series at the input 100 allows to cut DC voltage signals.

Any signals having a very low frequency (for example 5 Hz) are first attenuated by C17, and then they go to charge C30; however, the signals thus attenuated charge C30 too slowly and C11 has time to charge up to a voltage value such as to turn off Q22 and generate the reset signal. The charging time of C11 therefore corresponds to the maximum time that the watchdog waits before generating the reset signal, or, in other words, the minimum (or threshold) frequency that the test signal supplied by the microcontroller must have.

Test signals at too high a frequency, on the other hand, are able to pass capacitor C17, however C30 cannot charge enough to turn on transistor Q20 because the resistance R126 is chosen much smaller (for example ten times smaller) than the resistor R128, and this guarantees a discharge time of the capacitor C30 much lower than the charging time of the same.

The sizing, therefore, of the capacitors C30 and C17 and of the resistors R126 and R128 therefore allows to define the passband of the filter 10.

For applications in the household appliances sector, it is preferable to dimension the filter 10 in such a way that it allows test signals to pass with a frequency between 10 and 200 Hz, or more preferably between 10 and 100 Hz.

In fact, it has been verified that the malfunctions of a microcontroller of a household appliance are rarely able to generate signals with frequencies included in this band.

From the above description the advantages and characteristics of the present invention are clear; It is therefore clear that many variants are possible for those skilled in the art, without thereby departing from the scope of the invention as it results from the attached claims.

For example, the retract branch connecting the reset module output N4 to the monitoring module input N1 can be omitted.

In this case, once a malfunction of the microcontroller is detected, with consequent charge of C11, the output N4 passes from the high logic value to the low logic value and remains there.

The absence of the retraction branch, therefore, means that the reset signal is not an impulse, but a constantly low signal that keeps the reset input 22 of the microcontroller 2 low, which therefore does not restart, but is turns off.

A restart of the microcontroller is therefore possible only by removing power from the microcontroller and turning it back on (for example by means of a manual reset by the user), so that it resumes generating a test signal with a frequency included in the passband of filter 10.

This solution, without feedback branch, therefore allows to generate a reset signal that can be used both to drive the reset input of a microprocessor or microcontroller as mentioned above, and to open a safety switch, for example a relay that disconnects the power supply to the electrical appliance as shown in figure 3.

In figure 3 the electrical appliance, for example a household appliance, is powered through a single-phase power supply line (indicated with the references L and N) at alternating voltage, eg. 230V at 50Hz or 110V at 60Hz.

The electrical appliance is connected to the power supply line by means of an AC / DC power supply 3 which supplies a direct voltage VCC at the output which powers the microcontroller 2 and the watchdog 1.

The power supply 3 is connected to the alternating power supply line by means of a safety switch 4 (for example a circuit breaker or a triac).

Switch 4 is connected to reset module 30 in such a way that it opens when it generates a reset signal.

Operationally, in the event of a malfunction of the microcontroller 2, the watchdog 1 generates a reset signal which opens the switch 4, thus cutting off the power supply to the electrical device.

In the example of figure 3, the reset signal is sent both to the microcontroller and to the switch 4, however this signal can also be sent only to the switch 4.

The examples described above with reference to Figures 1-3 are in no way to be considered limiting of the present invention.

Although the examples described above referred to the monitoring of a microcontroller, it is clear that the watchdog circuit according to the present invention can also be used to control a microprocessor, for example a computer CPU.

It is also clear that the modules of the watchdog circuit can be mounted on one or more plates connected to each other, and that the sizing of the circuit components depends on the particular applications in which this must be used. For example, the bandpass filter can be separated from the rest of the circuit and connected to the monitoring module via a dedicated line.

The watchdog circuit according to the invention is preferably made with discrete components, however it is clear that part of it can be integrated in a chip.

In a preferred embodiment, particularly advantageous for use in a household appliance, the components of the watchdog circuit 1 have the values shown in Table 1 below.

