In ARM , Interrupt Priority is used. How is priority decided.
Is it done at hardware level or software level.
Hardware level - Is there an option in Interrupt Controller to set the interrupt priority.
Software level - Priority is calculated by the kernel and the corresponding interrupt handler is executed based on priority.
Thanks in advance.
The priority of interrupts is determined by the interrupt controller. And the interrupt controller should be programmable for this configuration.
For hardware interrupt, there is no software level priority, so far as I know.
Arm refers to interrupts as a subset of exceptions.
The first three exceptions have the highest priority, and their priority may not be changed. These are Reset, NMI, and Hard Fault.
Exception 4 to 15 priority can be set with the SCB, System Control Block.
Other exception priorities are set with the NVIC registers.
To learn more, read the Arm architecture manual. Chip manuals tend to gloss over the core details.
Related
Is there any way to disable all irq from Cortex M3 MCU except one ?
My issue is that I have a system running several kinds of irq with various priority levels and I want to disable all irq except one in a particular state.
I know I can disable all irq by using "__disable_irq()" instruction but I can't enable one irq after calling this instruction if I didn't call "__enable_irq()" before.
Thanks for your help,
Regards
Use the BASEPRI register to disable all interrupts below the specified priority level.
This is a core register, described in the Cortex-M3 Programming Manual.
CMSIS provides the __get_BASEPRI() and __set_BASEPRI() functions to manipulate its value.
Note that bits 7-4 are used, the priority value must be shifted left by 4. To disable all interrupts with priority 1 or lower, use
__set_BASEPRI(1 << 4);
and to enable all, set it to 0
__set_BASEPRI(0);
You should of course set interrupt priorities accordingly, ensuring that no other interrupt has priority 0.
Other than by disabling all the enabled interrupts you don't want, no.
__disable_irq() is implemented as CPSID I, which turns off all exceptions which can have a priority set (those configured in the NVIC), it achieves this by changing the PRIMASK register (setting bit 0) within the CPU. There is no way to tell this to only enable a specific interrupt.
I have started using ARM Cortex M0+ for GPIO Interrupts. I want to disable nesting feature from ARM Interrupts. Is there any way to do it.? I know by default, nesting is enabled in ARM, I want to disable it.
ARM Cortex-M0/M0+ do not support interrupt priority grouping into preemption priority(nestable) and sub-priority (non-nestable) available on the M3/M4/M7 for example.
If you wish to prevent interrupt nesting; it would be necessary to either;
set all interrupts to the same priority, or
disable and re-enable interrupts on entry and exit to all handlers.
The first of these options is the simplest, but gives no control over execution order (which seldom matters for asynchronous events, but may lead to non-deterministic behaviour and timing). The second does not actually prevent nesting, but does allow nesting only before the lower-priority interrupt has disabled the interrupts - before it has started processing the actual event. The result is behaviour similar to that of sub-priorities available on Cortex-M3 etc.
I have read this question, It is not a valid answer when one interrupt is executing it will not disable all the other interrupts always. It is based on the interrupt type (in some case we need to do manually in our program).
My Question is what happens when a interrupt occurs while executing a
interrupt ??? If Low priority interrupt is executing then High
priority interrupt occurs the what will happen ?
It depends on the system. If the microcontroller/interrupt controller supports nested interrupts and the application enables that feature then a higher priority interrupt will interrupt a lower priority interrupt. In this case the lower priority interrupt will resume when the higher priority interrupt is complete. But if the system does not support nested interrupts then the subsequent interrupt request will pend and be serviced when the active interrupt service routine is complete.
It is too broad for SO, I think and it is arch based.
I try to give you a brief overview, expecting some DV on it. ;)
Mainly, if the arch allow nested interrupts, the interrupt with lower priority is interrupted while executing to jump to the ISR of high level interrupt.
But you can have NMI (Non Maskable Interrupt) that have priority on all other interrupts and cannot be disable.
Usually (all I think) archs have also a global interrupt enable flag, so it must be enabled to allow other interrupts to be served. Also means that an ISR, when is executing, can disable other interrupts during its job.
You can think, for example, on an RTOS implementation: the scheduler can be easily developed using a Timer within its interrupt. This interrupt must have the lower priority and mustn't stops other interrupts (usually): this grant that interrupts are served a soon as possible not considering context switch of RTOS scheduler.
The question was
What happens when a interrupt occurs in RTOS while currently in any task or another ISR?
I have written two commercial RTOS's and there is no answer that satisfies all of the criteria. However, I CAN answer as broadly as the question:
Depending on what is allowed, it will act as a normal interrupt. The problem with this question is that "what happens" is a little broad - some RTOS's do work behind the scenes with interrupts. So, the problem is that the question is not specific enough.
"What happens" in regards to a task is that NOTHING happens in regards to a task. An interrupt is an interrupt and what the relation to a task is depends on the programming. Since I don't read minds, again, the question is not specific enough.
The BEST answer is 42 (HHGTTG)
There is no one answer, sometimes nothing happens the lower prio interrupt continues to completion, sometimes the higher one interrupts the lower. It depends first on the chip/system design, second it depends on the individual programmers across the board RTOS and application folks.
