JPH0638218B2 - Microcomputer power supply backup circuit - Google Patents

Description

Description: TECHNICAL FIELD The present invention relates to a power supply backup circuit for a microcomputer including a watchdog timer circuit.

<Prior Art> FIG. 2 shows a conventional example of a power supply backup circuit for a microcomputer. In FIG. 2, 2 is a commercial power supply, 4 is a full-wave rectifier circuit configured with a diode bridge, C1 is a smoothing capacitor, IC1 is a watchdog timer circuit configured with an integrated circuit, Q1 is a power transistor, and C2 is a watchdog. Backup capacitor, MC for microcomputer, C3 for microcomputer backup capacitor, RL1 for relay as actuator (load),
R1 to R6 are resistors, D1 is a backflow prevention diode, ZD1
Is a Zener diode, C4 is a capacitor, and INV is an inverter. The relay RL1 is used, for example, as a power switch for a gas proportional valve of a water heater.

The AC input from the commercial power supply 2 is converted into DC by the full-wave rectifier circuit 4 and the smoothing capacitor C1 to obtain the DC power supply Vcc. DC power supply Vcc consists of resistor R1 and backflow prevention diode D
Backup capacitor C for watchdog via 1
2 is charged, and at the same time, the microcomputer backup capacitor C3 is charged via the power transistor Q1.
Charging voltage V _DD of microcomputer backup capacitor C3
Is 5 as the power supply voltage of the microcomputer MC.
It is set to [V].

The watchdog timer circuit IC1 inputs the power supply voltage V _DD for the microcomputer from the terminal V _O and uses it as the built-in reference voltage V _DD.
Compared with ref ₀ (5 [V]), and depending on the difference, terminal V
By controlling the base voltage applied to the power transistor Q1 from _CONT , the microcomputer power supply voltage V _DD is always controlled to be automatically maintained at the reference voltage V ref _0, that is, 5 [V]. Also, the power supply voltage V _DD for the microcomputer is the resistance R
The voltage divided by 3, R4 is input from the terminal V _S , and when the divided voltage becomes lower than the built-in reference voltage Vref ₁ , a reset signal is output from the reset terminal RES to the microcomputer MC. To do. Reference voltage V
ref ₁ is a voltage (4.75 [V]) that is 5% less than the power supply voltage V _DD (5 [V]) for the microcomputer and is the lower limit voltage of the allowable range of the power supply voltage at which the microcomputer MC operates normally. It is set.

The microcomputer MC outputs a "H" signal to the inverter INV when driving the relay RL1 as an actuator. Then, the inverter IN
The output side of V becomes "L", the current from the DC power supply Vcc flows to the relay RL1, and the relay RL1 is driven.

When the microcomputer MC drives the relay RL1,
That is, while the signal of “H” is output to the inverter INV, 1 is output to the inverter INV.
A pulse signal of “L” having a pulse width of 100 μsec is output once every 00 msec. That is, while the output of the inverter INV is "L", it is "H" with a width of 100 µsec every 100 msec from the inverter INV.
The watchdog pulse of is output. Inverter INV
The watchdog pulse with a very short time width output from the above does not affect the driving state of the relay RL1.

By the way, since the DC power supply Vcc by the smoothing capacitor C1 makes the Zener diode ZD1 conductive higher than the Zener voltage of the Zener diode ZD1, the watchdog pulse output from the inverter INV passes through the Zener diode ZD1 and through the resistor R6. It is input to the clock terminal CK of the watchdog timer circuit IC1. The watchdog timer circuit IC1 monitors the state of the clock terminal CK, and if the watchdog pulse is no longer input as a result of the microcomputer MC malfunctioning for some reason, it outputs a reset signal from the reset terminal RES. It outputs and resets the microcomputer MC. As a result, the microcomputer MC returns to the initial state, outputs the signal of "L" to the inverter INV, and the output of the inverter INV becomes "H", and the driving of the relay RL1 is stopped.

Non-driving state of the relay RL1, that is, the inverter IN
When the commercial power supply 2 is turned off or a power failure occurs in the commercial power supply 2 when the output of V is “H”,
The microcomputer backup capacitor C3 performs power backup for the microcomputer MC, and the watchdog backup capacitor C2 performs power backup for the watchdog timer circuit IC1. The backup time in this case is about 15 seconds.

<Problems to be Solved by the Invention> However, conversely to the above, when a power failure occurs in the commercial power supply 2 in the driving state of the relay RL1, the output of the inverter INV is “L”, and therefore the smoothing capacitor is Although the voltage (Vcc) across C1 drops rapidly, the microcomputer power supply voltage V _DD is supplied from the microcomputer backup capacitor C3, and the microcomputer MC tries to back up despite the power failure of the commercial power supply 2. As for the relay RL1, it tries to keep the relay RL1 in a driving state.

