When do interrupts occur

Using the pointer to the interrupt handler table and the interrupt vector, the location of the kernel code to execute is determined. The interrupt vector is the IRQ for hardware interrupts read from an interrupt controller register and an argument to the interrupt assembly language instruction for software interrupts. Processing is switched to the appropriate portion of the kernel. Table Of Contents 1. Basics of How Operating Systems Work 1. Role of Interrupts 1. CPU Execution Mode 1.

Computer Architecture Next topic 1. Quick search. Created using Sphinx 1. A busy-wait interface will read the appropriate RIS bit over and over, until it is set. With interrupt synchronization, the initialization phase will arm the trigger flag by setting the corresponding IM bit. In this way, the active edge of the pin will set the RIS and request an interrupt. The interrupt will suspend the main program and run a special interrupt service routine ISR.

At the end of the ISR it will return, causing the main program to resume. In particular, five conditions must be simultaneously true for an edge-triggered interrupt to be requested:. The other ports have similar registers. We will begin with a simple example that counts the number of rising edges on Port F bit 4 Program The initialization requires many steps. In this case we will trigger on the rise of PF4.

We do not wish to count a rising edge that might have occurred during the power up phase of the system. There is no need to unlock PF4.

EdgeInterrupt example Code Demo. This initialization is shown to enable interrupts in step i. However, in most systems we would not enable interrupts in the device initialization. Rather, it is good design to initialize all devices in the system, then enable interrupts. All ISRs must acknowledge the interrupt by clearing the trigger flag that requested the interrupt. If two or more triggers share the same vector, these requests are called polled interrupts , and the ISR must determine which trigger generated the interrupt.

If the requests have separate vectors, then these requests are called vectored interrupts and the ISR knows which trigger caused the interrupt. One of the problems with switches is called switch bounce.

Many inexpensive switches will mechanically oscillate for up to a few milliseconds when touched or released. It behaves like an underdamped oscillator. In some cases this bounce should be removed. To remove switch bounce we can ignore changes in a switch that occur within 10 ms of each other. In other words, recognize a switch transition, disarm interrupts for 10ms, and then rearm after 10 ms.

Alternatively, we could record the time of the switch transition. If the time between this transition and the previous transition is less than 10ms, ignore it. If the time is more than 10 ms, then accept and process the input as a real event. If no device needs service, then the interrupt simply returns. On the left is busy-wait, and on the right is periodic polling. Periodic polling is appropriate for low bandwidth devices where real-time response is not necessary. This method frees the main program to perform other functions.

We use periodic polling if the following two conditions apply:. For a data acquisition system, it is important to establish an accurate sampling rate. The time in between ADC samples must be equal and known in order for the digital signal processing to function properly. Similarly for microcontroller-based control systems, it is important to maintain a periodic rate for reading data from the sensors and outputing commands to the actuators.

The SysTick timer is a simple way to create periodic interrupts. A periodic interrupt is one that is requested on a fixed time basis. This interfacing technique is required for data acquisition and control systems, because software servicing must be performed at accurate time intervals.

SysTick has a bit counter that decrements at the bus clock frequency. SysTick is the only interrupt on the TM4C that has an automatic acknowledge. Example Solution: To make sound we need to create an oscillating wave. In this example, the wave will be a simple square wave. The maximum the TM4C can produce on an output pin is 8 mA. If we place a resistor in series with the head phones, then the current will only be 3.

To generate the 1 kHz sound we need a 1 kHz square wave. There are many good methods to generate square waves. In this example we will implement one of the simplest methods: period interrupt and toggle an output pin in the ISR. Since this sound wave output is a real time signal, we set its priority to highest level, which is 0. See Program A squarewave output connected to a speaker will generate sound. EnableInterrupts ;. Observation: To make a quieter sound, we could use a larger resistor between the PA5 output and the speaker.

The DC motor has a frame that remains motionless called the stator , and an armature that moves called the rotor. A brushed DC motor has an electromagnetic coil as well, located on the rotor, and the rotor is positioned inside the stator. In Figure In this case, the rotor moves in a circular manner. When current flows through the coil, a magnetic force is created causing a rotation of the shaft. A brushed DC motor uses commutators to flip the direction of the current in the coil.