Or another way to say it is, what happens is what those individuals desired to happen in their design and implementation.
Is it a feature of Cortex-M series MCUs to be able to wake from a low power mode (sleep or deepsleep or other MCU specific modes) with interrupts configured but disabled?
The mechanism for entering the low power mode is executing WFI instruction.
The MCUs I have used from STM and Renesas, wake from the low power mode via an interrupt if the interrupt is configured correctly and even if all interrupts are disabled via the cpsid instruction.
I am trying to determine is this is what I should expect on all Cortex M series based MCUs or just specific to certain MCU vendors.
Every ARM Cortex-M will work this way as long as you use PRIMASK to disable ALL interrupts. This is what __disable_irq() from CMSIS does. According to the docs, raising priority with BASEPRI will not cause an interrupt with insufficient priority to wake the chip.
See here - http://infocenter.arm.com/help/index.jsp?topic=/com.arm.doc.dui0552a/BABFEFIG.html
Operation
WFI is a hint instruction that suspends execution until one of the following events occurs:
a non-masked interrupt occurs and is taken
an interrupt masked by PRIMASK becomes pending
a Debug Entry request.
I am new to arm & have some doubs related to IRQ & FIQ. Please try to clarify these.
How many number of FIQ & IRQ channel arm have ?
And what number of handlers can we write for each channel ?
Also if we can register multiple handler for single interrupt channel how arm comes to know which handler to run.
The distinction between IRQ and FIQ goes right the way back to early days of ARM when it was designed by Acorn. It was always the case that the IRQ line was attached to an interrupt controller that multiplexed a large number of interrupt sources together. This is precisely what happens in all modern ARMs
The rationale behind the FIQ was to provide an extremely low latency response with maximum priority (it can safely pre-empt the IRQ handler). The comparatively large number of shadow registers facilitate writing handlers that store the handler's state in CPU registers and not hitting the stack.
The shadow registers are almost of the opposite set to those commonly used by APCS for function call, so writing handlers in C, would cause a push and eventual pop of up to 8 non-shadowed registers. Having any kind of interrupt demultiplexing wipes out any performance advantage that FIQ might have given.
All of this means that there is only really any benefit in using FIQ for very specialised applications where really hard-real time interrupt response is required for one interrupting device, and you're willing to write your handler in assembler. You'll also be left with working out how to synchronise with the rest of the system - some of which would rely on disabling IRQ to keep data synchronised.
traditionally the arm has one interrupt line which you can send to one of two handlers FIQ or IRQ. FIQ has a larger bank of FIQ mode only registers so you have fewer that you need to store on the stack. From there you read the vendor specific registers if any to determine the source of the interrupt and then branch into separate handlers.
More recently there have bend arm architectures with many interrupts 128, 256 each with a separate handler. So generically asking about arm is not as varied but about like asking something generic about x86.
All of this information is easily available in the ARM architectural reference manuals for the different architectures and the pinouts to the core (what the vendor builds its chip around) is documented in the technical reference manuals for the various cores (also very easy to obtain). infocenter.arm.com has the architecture and technical reference manuals as well as amba/axi (the data bus that the vendor connects to). Your question is completely answered in those documents.
The ARM processor directly supports only ONE IRQ and ONE FIQ. ARM supports multiple interrupts through a peripheral called Interrupt Controller. ARM standard interrupt controllers are called GIC (Generic Interrupt Controller).
The GIC has a number of inputs for peripherals to connect their interrupt lines and two output lines that connect to IRQ and FIQ. Basically it acts as a MUX. A GIC driver will setup configurations such as interrupt priority, type (IRQ/FIQ), masking etc.
In traditional ARM systems there is one entry each for IRQ and FIQ in the Exception Vectors. Depending on which line the interrupt fired, IRQ or FIQ handler is called. The interrupt handler queries the GIC (GIC CPU interface registers, to be specific) to get the interrupt number. Based on this interrupt number, corresponding device handler is invoked.
Number of interrupts depends on the specific GIC implementation. So you would have to check the manual for the interrupt controller in your system to get those specifics.
Note: The interrupt handling is slightly different depending on which specific ARM core you are coding for.
Actually the question is a bit tricky. You must specify in the question to which architecture in ARM you work. ARM v7-A and ARM v7-R Architecture Reference Manual (ARM ARM) specifies one FIQ and one IRQ, as many already answered. But ARMv7-M (used in Cortex-M processors) integrates a interrupt controller in the processor, and thus offers one NMI (instead of FIQ) and up to 240 IRQ lines.
For more information: ARMv7 A and ARMv7-R Architecure reference manual: http://infocenter.arm.com/help/index.jsp?topic=/com.arm.doc.ddi0406c/index.html
ARMv7-M Architecture Reference Manual: http://infocenter.arm.com/help/index.jsp?topic=/com.arm.doc.ddi0403e.b/index.html
As an example, Cortex M4 specs sheet: http://www.arm.com/products/processors/cortex-m/cortex-m4-processor.php