JP2005092430A - Watchdog timer - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a watchdog timer that can detect a runaway of a plurality of processing parts and identify which processing part has run away after the detection, on a small circuit scale. <P>SOLUTION: If clear signals CLR1 and CLR2 are not input into clear terminals T11 and T12 respectively during a timeout time after the preceding clearing of a counter 31, a comparator 33 outputs a signal indicating that either program processing part associated with the clear terminal T11, T12 has run away. If the signal is output and an output of latch circuits 21 and 22 for holding the clear signals CLR1 and CLR2 does not indicate clearing, an AND circuit 53, 54 sets a corresponding latch circuit 51, 52. The latch circuit 51, 52 can output a value indicating which program processing part has run away after the runaway detection. <P>COPYRIGHT: (C)2005,JPO&NCIPI

Description

  The present invention relates to a watchdog timer, and in particular, any one of a plurality of processing units has runaway after a runaway although a single counter can detect that any of a plurality of processing units has runaway. This is related to a watchdog timer that can specify this.

  Conventionally, in a system (for example, a control system) that is strongly required to continue to operate normally, for example, a watchdog that detects a program runaway in order to perform a return process from the runaway and continue operating normally. Information processing apparatuses having a timer are widely used.

  As shown in FIG. 3, in the watchdog timer 114, the counter 131 counts up its count value C131 at a predetermined time interval, and the comparator 133 includes the count value C131 and the time setting register. By comparing with the count value C132 stored in advance 132, it is determined whether or not the time-up time corresponding to the count value C132 has elapsed since the counter 131 was reset last time. In this case, an NMI (non-maskable interrupt) signal is output to a CPU (not shown).

  Here, during the normal operation of the program α executed by the CPU, the clear signal CLR is activated (low in the example of FIG. 3) in a time shorter than the time-up time. Programmed to reset counter 131 repeatedly. However, if the program α goes out of control, the counter 131 cannot be reset. Therefore, the count value C131 of the counter 131 reaches the count value C132, and the watchdog timer 114 outputs an NMI signal.

  When the NMI signal is input, the CPU executes a program for returning from the runaway state regardless of the currently executing instruction. Thus, even when the program α is running out of control, the CPU can execute the return program Ω without any trouble, and can restore the program α to a normal state.

Furthermore, in Patent Document 1 to be described later, in order to monitor program runaway at an appropriate time-up time in each of different states of the appropriate time-up time, such as during system startup and normal operation, A configuration is described in which when the clear signal is given at the time of start-up, the frequency division ratio for generating the clock signal to be supplied to the counter 131 is changed.
JP 2001-306362 A (publication date: November 2, 2001)

  However, the conventional configuration described above solves all the problems of detecting a runaway of a plurality of processing units realized by a program or the like with a small circuit scale and specifying which processing unit has runaway after the detection. The problem is that you can't.

  More specifically, at present, the processing required for the information processing apparatus becomes complicated, and the CPU often executes a plurality of independent programs in a time-sharing manner. In this case, it is desired that the watchdog timer 114 monitors these plural programs α.

  However, if the watchdog timer 114 is provided for each program α..., The number of necessary counters increases. Therefore, when a reduction in the circuit scale is required, a single counter can be used for a plurality of programs α. It is required to monitor the runaway.

  Here, if a clear terminal is provided for each program to be monitored, the clear signal by each program is made independent, and the counter 131 is reset when all clear signals are true, the watchdog timer 114 is Since it is possible to detect that one of the programs has runaway, the circuit scale can be reduced compared to a configuration in which a single counter monitors the runaway of a single program.

  However, in this configuration, after detecting a runaway, it is not possible to specify which program has runaway. Therefore, for example, to perform recovery processing only for the runaway program, display the runaway program, and verify the program intensively to identify the cause of the runaway, and perform processing according to the runaway program I can't.

  The present invention has been made in view of the above problems, and its purpose is to detect runaway of a plurality of processing units with a small circuit scale and to identify which processing unit has runaway after detection. The realization of a possible watchdog timer.

