JPS62296247A - Watch dog timer - Google Patents

Abstract

PURPOSE:To considerably reduce the probability that the count-clear of a watch dog timer is mistakenly executed by setting the count clear value of the watch dog timer to be cleared by means of the writing of specific data, and setting specific data to be changed with every clearance. CONSTITUTION:The timer 1 ticks the time and it generates an interruption request signal 2 when the count value overflows. A comparator 3 compares data transmitted from a central processing unit 4 and holding data in a register 5, and generates a count clear signal 6 when both data coincide. The count clear signal 6 clears the count value of the timer 1 and changes the holding data in the register 5. Thus, the timer 1 is count-cleared only when data equivalent to that in the register 5 is transmitted to the comparator 3 from the central processing unit 4.

Description

Detailed Description of the Invention 3. Detailed Description of the Invention [Field of Industrial Application] The present invention relates to a watchdog timer for detecting runaway of a microcomputer.

[Prior Art] Conventionally, as a means for detecting runaway of a microcomputer, there is a method of using a timer whose count value can only be cleared and cannot be stopped using software. This evening...
I now call it the watchdog timer.

Runaway detection using a watchdog timer is possible by programming the watchdog timer so that the count value is cleared before it overflows.

In the event of a runaway, the watchdog timer is not cleared for a certain period of time, so the watchdog timer overflows and generates an interrupt, and this interrupt is used to detect the runaway.

[Problem that the invention seeks to solve]

In the conventional watchdog timer described above, the watchdog clear instruction is a specific instruction or a general data write instruction, and the above instruction is accidentally generated due to program runaway. If the own code occurs, the watchdog timer is erroneously counted and cleared, so there is a problem that the runaway detection function cannot be performed at all.

[Means for solving problems]

The watchdog timer of the present invention includes a timer that generates an interrupt request when the count value overflows, a register that holds data, and a register that checks whether the data held in this register matches the data sent from the central processing unit. and a comparator that generates a count/clear signal that, when detected, counts and clears the timer and changes the data in the register.

〔Example〕

Next, the present invention will be explained with reference to the drawings.

FIG. 1 is a block diagram of one embodiment of the present invention.

Timer 1 is keeping time, and when the count 1 to 20 exceeds 20, it generates an interrupt request signal 2. The comparator 3 compares the data sent from the central processing unit 4 with the data held in the register 5, and generates a clear signal 6 when both data match. The count value of timer l is cleared by count clear signal 6,
Also, the data held in register 5 is changed.

With the above configuration, the timer 1 is cleared only when data equal to the register 5 is transferred from the central processing unit 4 to the comparator 3.

Furthermore, the data held in the register 5 is changed each time it is cleared.

The data held in the register 5 may be converted so that, for example, "0" and "1" are alternately exchanged, or may be a 2-bit count value.

Since the central processing unit 4 recognizes this change in the data held in the register 5, it sends data according to the change to the comparator 3 at regular intervals.

When the program is executed normally, the comparator 3 generates a count clear signal 6 because this data matches the data held in the register 5.
Since timer 1 is cleared before overflow, it does not generate interrupt request signal 2.

If a runaway program occurs, the central processing unit 4 generally does not send out the same data as the data held in the register 5, and the timer l generates an interrupt request signal 2, which makes it possible to detect the program runaway. can.

However, even if the central processing unit 4 is running out of control, it is possible that the central processing unit 4 may accidentally send a positive value to the comparator 3. In order to avoid such a coincidence, the data held in the register 5 is changed one after another. As a result, in order for the central processing unit 4 to send out a positive value, it must follow this change, but this is difficult during a runaway program.

〔Effect of the invention〕

As explained above, the present invention clears the current value of the watchdog timer by writing certain specific data, and changes the specific data each time it is cleared. The watchdog timer may be incorrectly counted due to a runaway program.
This has the effect of greatly reducing the probability of clearing.

[Brief explanation of the drawing]

FIG. 1 is a block diagram of an embodiment of the present invention. 1...Timer, 2...Interrupt request signal, 3...Comparator, 4...Central processing unit, 5...Register, 6 ...Count/clear signal.

Claims (1)

[Claims] A timer that generates an interrupt request due to an overflow of the count value, a register that holds data, and clearing the count of the timer when a match is detected between the data held in the register and the data sent from the central processing unit. and a comparator that generates a count/clear signal that changes data in the register.