Книга: Real-Time Concepts for Embedded Systems
10.6 The Nature of Spurious Interrupts
10.6 The Nature of Spurious Interrupts
A spurious interrupt is a signal of very short duration on one of the interrupt input lines, and it is likely caused by a signal glitch.
An external device uses a triggering mechanism to raise interrupts to the core processor. Two types of triggering mechanisms are level triggering and edge triggering. Figure 10.11 illustrates the variants of edge triggers (rising edge or falling edge). This kind of triggering is typically used with a digital signal.
Figure 10.11: Edge triggering on either rising or falling edge.
In contrast, level triggering is commonly used in conjunction with an analog signal. Figure 10.12 illustrates how level triggering might be implemented in a design. It is important to note that when using level triggering, the PIC or microcontroller silicon typically defines the trigger threshold value.
Figure 10.12: Level triggering.
How do spurious interrupts occur? In real-world situations, digital and analog signals are not as clean as portrayed here. The environment, types of sensors or transducers, and the method in which wiring is laid out in an embedded design all have a considerable effect on how clean the signal might appear. For example, a digital signal from a switch might require debouncing, or an analog signal might need filtering. Figure 10.13 provides a good illustration of how both digital and analog signals can really look. While electronic methods for debouncing and filtering fall beyond the realm of this book, it is important nonetheless to understand that input signals, whether for interrupts or other inputs, might not be as clean as a developer might envision them. These signals, therefore, can represent a potential source for sporadic behavior.
Figure 10.13: Real signals.
As can be seen, one reason for the occurrence of spurious interrupts is unstableness of the interrupt signal. Spurious interrupts can be caused when the processor detects errors while processing an interrupt request. The embedded systems programmer must be aware of spurious interrupts and know that spurious interrupts can occur and that this type of interrupt must be handled as any other type of interrupts. The default action from the kernel is usually sufficient.
Оглавление книги
- 4.4.4 The Dispatcher
- About the author
- Chapter 7. The state machine
- Appendix E. Other resources and links
- Example NAT machine in theory
- The final stage of our NAT machine
- Compiling the user-land applications
- The conntrack entries
- Untracked connections and the raw table
- Basics of the iptables command
- Other debugging tools
- Setting up user specified chains in the filter table