However, since the Zener diode ZD1 is cut off in about 2 seconds after the power failure due to the rapid drop of the voltage across the smoothing capacitor C1, the microcomputer MC
The watchdog pulse output from the inverter INV through the inverter INV does not pass through the Zener diode ZD1. In this case, the watchdog timer circuit IC1
Since a watchdog pulse is not input to the clock terminal CK, the reset terminal RES outputs a reset signal to the microcomputer MC. Since this reset signal resets the microcomputer MC (at this time, the drive of the relay RL1 is also stopped), the backup of the microcomputer MC when the commercial power source 2 fails is meaningless. Becomes

That is, in the case of a power failure in the non-driving state of the relay RL1, backup for about 15 seconds was possible, whereas in the case of a power failure in the driving state of the relay RL1, a watchdog pulse was sent to the clock terminal CK. Then, the microcomputer MC itself is reset after about 2 seconds, and the relay RL1 is switched from the driving state to the non-driving state.

Therefore, even if the commercial power supply 2 recovers from the power failure state after a few seconds, the relay RL1 does not return to the state immediately before the power failure. In other words, even if the power outage is short for a few seconds, since the microcomputer MC has already been reset, the operation operation of the device must be restarted from the beginning. This is nothing but the fact that the backup to the microcomputer MC is not properly performed.

The present invention has been made in view of such circumstances, and it is possible to provide a microcomputer with a substantially constant backup time regardless of whether a load is driven or not driven. With the goal.

<Means for Solving the Problems> The present invention has the following configuration in order to achieve such an object.

That is, the present invention provides a microcomputer that outputs a watchdog pulse at the same time as a signal that gives a drive command to a load, a watchdog pulse transmission circuit that transmits the watchdog pulse via a Zener diode, and the watchdog pulse transmission circuit. And a load, a watchdog timer circuit that outputs a reset signal to the microcomputer when the input of the watchdog pulse transmitted through the watchdog input is not received within a predetermined period To the watchdog timer transmission circuit, a backup capacitor for the microcomputer, and a backup capacitor for the watchdog timer circuit. Connects to the backup capacitor for the circuit, is characterized in that retaining the continuity of the Zener diode by the output voltage of the backup capacitor.

<Operation> The operation of the above configuration of the present invention is as follows.

The stored charge of the backup capacitor for the watchdog timer circuit, which holds the Zener diode in the conductive state, is not consumed by the load. Even if a power failure occurs in the drive state of the load, the voltage drop of the backup capacitor for the watchdog timer circuit is gentle. Since the watchdog pulse sent from the microcomputer at the time of driving the load is given to the watchdog timer circuit via the Zener diode in the conductive state, the watchdog timer circuit does not output the reset signal to the microcomputer. That is,
The microcomputer continues the current operating state including the drive command for the load.

Power backup for such a microcomputer continues until the voltage of the backup capacitor for the watchdog timer circuit falls below the Zener voltage.
Since the charge stored in the backup capacitor is not consumed by the load, it is possible to secure a substantially constant and sufficiently long backup time regardless of whether the load is driven or not driven.

<Example> Hereinafter, an example of the present invention is described in detail based on a drawing. FIG. 1 shows a power supply backup circuit of a microcomputer according to an embodiment of the present invention.

Full-wave rectifier circuit 4 with diode bridge
Has two input terminals connected to both ends of the commercial power supply 2 and two output terminals connected to both ends of the smoothing capacitor C1. Resistors R1 and R1 are provided at both ends of the smoothing capacitor C1.
A series circuit of a backflow prevention diode D1 and a watchdog backup capacitor C2 is connected. P
The NP type power transistor Q1 has its emitter connected to the connection point between the backflow prevention diode D1 and the watchdog backup capacitor C2, and its collector connected to one end of the microcomputer backup capacitor C3 via the backflow prevention diode D2 and the resistor R7. Connected and the base is a watchdog timer circuit IC1 (LA5693 made by Sanyo Electric)
_Is connected to the terminal V _CONT of. A resistor R2 for bias is connected between the emitter and the base. The other end of the microcomputer backup capacitor C3 is connected to the ground line.

The cathode of the backflow prevention diode D2 is connected to the power supply input terminal of the microcomputer MC via the resistor R8, and the connection point P1 between the power supply input terminal and the resistor R8 is the microcomputer power supply voltage V _DD (5 [V]). Is set to. The connection point P1, which is the power supply voltage V _DD for the microcomputer, is
It is connected to the terminal V _O of the watchdog timer circuit IC1 and is also connected to the ground line via the voltage dividing resistors R3 and R4. Connection point of the voltage dividing resistors R3, R4 is connected to the terminal _{V S} of the watchdog timer circuit IC1. The positive terminal of the microcomputer backup capacitor C3 is connected to the resistor R via the backflow prevention diode D3.
8 and R3 are connected to a connection point P1.