In this way, the coil on the right always has an up force, and the one on the left always has a down force. Hence, a constant current generates a continuous rotation of the shaft. When the current is removed, the magnetic force stops, and the shaft is free to rotate. In a pulse-width modulated DC motor, the computer activates the coil with a current of fixed magnitude but varies the duty cycle in order to adjust the power delivered to the motor.

Working of a DC Motor. A brushed DC motor uses a commutator to flip the coil current. In the LED interface the microcontroller was able to control electrical power to the LED in a binary fashion: either all on or all off. Sometimes it is desirable for the microcontroller to be able to vary the delivered power in a variable manner. One effective way to do this is to use pulse width modulation PWM.

The basic idea of PWM is to create a digital output wave of fixed frequency, but allow the microcontroller to vary its duty cycle.

The duty cycle is defined as the fraction of time the signal is high:. Hence, duty cycle varies from 0 to 1. We interface this digital output wave to an external actuator like a DC motor , such that power is applied to the motor when the signal is high, and no power is applied when the signal is low.

The average value of a PWM signal is linearly related to its duty cycle and is independent of its frequency. Under conditions of constant speed and constant load, the delivered power to the motor is linearly related to duty cycle. Unfortunately, as speed and torque vary, the developed emf will affect delivered power. Nevertheless, PWM is a very effective mechanism, allowing the microcontroller to adjust delivered power.

The inductance in the coil L arises from the fact that the wire is wound into coils to create the electromagnetics. The coil itself can generate its own voltage emf because of the interaction between the electric and magnetic fields. If the coil is a DC motor, then the emf is a function of both the speed of the motor and the developed torque which in turn is a function of the applied load on the motor.

Because of the internal emf of the coil, the current will depend on the mechanical load. For example, a DC motor running with no load might draw mA, but under load friction the current may jump to 1 A. The system uses a transistor to turn the motor on and off.

There are lots of motor driver chips, but we will use an NPN Darlington transistor, e. If the port output is low, no current can flow into the base, so the TIP transistor is off, and the collector current, I C , will be zero. We select the value of the R b resistor to operate the transistor in its saturated mode.

We start with the desired motor current, which will be the collector current I c. The voltage across the coil will be the V - V CE. Next, we calculate the needed base current I b given the current gain of the NPN.

Finally, given the output high voltage of the microcontroller V OH is about 3. The inequality means we can choose a smaller resistor, creating a larger I b. Because the h fe of the transistors can vary a lot, it is a good design practice to make the R b resistor 0. Since the transistor is saturated, the increased base current produces the same V CE and thus the same coil current.

V BEsat. Up to 5A. Design an interface for an 8-V mA geared DC motor. SW1 will make the motor spin faster and SW2 will make it spin slower.

Just like the stepper motor, when designing with motors we will not use the 3. Although the current is only mA when spinning unloaded, it will increase to 1 A if the motor experiences mechanical friction. The needed base current is. Motors and transistors vary a lot, so it is appropriate to experimentally verify the design by measuring the voltages and currents. Interrupts are triggered on the falling edges of PF4 and PF0. The period of the PWM output is chosen to be about 10 times shorter than the time constant of the motor.

The electronic driver will turn on and off every 1ms, but the motor only responds to the average level. The software sets the duty cycle of the PWM to adjust the delivered power. In this article, we will look at what happens after an interrupt occurs. When an interrupt occurs, it causes the CPU to stop executing the current program. The control then passes to a special piece of code called an Interrupt Handler or Interrupt Service Routine. The interrupt handler will process the interrupt and resume the interrupted program.

But have you ever wondered how the initial program is resumed? How does the CPU know where to continue from, and what values were being computed when the it occurred? This happens by means of a context switch, which forms a vital part of interrupt handling.

A context switch is the process of storing and restoring state the context of a CPU, so that execution can be resumed from the same point at a later time. When it is generated, the processor saves its execution state via a context switch, and begins executing the interrupt handler at the interrupt vector. In a switch, the state of the first process must be saved, so that when the scheduler gets back to executing the first process, it can restore this state and continue.

The state of the process includes all registers that the process may be using, including the program counter PC. Note: Usually, the value of the PC is preserved by the underlying hardware, so you might not bother about it. Article Contributed By :. Easy Normal Medium Hard Expert. Writing code in comment? Please use ide. Load Comments. What's New. We use cookies to ensure you have the best browsing experience on our website.