  In order to solve the above-described problem, the watchdog timer according to the present invention is a process that can be set as a monitoring target in a watchdog timer having a timing unit that outputs an output signal indicating a runaway when a predetermined timeout time elapses. A plurality of clear terminals provided for each section, a reset means for resetting the time counting means when a clear signal is input to all the clear terminals corresponding to the processing target to be monitored, and for each clear terminal A latch circuit and a latch circuit control means provided, wherein the latch circuit control means outputs an output signal indicating runaway by the time counting means and no signal indicating clear is input to the clear terminal The latch circuit corresponding to the clear terminal is set and printed at a timing independent of the output signal indicating the runaway. Until a reset signal is input, it is characterized in that to hold the value in the latch circuit.

  Further, in addition to the above configuration, an input terminal to which a control signal indicating whether or not each processing unit is to be monitored is input, and the reset unit is configured to monitor the monitoring based on a signal to the input terminal. A target processing unit may be specified.

  Further, in addition to the above configuration, the latch circuit control means may reset a latch circuit corresponding to a processing unit indicating that an input signal to the input terminal is not monitored.

  As described above, the watchdog timer according to the present invention includes the time counting means, the clear terminal, the reset means, the latch circuit, and the latch circuit control means, and the reset means corresponds to the processing unit to be monitored. When the signal indicating clear is input to all the clear terminals, the time measuring means is reset. Therefore, until the timeout time elapses, all of the processing units to be monitored are operating normally, and a signal indicating clear is input to the clear terminals provided corresponding to each of the processing units. During this period, no output signal indicating runaway is output. In addition, the said process part is realizable when arithmetic means, such as CPU, runs the program memorize | stored in ROM, RAM, etc.

  On the other hand, if a processing unit runs away, the processing unit cannot input a signal indicating clear to the corresponding clear terminal even if the time-out period elapses after the time counting unit was reset last time. The means outputs an output signal indicating a runaway.

  Thereby, the watchdog timer can detect whether or not a plurality of processing units have runaway despite the fact that the circuit scale is reduced as compared with the configuration in which the time measuring unit according to the monitoring target is provided. A control unit that is set to perform a predetermined process during a runaway, such as a control unit realized by a program that restores each processing unit to normal operation, can be notified of the runaway of the processing unit to be monitored, Processing corresponding to the runaway of the processing unit can be performed.

  Further, in the above configuration, a latch circuit is provided corresponding to each clear terminal, and the latch circuit control means outputs an output signal indicating that the time measuring means is out of control, and also indicates clear to the clear terminal. When no signal is input, the latch circuit corresponding to the clear terminal is set and the latch circuit is input until a reset signal applied at a timing independent of the output signal indicating the runaway is input. Hold the value.

  As a result, each latch circuit can output a value indicating whether or not the corresponding processing unit has runaway from the time when the runaway is detected until the reset signal is input. Therefore, each latch circuit refers to each latch circuit, for example, determines whether each processing unit is running out of control, and presents the processing unit that is running away to the user, Processing depending on which processing unit is running out of control in a control unit that is set to perform processing according to the processing unit that is running away, such as a control unit that performs recovery processing only on the part Can be implemented.

  In addition, as described above, the watchdog timer according to the present invention includes an input terminal to which a control signal indicating whether or not each processing unit is to be monitored is input, so that control applied to the input terminal is performed. Depending on the signal, it is possible to change which of the processing units that can be set as the monitoring target is the monitoring target.

  In this configuration, for example, the operation of the processing unit to be monitored is changed to clear not only the clear terminal corresponding to itself but also the clear terminal corresponding to the processing unit not to be monitored. It is possible to change which of the processing units that can be set as the monitoring target is to be monitored without changing the operation of the target processing unit. Therefore, it is possible to realize a watch dog timer that can easily change the monitoring target.

  Further, in the watchdog timer according to the present invention, as described above, the latch circuit control unit resets the latch circuit corresponding to the processing unit that is excluded from the monitoring target. Is the only latch circuit corresponding to the processing unit in which the runaway is detected. Therefore, it is possible to realize a watchdog timer that can accurately notify only the processing unit in which the runaway is detected, as compared with the configuration in which the output of the latch circuit that is not monitored is indefinite.

  An embodiment of the present invention will be described with reference to FIGS. 1 and 2 as follows. That is, the information processing apparatus 1 according to the present embodiment is an apparatus capable of detecting a program runaway by a watchdog timer, and as shown in FIG. 2, the first and second program processing units 11 as monitoring targets.・ When both program processing units 11 and 12 are operating normally, a control value is repeatedly written by each program processing unit 11 and 12 in a shorter time than a predetermined time-up time. If the value indicating normality is not written in the control register 13 even though the time-up time has elapsed since the register 13 and the value indicating normality were written last time, the program processing to be monitored Watchdog timer 14 that determines that one of the parts is running out of control, and the runaway is detected from the watchdog timer 14 When it is notification, for example, to initialize the respective program processing unit 11, 12, and a control unit 15 for performing predetermined processing. The program processing units 11 and 12 correspond to the processing units described in the claims.

