If I have a set of peripherals in an AVR microcontroller with equal priority, does the microcontroller use round-robin as a suitable arbitration mechanism for interrupting the sub system?
Or else, how can it manage interrupts with the same priority that happen at same time?
It depends.
For example "classical" AVR microcontrollers have simple one-level interrupt controller. That means, when interrupt is running, interrupt flag in SREG is cleared, thus blocking any other interrupt from running. IRET instruction enables this flag back again, and
after one instruction from the main code is executed, next interrupt is ready to be executed.
When several interrupt requests are asserted simultaneously, then only the one with the lowest interrupt vector address is chosen.
For example, refer to the ATMega328P datasheet (section 6.7 Reset and Interrupt Handling, page 15):
The lower the address the higher is the priority level.
Thus, if interrupt request flag is not cleared, or reasserted before return of the interrupt handler, the same interrupt will run again, and interrupt handlers with higher interrupt vector addresses might be never executed.
But in the newest versions of the architecture there is a more advanced interrupt controller, which allows to enable Round Robin scheduling, and assign to one of the interrupts a higher level (allowing it to be executed even if another interrupt handler is running).
For example in ATmega3208 (refer to the datasheet, section 12. CPU Interrupt Controller):
All interrupt vectors other than NMI are assigned to priority level 0 (normal) by default. The user may override this by assigning one of these vectors as a high priority vector. The device will have many normal priority vectors, and some of these may be pending at the same time. Two different scheduling schemes are available to choose which of the pending normal priority interrupts to service first: Static and round robin
So, the answer is: carefully read the datasheet on the part you're working with.
Section 9 of the ATmega328PB datasheet is entitled "AVR CPU Core" and it says:
All interrupts have a separate interrupt vector in the interrupt vector table.
The interrupts have priority in accordance with their interrupt vector position. The lower the interrupt vector address, the higher the priority.
Related
I know that you can either disable interrupts while the interrupt is going to be processed or you can use a priority scheme. But what happens when the 2 are too close to one another
Even if, supposedly theoretically, multiple interrupts occur at the same time physically, interrupts are sampled at the processor's clock rate and then multiple pending interrupt requests handled according to predefined or preconfigured interrupt priorities.
This is usually done by selecting the one with the topmost priority and keeping the interrupt service routine itself from being interrupted, but some processors takes it furthur and allow storing context for each interrupt and makes lower priority ISRs themselves preemptible by higher priority interrupt requests.
I want to understand what exactly an interrupt is for my 6502 work-alike processor project in Logisim.
I know that an interrupt does the following steps:
Stops the current program from processing
Saves all unfinished data into the stack
Does "SOMETHING"
Loads back the unfinished data and let's the program keep running normally.
My question is: what happens during that "SOMETHING" step? Does the program counter get redirected to a special program to be executed? Something like reading the pressed button's ASCII code and saving that into a register or some memory location? If so, where is that special program usually stored in the memory? And can you make such a CPU that will handle different kinds of interrupts? Maybe if you press the button "a" then it's ASCII will be stored in A register, but if you press the button "b" then it will be stored in X register?
Any help is greatly appreciated.
Edit: Thanks to everybody for answers. I learned a lot and now can proceed with my project.
My question is: what happens during that "SOMETHING" step? Does the program counter get redirected to a special program to be executed?
What happens with a 6502 maskable interrupt is this:
the interrupt is raised (by this I mean the interrupt pin on the chip is forced low.
when it's time to execute a new instruction, the 6502 checks if the interrupt pin is low and the interrupt mask in the status register is not set. If either is not thew case i.e. if the interrupt pin is high or the interrupt mask is high, the CPU just carries on.
Assuming an interrupt is required, the CPU saves the PC on the stack
The CPU then saves the status register on the stack but with the B bit set to 0. The B bit is the "break" bit. It would be set to 1 for a BRK instruction and that is the only way to tell the difference between a hardware interrupt and a BRK instruction.
The CPU then fetches the address at locations $FFFE and $FFFF and stuffs it into the PC, so execution begins again at that address.
That's all it does. Everything else is up to the programmer until the programmer executes an RTI, then the status word and the return address are pulled off the stack and restored into their respective registers. It is the programmer's responsibility to save any registers and other data.
Does the program counter get redirected to a special program to be executed? Something like reading the pressed button's ASCII code and saving that into a register or some memory location?