Component Value

R128 10 kâ „¦

R126 1 kâ „¦

R127 33 kâ „¦

R72 33 kâ „¦

R23 56 â „¦

R75 22 kâ „¦

R74 22 kâ „¦

R80 4.7 kâ „¦

R79 10 kâ „¦

R76 100 â „¦

R77 10 kâ „¦

R78 10 kâ „¦

R81 4.7 kâ „¦

R70 10 kâ „¦

R73 100 â „¦

R71 10 kâ „¦

C17 2.2 µF

C30 470 nF

C11 10 µF

C19 1µF

C16 1µF

Q20 BC 817-25

Q21 BC 807-25

Q22 BC 807-25

Q19 BC 817-25

Q1 BC 807-25

D21 1N5148W

D22 1N5148W

D19 1N5148W

From the above description it is then clear that the invention is not limited only to the watchdog circuit, but is also related to a method for restarting a microprocessor (in particular a microcontroller) comprising a reset input and an output. According to the invention, the microprocessor generates a test signal on said output and a watchdog circuit, e.g. realized as described above, it filters it through a bandpass filter, in such a way as to transmit a reset signal to said reset input of the microprocessor if said test signal does not have a frequency included in the pass band of the filter.

Claims (10)

CLAIMS 1. Watchdog circuit (1) comprising an input (100) for receiving test signals from a monitored equipment, a monitoring module (20) and a reset module (30), in which said reset module (30) is adapted to generate a reset signal when activated by said monitoring module (20), and in which said monitoring module (20) is adapted to activate said reset module ( 30) if it does not receive in input a test signal with a frequency greater than a threshold frequency, characterized in that it comprises a band pass filter (10) connected between said input (100) and said monitoring module (20). 2. Circuit according to claim 1, wherein said filter (10) has a pass band between 10Hz and 200 Hz, in particular between 10Hz and 100 Hz. Circuit according to claim 1 or 2, wherein said band pass filter (10) comprises a first capacitor (C17) connected in series to said input (100), and a second capacitor (C30) connected to said first capacitor (C17) by means of a resistive network, such that the discharge time of said second capacitor (C30) is less than the charging time of said second capacitor (C30). 4. A circuit according to claim 3, wherein said resistive network comprises a first and a second branch connected in parallel between said first capacitor (C17) and said second capacitor (C30), wherein said first branch has an equivalent resistance higher than the equivalent resistance of said second branch, and wherein said second branch comprises a first diode (D22), the cathode of said first diode being connected to said first capacitor (C17). 5. Circuit according to any one of the preceding claims, wherein said monitoring module comprises a transistor (Q20) connected to a third capacitor (C11), so that when said transistor (Q20) is conducting said third capacitor (C11 ) is discharged, and in which said reset module (30) is able to be activated when at the ends of said third capacitor (C11) there is a voltage drop higher than a predetermined voltage. 6. Circuit according to any one of the preceding claims, further comprising a retraction branch between the output (N4) of said reset module (30) and the input (N1) of said monitoring module (20), said branch of retraction being adapted to supply at the input to said monitoring module (20) a signal such as to deactivate said reset module. 7. A circuit according to claim 6, wherein said feedback branch comprises a second diode (D19) connected with the cathode to the output (N4) of said reset module (30) and an inverter module connected to the anode of said second diode (19) and to said input (N1) of said monitoring module (20), to reverse the polarity of a voltage signal present at the anode of said diode (D19). 8. Household appliance comprising a microcontroller (2) and a watchdog circuit (1), in which said microcontroller (2) comprises a reset input (22) and an output (21), in which an input (100) of said watchdog circuit is connected to said output (21) of the microcontroller (2), and in which an output (OUT) of said watchdog circuit is connected to said reset input (22) of the microcontroller (2), characterized in that said watchdog circuit is a circuit according to any one of claims 1 to 7. 9. Household appliance according to claim 8, further comprising a power supply (3) adapted to be connected to a power supply network (L, N) by means of a switch (4) and to supply a power supply voltage to said microcontroller (2), wherein said switch (4) is connected to said reset module (30) in such a way as to open when said reset module (30) generates said reset signal. 10. Method for restarting a microprocessor comprising a reset input and an output, in which the microprocessor generates a test signal on said output and in which a watchdog circuit transmits a reset signal to said reset input in the event of a malfunction of the microprocessor , the method being characterized by filtering said test signal through a bandpass filter, and transmitting a reset signal to said reset input of the microprocessor if said test signal does not have a frequency included in the passband of said bandpass filter.