  The time-up time is set to be sufficiently longer than the period in consideration of fluctuations in the period in which the program processing units 11 and 12 write values indicating clear. Various values are selected for these times and periods depending on the application. In this embodiment, as an example, the time-up time is set to 1 [s] and the repetition period is set to 10 [ms]. ing.

  Here, in the first and second program processing units 11 and 12 and the control unit 15, the calculation unit such as a CPU executes a program stored in a storage unit such as a ROM or a RAM, and a touch panel is used as necessary. Functional blocks realized by controlling input / output means such as a liquid crystal display device or communication means such as an interface circuit, wherein the first and second program processing units 11 and 12 are, for example, memory areas A value indicating normality is repeatedly written into the control register 13 assigned to an area accessible to arithmetic means such as a CPU, such as the I / O area. In addition, the watchdog timer 14 notifies the arithmetic means such as a CPU of the occurrence of a runaway by an interrupt signal such as an NMI (non-maskable interrupt) signal, and the arithmetic means Regardless, the program for realizing the control unit 15 is executed. As a result, the control unit 15 can operate without any trouble even when the first and second program processing units 11 and 12 run away.

  Furthermore, the watchdog timer 14 of the information processing apparatus 1 according to the present embodiment is provided with a status register 16 in which a different storage area is provided for each of the program processing units 11 and 12 to be monitored. The dog timer 14 detects the runaway of the program processing units 11 and 12 in each storage area of the status register 16 when it detects that any of the program processing units 11 and 12 to be monitored is running out of control. You can write whether or not. The status register 16 is configured to hold the stored value while the control unit 15 performs the predetermined processing. The control unit 15 runs away from the watchdog timer 14. After receiving the detection notification, the value of the status register 16 (WDT1P / WDT2P, which will be described later) can be referred to determine which of the program processing units 11 and 12 is out of control. 12 can be performed.

  In the following, as an example, a mode for detecting only the runaway of the first program processing unit 11 separately from the mode for detecting that either of the first and second program processing units 11 and 12 is running away, and The configuration when the information processing apparatus 1 has a mode for detecting only the runaway of the second program processing unit 12 will be described in detail with reference to FIG. In the following, for convenience of explanation, each is referred to as a third, first, and second mode.

  The control register 13 according to the present embodiment stores a bit area B1 for storing whether or not the runaway of the first program processing unit 11 is monitored and whether or not the runaway of the second program processing unit 12 is monitored. When the watchdog timer 14 is set to the first mode, the control unit 15 sets only the value of the region B1 to true. Further, when setting the second mode or the third mode, the control unit 15 sets only the value of the region B2 and the values of both the regions B1 and B2, respectively.

  Further, the control register 13 is written with a bit region B11 in which a value (true) indicating repeated normality is written by the first program processing unit 11 and a value (true) indicating repeated normality by the second program processing unit 12. A bit area B12 is provided.

  As shown in FIG. 1, the values of the respective areas B1, B2, B11, and B12 are set as control signals ENB1, ENB2, CLR1, and CLR2 via the input terminals T1 and T2 and the clear terminals T11 and T12, respectively. Input to the dog timer 14.

  On the other hand, the watchdog timer 14 is provided with latch circuits 21 and 22 for latching the control signals CLR1 and CLR2 as clear signals. In the present embodiment, both latch circuits 21 and 22 are realized by, for example, D flip-flops, and the D terminals of both latch circuits 21 and 22 are constantly maintained at a value indicating true, and the clear signal CLR1 and CLR2 are applied to the clock terminals of the corresponding latch circuits.

  Further, the watchdog timer 14 receives the following members 31 to 33 as clocking means, that is, a clock signal CLK having a predetermined cycle, and counts up a count value at a time interval indicated by the clock signal CLK. The counter 31, the time setting register 32 in which the count value corresponding to the time-up time is stored, the count value C32 stored in the time setting register 32 and the count value C31 of the counter 31 are compared. And a comparator 33 that changes the NMI signal to a value indicating true when the time-up time has elapsed since the last time the counter 31 was reset. The count value C32 may be fixed to a predetermined value, or may be set by the control unit 15 accessing the time setting register 32.