That is correct. In 6502 based computer systems, there are three vectors at the top of memory:
$FFFA - $FFFB : Non maskable interrupt (as above except the I bit in the status register is ignored).
$FFFC - $FFFD : Reset vector used when the CPU detects a reset
$FFFE - $FFFF : Normal interrupt vector.
The above are usually in ROM because the reset vector (at least) has to be there when the CPU powers up. Each address will point to a routine in the machine's operating system for handling interrupts.
Typically, the interrupt routine will first do an indirect jump through a vector stored in RAM. This allows the interrupt routine to be changed when the machine is running.
Then the interrupt routine has to determine the source of the interrupt. For example, on the Commodore PET thew interrupt might originate from the VIA chip or either of the PIA chips and each of those may raise an interrupt for various reasons e.g. one of the PIA chips raises an interrupt when the monitor does a vertical blank i.e. when it finishes scanning the screen and goes back to the top line. During this interrupt, the PET executes a routine to scan the keyboard and another routine to invert the cursor. Another interrupt might occur when the VIA timer hits zero and the programmer can insert an interrupt routine to, for example toggle an output line to generate a square wave for sound.
Some answers to questions in the comments.
program counter goes to address $FFFE to get relocate to the address
No, the program counter is set to whatever is at that address. If you have:
FFFE: 00
FFFF: 10
the program counter will be set to $1000 (6502 is little endian) and that's where the interrupt routine must start. Also, the vector for NMI is at $FFFA. The normal interrupt shares $FFFE with the BRK instruction, not the NMI.
What exactly the reset vector does? Does it reset the cpu?
The reset vector contains the location of the code that runs after the processor has been powered on or when a reset occurs.
What's the difference between NMI and IRQ? Then I also would like to know what's up with masking? Is it the way to set the "I" flag in Processor Status Register high or low?
The 6502 status register contains seven flags. Mostly they are to do with the results of arithmetic instructions e.g. Z is set if the result of an operation is zero, C is set when an operation overflows eight bits and for shifts. The I flag enables and disables the normal interrupt (IRQ). If it's zero, interrupts on IRQ will be respected. If it's 1, interrupts are disabled. You can set it and disable it manually with the SEI and CLI instructions and it is set automatically when an interrupt occurs (this is to prevent an interrupt from interrupting an interrupt).
NMI is a non maskable interrupt. The difference is that it ignores the state of the I flag and uses a different vector.
And finally, what are vectors? Are they synonymous for indirect addresses?
Yes.
Oh, and if you do know, how are interrupt addresses starting from $FFFA stored in ROM instead of RAM in real 6502?
You have to arrange for the address decoding logic to point those address at ROM instead of RAM. In fact, in Commodore systems the whole block from $F000 is ROM containing part of the operating system. The same probably applies to most other 6502 based systems.
There are four types of interrupt on the 6502: RESET, NMI, IRQ and BRK. The first three are hardware interrupts and the last is a software interrupt. The hardware interrupts have physical input voltages on pins on the microprocessor itself. The software interrupt is caused by a BRK instruction.
All interrupts are 'vectored'. That means when they occur the program counter (PC) is immediately loaded from an address stored in memory, and instruction execution continues from that address.
The addresses are stored as two bytes little endian format at the end of the 64k memory space. They are (in hex):
NMI $FFFA/$FFFB
RESET $FFFC/$FFFD
IRQ $FFFE/$FFFF
BRK $FFFE/$FFFF
In the case of NMI, IRQ and BRK, the current PC address is pushed on to the stack, before loading the interrupt address. The processor status register is also pushed on to the stack.
Pushing the registers on to the stack, is enough information to resume execution after the interrupt has been serviced (processed). The A, X and Y registers however, are not pushed automatically on to the stack. Instead the interrupt service routine should do this if necessary - and pull them back off the stack at the end of the service.
Notice that the IRQ and BRK vectors have the same address. In order to distinguish what happened in your service code, you need to examine the Break Bit of the pushed processor status register. The Break Bit is set if the interrupt came from a BRK instruction.
The currently executing instruction will always be completed before servicing the interrupt.
There are many subtleties to interrupt processing. One of which is which type of interrupt wins in the case that they happen (asserted) at the same time. Another is the point at which an interrupt occurs during the instruction cycle. If the interrupt occurs before the penultimate cycle of the instruction, then it will be serviced on the next instruction. If on or after the penultimate cycle, then it will be delayed until one instruction after.