One end of the relay RL1 as an actuator is connected to one end (DC power supply Vcc) of the smoothing capacitor C1 and the other end is connected to the output terminal of the inverter INV. The input terminal of the inverter INV is connected to the output terminal OUT of the microcomputer MC.

A series circuit of a resistor R5, a zener diode ZD1, a resistor R6 and a capacitor C4 is connected across the watchdog backup capacitor C2, and a connection point between the resistor R5 and the cathode of the zener diode ZD1 is connected to the output terminal of the inverter INV. Has been done. The connection point between the resistor R6 and the capacitor C4 is connected to the clock terminal CK of the watchdog timer circuit IC1, and the reset terminal RES of the watchdog timer circuit IC1 is connected to the microcomputer MC. C5 is a capacitor that determines the time constant of the watchdog pulse monitoring period in the watchdog timer circuit IC1, and G is a ground terminal.

A power backup circuit for the microcomputer MC is configured by the microcomputer backup capacitor C3, the backflow prevention diodes D2, D3, and the resistors R7, R8. Watchdog timer circuit IC1, power transistor Q1, voltage dividing resistors R3, R4, resistors R5, R
6, Zener diode ZD1, capacitors C4, C5
The watchdog voltage monitoring circuit is configured with. A watchdog pulse transmission circuit is formed by the Zener diode ZD1 and the resistor R6.

The main differences between this embodiment and the conventional example are as follows.

As a power source for keeping the Zener diode ZD1 conductive, a smoothing capacitor C1 is used in the conventional example.
The voltage of the backup capacitor C2 for the watch dog is used in this embodiment.

A backflow prevention diode D2, resistors R7 and R8, and a backflow prevention diode D3 are connected to the collector of the power transistor Q1.

Among these, it is the person who follows the gist of the present invention.

Next, the operation of the power supply backup circuit of the microcomputer configured as described above will be described.

When a main power switch (not shown) is turned on, the AC input from the commercial power supply 2 is converted into DC by the full-wave rectifier circuit 4 and the smoothing capacitor C1 to obtain the DC power supply Vcc. The DC power supply Vcc charges the watchdog backup capacitor C2 via the resistor R1 and the backflow prevention diode D1, and also makes the power transistor Q1 conductive due to the voltage drop across the resistor R2.

The collector current of the power transistor Q1 is applied to the power supply input terminal of the microcomputer MC via the backflow prevention diode D2 and the resistor R8, and immediately starts up and starts up the microcomputer MC. On the other hand, the backup capacitor C3 for the microcomputer is gradually charged via the backflow prevention diode D2 and the resistor R7. In this case, since the backflow prevention diode D3 exists between the points P1 and P2, the microcomputer MC can be immediately started up while gradually charging the microcomputer backup capacitor C3. is there. Therefore, a large-capacity capacitor of several hundreds of thousands μF can be used as the backup capacitor C3 for the microcomputer, and the backup time for the microcomputer MC can be extended.

It should be noted that the watchdog timer circuit IC1 uses the terminal V 0 based on the microcomputer power supply voltage V _DD input from the terminal V _O.
By controlling the base voltage of the _CONT, that holds the power supply voltage V _DD microcomputer constant (5 V), and the power supply voltage V voltage input from the terminal V _S is the microcomputer
Similar to the conventional example, a reset signal is output from the reset terminal RES to the microcomputer MC when the voltage is lower than the voltage (4.75 [V]) that is 5% lower than _DD (5 [V]). Is.

The microcomputer MC outputs a "H" signal to the inverter INV when driving the relay RL1 as an actuator. Then, the inverter IN
The output side of V becomes "L", and the current from the DC power supply Vcc flows to the relay RL1 to drive the relay RL1.

At the same time, the microcomputer MC outputs a pulse signal of "L" having a pulse width of 100 μsec to the inverter INV once every 100 msec as a condition that does not affect the driving state of the relay RL1. That is, while the output of the inverter INV is "L", the watchdog pulse of "H" having a width of 100 µsec is output from the inverter INV every 100 msec.