  A logic circuit (reset means) 41 is provided between the reset terminal R of the counter 31 and the latch circuits 21 and 22, and the control signals ENB1 and ENB2 are values indicating the third mode. In addition, the counter 31 can be reset when the output signals of both the latch circuits 21 and 22 are true. In the example of FIG. 1, the reset signal of the counter 31 is low active, the control signal ENB1 • ENB has a value indicating the third mode (both true), and both the latch circuits 21 • Only when the output signal 22 is true, the AND circuit 42 that outputs true and the output signal of the AND circuit 42 are input, and when any of the input signals is true, the output signal is made active (low). And an OR circuit 43 having a negative logic output.

  Further, in the present embodiment, since the first and second modes are provided, the logic circuit 41 is such that the control signals ENB1 and ENB2 are values indicating the first mode, and the output signal of the latch circuit 21 is true. The counter 31 can be reset both when the control signal ENB1 and ENB2 are values indicating the second mode and when the output signal of the latch circuit 22 is true. For example, as shown in FIG. 1, as the logic circuit 41, between the OR circuit 43 and the latch circuits 21 and 22, the control signals ENB1 and ENB2 indicate values indicating the first mode (true / false). In addition, only when the output signal of the latch circuit 21 is true, the AND circuit 44 that outputs true to the OR circuit 43, and the control signals ENB1 and ENB2 are values (false / true) indicating the second mode, In addition, an AND circuit 45 that outputs true to the OR circuit 43 only when the output signal of the latch circuit 22 is true is provided.

  The output of the OR circuit 43 is also input to the reset terminals R of the latch circuits 21 and 22. In the latch circuits 21 and 22, the counter 31 is reset by the outputs of the latch circuits 21 and 22. It is reset at the time.

  Note that, in the watchdog timer 14 according to the present embodiment, even when either the control signal RST for resetting the entire information processing apparatus 1 or the control signal WDTRST for resetting the watchdog timer 14 becomes true, the counter 31 can be reset, and in the example of FIG. 1, control signals RST and WDTRST input via the terminals T21 and T22 are input to the OR circuit 43. In FIG. 1, the fact that both control signals RST and WDTRST are low-active is indicated by adding “*” to the end of the symbol.

  Further, the watchdog timer 14 according to the present embodiment includes latch circuits 51 and 52 provided corresponding to the program processing units 11 and 12 as the status register 16 shown in FIG. Further, in the watchdog timer 14, when the output of the comparator 33 is true and the output of the latch circuit 21 does not indicate clear (both the output of the comparator 33 and the inverted output of the latch circuit 21 are true). Only when the output of the AND circuit 53 that sets the latch circuit 51 corresponding to the latch circuit 21 and the comparator 33 is true and the output of the latch circuit 22 does not indicate clear (of the comparator 33). An AND circuit 54 that sets the latch circuit 52 corresponding to the latch circuit 22 is provided only when both the output and the inverted output of the latch circuit 22 are true. The latch circuits 51 and 52 are assigned to areas accessible by arithmetic means such as a CPU that implements the control unit 15, for example, and the control unit 15 determines the values (WDT1P WDT2P) can be read out. Both the latch circuits 51 and 52 are realized by, for example, D flip-flops, and the D terminals of both latch circuits 51 and 52 are always maintained at a value indicating true, and the outputs of the AND circuits 53 and 54 Signals are applied to the clock terminals of the corresponding latch circuits.

  Further, a reset signal independent of the output of the comparator 33 is inputted to the reset terminal R of both the latch circuits 51 and 52 so that the value can be held even after the output of the comparator 33 becomes true. It is configured.

  In the present embodiment, when either of the control signals RST / WDTRST is true or when the control signal ENB1 is false, the OR circuit 55 that outputs a true value and the both control signals RST / WDTRST Is true, or when the control signal ENB2 is false, an OR circuit 56 that outputs a true value is provided, and the output of the OR circuit 55 is supplied to the reset terminal R of the latch circuit 51. The output of the OR circuit 56 is used as the output of the latch circuit 52. In the example of FIG. 1, both control signals RST and WDTRST and reset signals to both latch circuits 51 and 52 are both low active, and both control signals ENB1 and ENB2 are high active. The circuits 53 to 56 correspond to the latch circuit control means described in the claims.

