Fail-Safe Circuit Using Flip-Flop Masking for Microcomputer Reset
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Solution Overview
Problem
In control circuits for vehicles, there is a challenge in ensuring that a switch can be safely turned on/off by an external device even when the microcomputer controlling it is in an abnormal state or when the power supply for backup control is lost, particularly due to the temporary loss of control and power supply issues during initialization and malfunction.
Innovation Solution
A fail-safe circuit is implemented, including a control circuit, a watchdog circuit, and a flip-flop circuit with a mask circuit, where the flip-flop is set and reset based on reset signals and watchdog pulses, allowing the switch to be controlled by an external instruction signal even during microcomputer resets and power supply failures.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Reliability
If a watchdog circuit is used to monitor microcomputer operation and generate reset signals, then system reliability is improved, but the control circuit becomes temporarily uncontrollable during initialization after reset
Solution Approach 1:
An SR flip-flop circuit is introduced as an intermediary between the watchdog circuit and the control circuit. The flip-flop maintains the control signal state during the microcomputer's initialization period after reset, preventing temporary loss of control. The flip-flop's set/reset mechanism allows it to hold the previous control state while the microcomputer restarts, thus mediating between the watchdog's reset function and continuous control availability.
2Reliability
If backup control is implemented using an OR gate and AND gate configuration, then control safety is improved during microcomputer failure, but device complexity increases
Solution Approach 1:
The patent merges the backup control function with the main control path by using the SR flip-flop's Q output that naturally feeds into the existing OR gate configuration. The flip-flop is integrated into the control signal path such that its output combines with the microcomputer's control signal through the OR gate, eliminating the need for separate backup control circuits while maintaining safety functionality.
Solution Approach 2:
The SR flip-flop serves multiple functions: it acts as a state memory element, a backup control mechanism during microcomputer failure, and a signal synchronization device. This multi-functionality reduces the need for additional dedicated backup circuits, thereby reducing overall device complexity while maintaining control safety.
3Reliability
If the control circuit is reset by the watchdog circuit during abnormal operation, then system reliability is improved, but the switch becomes temporarily uncontrollable during the reset initialization period
Solution Approach 1:
The SR flip-flop is configured to maintain the control signal state in advance before the microcomputer completes its initialization. By setting the flip-flop's state based on the previous valid control signal before reset occurs, the system prepares the control path ahead of time, ensuring continuous controllability during the initialization period without waiting for the microcomputer to fully restart.
Data Source
AI summary
A fail-safe circuit enables a switch to turn on/off according to a signal from an external device if a microcomputer that controls the turning on/off of the switch falls into an abnormal state and is reset, and if the power supply for a circuit that backs up the control of the switch is lost. The fail-safe circuit includes a microcomputer that controls the turning on/off of a switch based on an instruction signal from an input terminal, a watchdog circuit that generates a reset signal based on a watchdog pulse from the microcomputer, and a transistor for masking a watchdog pulse for resetting a flip-flop circuit that is set by the reset signal. If a voltage supplied by a power supply circuit is lost, a transistor turns off, and therefore the switch turns on/off according to an instruction signal supplied to an output terminal via a resistor and a diode.