I'm studying the FreeRTOS scheduler (port.c) and have a question about Systick handler. I understand xPortSysTickHandler() is supposed to be called at RTOS ticks, but I don't see where it gets specified by saying xPortSysTickHandler() is the Systick timer interrupt callback function.
Especially, I didn't see that in vPortSetupTimerInterrupt(), which seems a bit strange to me since this function does configure, such as load register value, of the timer.
Does it gets specified as the callback somewhere else? Or is there something I'm missing?
Thanks!
It is installed directly into the interrupt vector table. If the vector table uses CMSIS names for the handlers then you can map the CMSIS name to the name of the FreeRTOS systick handler in FreeRTOSConfig.h, as per the FAQ - see the red "special note to ARM Cortex-M users" here: https://www.freertos.org/FAQHelp.html
Related
I am doing some bare metal C development on an ARM Cortex M3 SoC, and I wanted to check and see if it is possible to add a new user-defined interrupt handler to the NVIC. I am adding my own IRQ with the plan of triggering it via software, either via NVIC_SetPendingIRQ() or via the NVIC->STIR register. Neither seem to work.
I have added my interrupt vector name to the end of the vector list in the CMSIS startup assembler file, and added the corresponding enum to the system header, and while debugging and executing the function call NVIC_EnableIRQ(), it doesn't correctly update the NVIC->ISER (Interrupt Set Enable Register). So I guess, the question is, can you even add your own interrupt? There are 256 total interrupts than can be used in the ARM Cortex M3, and I just followed how the others were added so I figured it wouldn't be an issue.
Thank you.
The datasheet for your SoC should say how many interrupts are supported by the NVIC. While 240 is the maximum possible number for a Cortex-M3 device in general, the actual number on your chip is defined by the implementation and it makes sense to have that number be as small as possible to reduce costs.
In general, there is no way to add interrupts in software, but you might be able to use the SVCall interrupt, which is designed to be triggered by software. Or you could find some other interrupt you aren't using in your system, and which is not being activated by hardware, and try to use that for your purposes.
References:
Nested Vectored Interrupt Controller in the Cortex-M3 Devices Generic User Guide
The SVC instruction invokes the SVCall handler with an 8-bit service number available to the handler which can be used to invoke a handler from a look-up table (essentially a secondary vector table for software interrupts).
An example of that can be found at https://developer.arm.com/documentation/ka004005/latest, except there it uses a switch rather than a look-up table - to the same effect.
I am using HAL library for my project with STM32 microcontroller.
In the sample code provided by STM, they use HAL_GPIO_EXTI_Callback for a push button interrupt. However, I learned in class that we need to use the IRQHandler to handle the interrupt.
So my question is when we use HAL_GPIO_EXTI_Callback and HAL_GPIO_EXTI_IRQHandler to handle interrupt?
In ST's example projects, there are several function calls for a single interrupt:
the interrupt handler itself is called EXTI15_10_Handler. It is the function whose address is specified in the vector table. It is the generic handler for the interrupt line EXTI15_10 and by default it is "routed" to a default function doing nothing. As it is a weak symbol you can redefine it as you want.
if you are using EXTI15_10 interrupt line with standard GPIOs, EXTI15_10_Handler should call HAL_GPIO_EXTI_IRQHandler. This is a function provided by the HAL.
the latter function is clearing the IT flag and then calls HAL_GPIO_EXTI_Callback, which is also a weak function that is doing nothing by default. This is the function that you need to implement to process your GPIO.
I'm using the std library. I don't know what the difference between I2C_FLAG_TXE and I2C_IT_TXE is.
Why when in interrupt function we don't use I2C_ClearFlag instead I2C_ClearITPendingBit? When do we used I2C_ClearFlag?
I'm starting learn stm32f4. I have very little experience.
I'm using the std library. I don't know what the difference between
I2C_FLAG_TXE and I2C_IT_TXE is.
From the processor point of view - the interrupt flag which has to be cleared by interrupt routine is set when the processor enters the interrupt.
Interrupt pending flag - indicates that the event which triggers the interrupt occurred but for some reason the interrupt routine has not been invoked yet.
It is good to know your hardware before using any libraries.
I am starting to use FreeRTOS and I would like a interrupt to preempt whatever task was about to run and run the task I need to run critically.
Is there a way to do this in FreeRTOS? (Is this achieved through task priority?)
NO! Both the above answers are DANGEROUS.
Do NOT use taskENTER_CRITICAL() or taskEXIT_CRITICAL() inside an ISR - it is unusual to need a critical section in an ISR but if you do then use taskENTER_CRITICAL_FROM_ISR()/taskEXIT_CRITICAL_FROM_ISR(). (possible the AVR32 port is an exception to that rule?)
Do NOT use xTaskResumeFromISR() to synchronise a task with an interrupt. The link already posted to the documentation for that function even says this.
If my understanding of your question is correct you want the ability to have an interrupt unblock a task, and then if that task is the highest priority task in that is able to run, have the interrupt return directly to the unblocked task. If my understanding is right then there is an example of how to do that in an efficient way on the following page: http://www.freertos.org/RTOS_Task_Notification_As_Counting_Semaphore.html
The short answer would be: Yes, this is achieved through task priority.
The FreeRTOS kernel will consider swapping in any task in ready-state after an ISR has completed so it would preempt the current running task if a higher priority task is now ready.
It should be mentioned that this is really only true if the handler is called through FreeRTOS. On a Cortex-A processor there is a common IRQ entry-point in the IRQ or FIQ exception handler which is most likely handled by FreeRTOS or otherwise by an IRQ dispatcher which is easily wrapped by FreeRTOS, usually by a function in the port-layer called vApplicationIRQHandler().
On a Cortex-M this is not necessarily the case as the vector is usually manipulated by the vendor's MCU API. On a Cortex-M I'd safe-guard against this using portYIELD_FROM_ISR() in the ISR which should be implemented to provide the kernel with an opportunity to perform a context switch.
You can use xTaskResumeFromISR to do this.
There is a number of conditions to be met for the yielded task not to be interrupted by other tasks (like it's priority must be high enough) and a number of other conditions to be met to ensure that no interrupt can go un-serviced (like the yielded task must guarantee to be done before the next interrupt)
1. enable preemption:
This is very simple to do.
All the configuration options of FreeRTOS are under "FreeRTOSConfig.h"
#define configUSE_PREEMPTION 1
You can set this to 1 to use the preemptive RTOS scheduler, or 0 to use the cooperative RTOS scheduler.
Check this link for more info
2. Use critical section inside ISR
void taskENTER_CRITICAL( void );
//action
void taskEXIT_CRITICAL( void );
RTOS wont do anythis extra inside this critical part
ref: here
I'm studying operating systems and I encountered both the terms ISR and interrupt handler. Are they two words for the same mechanism? If not, what is the difference?
There is no difference in Interrupt handler and ISR.
Wiki says that:
In computer systems programming, an interrupt handler, also known as an interrupt service routine or ISR, is a callback function [...]
ISR is callback for a specific service pertaining to a device/operation/source. There could be multiple ISRs present in a system depending on addresses available in Interrupt Vector table. Where is Interrupt handler is a common routine which is triggered whenever any interrupt comes. Its job is to understand the source of the interrupt and trigger appropriate ISR mapped in Interrupt Vector table.
When interrupt occurs,
interrupt handler performs minimal operations required to respond to the device where as updating the buffer and all other operations are taken care by ISR