  In the above configuration, when the runaway of the first and second program processing units 11 and 12 shown in FIG. 2 is monitored, the control unit 15 indicates the value indicating the third mode in the areas B1 and B2 of the control register 13 (both true). And the control signals ENB1 and ENB2 shown in FIG. 1 are set to true.

  Here, the counter 31 of the watchdog timer 14 shown in FIG. 1 is configured to output an NMI signal indicating a runaway when a timeout time elapses from the time when the watchdog timer 14 is reset.

  However, while the program processing units 11 and 12 are operating normally, the values (true) indicating normality in the regions B11 and B12 of the control register 13 at a time interval shorter than the timeout time of the watchdog timer 14. Since the clear signals CLR1 and CLR2 are set to true, the output signals of both the latch circuits 21 and 22 become true before the time-out period elapses from the time of the previous reset, and the counter 31 Is reset. Therefore, the time-out of the watchdog timer 14 does not occur while both the program processing units 11 and 12 are operating normally.

  On the other hand, when one of the program processing units 11 and 12 (for example, the first program processing unit 11) runs out of control, the program processing unit selects the corresponding region (B11 / B12) of the control register 13 ( In this case, the reset of the area B11) cannot be repeated. Therefore, both the output signals of the latch circuits 21 and 22 do not become true even if the timeout time elapses from the time of the previous reset. As a result, the comparator 33 outputs an NMI signal indicating a runaway when the timeout time has elapsed. Thereby, the control unit 15 grasps that the timeout of the watchdog timer 14 has occurred, and performs a predetermined process such as initializing both the program processing units 11 and 12, for example. Therefore, even if either of the program processing units 11 and 12 goes out of control, the operations of the program processing units 11 and 12 can be returned to normal operations.

  Further, when an NMI signal indicating a runaway is output, the latch circuits 51 and 52 latch the inverted outputs of the corresponding latch circuits 21 and 22, and until a reset signal independent of the NMI signal is input. During this time, the latched value continues to be held. For example, when only the first program processing unit 11 runs away as in the above example, since only the output of the latch circuit 22 is true, the output of the latch circuit 51 becomes a value (true) indicating the runaway, and the latch circuit The output of 52 becomes a value (false) indicating that no runaway occurs.

  Therefore, by reading the outputs WDT1P and WDT2P of the latch circuits 51 and 52 as the status register 16 shown in FIG. 2, the control unit 15 determines which of the program processing units 11 and 12 to be monitored has runaway. For example, it is possible to perform processing according to the runaway state of each program processing unit, such as initializing only the runaway person or presenting the runaway person to the user.

  As a result, since the runaway of the plurality of program processing units 11 and 12 is monitored by one counter 31, the circuit scale is reduced compared with the case where each monitoring counter is provided separately. , It can be determined whether or not each runaway, and more appropriate measures can be taken.

  On the other hand, when only the runaway of the first program processing unit 11 is monitored, the control unit 15 writes a value (true / false) indicating the first mode in the areas B1 and B2 of the control register 13, and the control shown in FIG. The signals ENB1 and ENB2 are set to true / false.

  In this state, the counter 31 is reset when the clear signal CLR1 from the first program processing unit 11 is true regardless of the clear signal CLR2 from the second program processing unit 12. Therefore, when the first program processing unit 11 runs away and the timeout time elapses from the previous reset time, an NMI signal indicating the runaway is output. Thereby, even in the first mode, even if the first program processing unit 11 to be monitored runs away, the first program processing unit 11 can be returned to normal operation.

  In this case, since the control signal ENB2 is false, the latch circuit 52 corresponding to the one not to be monitored is reset. Further, the value WDT1P of the latch circuit 51 corresponding to the monitoring target is set to a value corresponding to the output of the latch circuit 21 at the time of runaway detection until it is reset. Therefore, the runaway program processing unit 11 can be specified only by referring to the values WDT1P and WDT2P of the latch circuits 51 and 52 regardless of the mode.

  Similarly, when only the runaway of the second program processing unit 12 is monitored, a value (true / false) indicating the second mode is written in the areas B1 and B2 of the control register 13 and cleared by the first program processing unit 11 Regardless of the signal CLR1, the counter 31 is reset when the clear signal CLR2 by the second program processing unit 12 is true.