By the way, backup capacitor C for watchdog
The Zener diode ZD1 is made conductive by the voltage across the terminal 2, and the watchdog pulse output from the inverter INV passes through the Zener diode ZD1 and is input to the clock terminal CK of the watchdog timer circuit IC1 via the resistor R6. Watchdog timer circuit I
C1 monitors the state of the clock terminal CK. When the microcomputer MC becomes abnormal due to some cause and the watchdog pulse is no longer output, a reset signal is output from the reset terminal RES to reset the microcomputer MC. As a result, the microcomputer MC returns to the initial state, and the inverter I
It outputs the signal of "L" to NV, and the inverter IN
Since the output of V becomes "H", the driving of the relay RL1 is stopped. When a power failure occurs in the commercial power source 2, power is supplied to the microcomputer MC by the smoothing capacitor C1 → the resistor R1 → the backflow prevention diode D1.
→ Power transistor Q1 → Instead of the route of resistor R8,
This is performed in the path from the microcomputer backup capacitor C3 to the backflow prevention diode D3.

At the time of this power failure, "H" is output from the microcomputer MC to the inverter INV, and the inverter IV
It is assumed that the output of NV is "L" and the relay RL1 is in a driving state.

Since the output of the inverter INV is "L", the smoothing capacitor C1 is discharged rapidly, but this has no effect on the conduction state of the Zener diode ZD1. This is because in the case of the present embodiment, unlike the conventional example, the Zener diode ZD1 is held in the conductive state by the voltage of the watchdog backup capacitor C2, and the watchdog backup capacitor C2 is maintained.
This is because a current does not flow from the relay RL1 to the relay RL1 due to the presence of the backflow prevention diode D1.

When the relay RL1 is in a driving state, the microcomputer MC outputs a watchdog pulse to the inverter INV, which is a Zener diode ZD1.
For each 100 msec, a watchdog pulse of "H" with a width of 100 μsec is applied to the watchdog timer circuit IC.
Since it is input to the clock terminal CK of 1, the reset signal is not output to the microcomputer MC, and the microcomputer MC continues to maintain the current operating state.

Such backup of the microcomputer MC is performed until the voltage of the watchdog backup capacitor C2 drops below the Zener voltage of the Zener diode ZD1, and the backup time is about 11 seconds, which is 2 seconds in the case of the conventional example. It was possible to make it about 5.5 times longer than that of. The time until the voltage of the watchdog backup capacitor C2 becomes equal to or lower than the Zener voltage is substantially the same when the relay RL1 is in the driving state and the non-driving state. That is, a substantially constant backup time can be secured for the microcomputer MC regardless of whether the load is driven or not driven.

If the commercial power supply 2 is in a power failure state for about several seconds, the backup to the microcomputer MC is performed without any problem, and the microcomputer MC can continue the operation even after the commercial power supply 2 is recovered from the power failure. it can.

<Effects of the Invention> According to the present invention, the following effects are exhibited. A backup for the watchdog timer circuit isolated from the load rather than the power supply to the load to keep the Zener diode for conducting the watchdog pulse sent from the microcomputer when driving the load to the watchdog timer circuit in the conductive state Since it is configured to turn on the Zener diode by the output voltage of the capacitor, even if a power failure occurs in the drive state of the load, the watchdog pulse is reliably transmitted to the watchdog timer circuit to avoid resetting the microcomputer, The microcomputer can be backed up by power. This kind of power backup continues until the voltage of the backup capacitor for the watchdog timer circuit becomes equal to or lower than the Zener voltage, but since the accumulated charge of this backup capacitor is not consumed by the load, it does not matter whether the load is driven or not. However, it is possible to secure a constant and sufficiently long backup time.

[Brief description of drawings]

FIG. 1 is a circuit diagram of a power supply backup circuit of a microcomputer according to an embodiment of the present invention, and FIG. 2 is a circuit diagram of a power supply backup circuit of a conventional microcomputer. IC1 ... Watchdog timer circuit MC ... Microcomputer C2 ... Watchdog backup capacitor C3 ... Microcomputer backup capacitor ZD1 ... Zener diode

Claims (1)

[Claims] 1. A microcomputer that outputs a watchdog pulse at the same time as a signal that gives a drive command to a load, a watchdog pulse transmission circuit that transmits the watchdog pulse via a Zener diode, and the watchdog pulse transmission circuit. The watchdog timer circuit that monitors the input of the watchdog pulse transmitted via the watchdog timer and outputs a reset signal to the microcomputer when the watchdog pulse is not input within a predetermined period, and the load, the microcomputer, and the watchdog timer circuit. The watchdog timer circuit includes a power supply unit for supplying power to the microcomputer, a backup capacitor for the microcomputer, and a backup capacitor for the watchdog timer circuit. Connects to the backup capacitor use, power backup circuit of the microcomputer, characterized by holding the conduction of the Zener diode by the output voltage of the backup capacitor.