  In the above description, the case where the counter 31 counts up is described as an example. However, the present invention is not limited to this. For example, a configuration in which the counter 31 counts down may be used. In any case, the same effect can be obtained if an output signal indicating runaway can be output when the time-up time has elapsed since the last reset.

  Furthermore, although the case where there are two program processing units to be monitored has been described above as an example, the present invention is not limited to this. By providing the same members as the latch circuits 21 and 51, the AND circuits 42 and 53, and the OR circuit 55 for each program processing unit to be monitored, the runaway of three or more program processing units can be monitored without any trouble.

  In this embodiment, the monitoring target is switched by setting a value indicating whether monitoring is required in the bit areas B1 and B2 of the control register 13, but the present invention is not limited to this. For example, a plurality of monitoring targets may be fixed in advance. In this case, a circuit for processing the control signals ENB1 and ENB2 (a circuit corresponding to a combination other than the combination to be monitored among the bit areas B1 and B2 and the AND circuits 42 to 44) is unnecessary.

  However, as in the present embodiment, when the bit regions B1 and B2 of the control register 13 and a circuit for processing the output signals (control signals ENB1 and ENB2) are provided and the monitoring target can be switched, Without changing the operation of the program processing units 11 and 12 to be monitored, the control unit 15 can change the monitoring target only by changing the bit areas B1 and B2, so that it is possible to reduce time and labor when changing the monitoring target.

  Further, as in the present embodiment, by resetting the latch circuits 51 and 52 that are not the monitoring target, the program processing unit to be monitored is the latch corresponding to each case. By simply referring to the values of the circuits 51 and 52, the runaway program processing unit can be identified. In the present embodiment, the values of the latch circuits 51 and 52 are collected as the status register 16 and can be accessed by the control unit 15 at a time. In the example of FIG. 1, when the program processing unit is removed from the monitoring target, there is only one remaining program processing unit. However, there are three or more program processing units, and a certain program processing unit may be excluded from the monitoring target. In the case where there are a plurality of monitoring targets, it is possible to determine which of the plurality of monitoring targets is out of control only by reading out the outputs of all the latch circuits.

  Detect that one of the multiple processing units has runaway, for example, can perform processing for runaway, such as recovery processing from a runaway state, and after detection can identify which of them has runaway, For example, in order to improve safety and convenience, such as in the field of control systems, the present invention can be suitably used for applications that require some processing even after a processor runs away.

1, showing an embodiment of the present invention, is a circuit diagram illustrating a configuration of a main part of a watchdog timer. FIG. It is a block diagram which shows the principal part structure of the whole information processing apparatus provided with the said watchdog timer. It is a circuit diagram which shows a prior art and shows the principal part structure of a watchdog timer.

Explanation of symbols

11 First program processing unit (processing unit)
12 Second program processing unit (processing unit)
14 Watchdog timer 31 Counter (time measuring means)
32 hour setting register (time keeping means)
33 Comparator (time measuring means)
41 logic circuit (reset means)
51/52 Latch circuit 53/54 AND circuit (Latch circuit control means)
55/56 OR circuit (latch circuit control means)
T1 input terminal T2 input terminal T11 clear terminal T12 clear terminal

Claims (3)

In a watchdog timer having a timing means for outputting an output signal indicating runaway when a predetermined timeout time elapses,
Multiple clear terminals provided for each processing unit that can be set as a monitoring target,
A reset means for resetting the time measuring means when a signal indicating clear is input to all the clear terminals corresponding to the processing unit to be monitored;
A latch circuit and a latch circuit control means provided for each of the clear terminals,
The latch circuit control means outputs an output signal indicating runaway by the timing means, and sets a latch circuit corresponding to the clear terminal when a signal indicating clear is not input to the clear terminal. A watchdog timer, wherein a value is held in the latch circuit until a reset signal applied at a timing independent of an output signal indicating runaway is input. An input terminal to which a control signal indicating whether or not each processing unit is to be monitored is input;
2. The watch dog timer according to claim 1, wherein the reset means specifies the processing unit to be monitored based on a signal to the input terminal.   3. The watch dog timer according to claim 2, wherein the latch circuit control means resets a latch circuit corresponding to a processing unit indicating that an input signal to the input terminal is not monitored.

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