IRQ interrupts can be 'switched off' or ignored by setting a bit in the processor status register, using the SEI instruction.
Typically the interrupt service routine needs to determine the cause of the interrupt (disc drive, keyboard etc.) and to make sure the interrupt condition is cleared and perform any processing (e.g. putting key presses into a buffer). It can normally do this by reading/writing specific memory locations which are mapped to hardware.
There is more information at this link: https://www.pagetable.com/?p=410
Some more information on how interrupts work in a real 8 bit machine (pages 59, 86, 295): BBC Microcomputer Advanced User Guide
And more information on the physical chip package where you can see the NMI, RES(ET) and IRQ pins on the chip package itself (pages 2,3): 6502 Datasheet
I guess you ask for hardware interrupt (IRQ or NMI). At your step 2 in stack (not in stack register) are stored program counter and flags register. Later you call RTI to resume program execution. The program counter is loaded with start address of "something" which is interrupt subroutine or program to process the interrupt. It has to store A, X, Y registers if need to modify their values and restore them before RTI. The IRQ interrupt can be masked (delayed) with I flag and NMI is non-maskable i.e. it is always processed. They have different addresses for subroutine.
An interrupt is the signal to the running processor by means of hardware or software so that the processor will give attention to that action and does the action according to the interrupt message.
There are three kinds of interrupt:
Internal interrupt :- which include the clock cycle interrupt ,in which cpu has to perform the certain action until the particular time and has to go to perform for the another operation.
Software interrupt:- This interrupt is occurred when the problem or errors occurs in the software itself. For example user tries to divide something by zero and error occurs. And there is interrupt.
External interrupt:- External interrupt is caused by IO devices for example mouse and keyboards.
Cpu is designed to handle such type of interrupt and resumes the process before the interrupt occurred.
Interrupts can be enabled for a specific pin(s) on a digital I/O port, correct? How would the ISR determine which pin caused the interrupt?
Because the vector table has only one slot for the Port1 ISR. So the same ISR function gets called no matter which input pin on Port1 needs attention unless I'm wrong...
As other people have suggested in comments this can be MCU dependent, but for ARM(The core behind MSP432) generally the answer is it doesnt know, it looks for it.
ARM has a vectored interrupt system, which means that every source has its own vector of interrupt, so CPU can easily find out which source is triggering thr interrupt. so far so good.
but then it happens that a device can trigger multiple interrupts, like GPIO as you said, in this case, CPU knows that which port has triggered interrupt so fires it's ISR but then it is ISR responsibility to poll device registers to figure out exact interrupt source, there are many of this peripherals with multiple interrupt, timers, DMAs just to name a few.
This is exactly why normally peripherals have an interrupt enable bit, that lets them trigger interrupts, but they also have bit masks that controls what exactly can trigger that interrupt internally,
Also have a look at this link for an in action example, specially at their ISR that does exactly the same as described above
In a typical MCU, there are hundreds, or at a stretch even thousands of potential interrupt sources. Depending on the application, only some will be important, and even fewer will be genuinely timing critical.
For a GPIO port, you typically enable only the pins which are interesting to generate an interrupt. If you can arrange only one pin of a port to be generating the interrupt, the job is done, your handler for that port can do the work, safely knowing that it will only be called when the right pin is active.
When you care about the cause within a single peripheral, and don't have the luxury of individually vectored handlers, you need to fall back on the 'non vectored' approach, and check the status registers before working out which eventual handler function needs to be called.
Interestingly, you can't work out which pin caused the interrupt - all you can see is which pins are still active once you get round to polling the status register. If you care about the phasing between two pulses, you may not be able to achieve this discrimination within a single GPIO unless there is dedicated hardware support. Even multiple exception vectors wouldn't help, unless you can be sure that the first exception is always taken before the second pin could become set.
What is the difference between INTDisableInterrupts() and INTEnableSystemMultiVectoredInt() and asm volatile ("di")
In Pic32, there are "normal" interrupts and "Vectored" interrupts. If you aren't familliar with Pic32, "vectored" means that each interrupt has it's own interrupt handler function. You can have a function for UART interrupt and another function for RS232 (UART),...
You do not have to put everything in a 'high priority' and a 'low priority' interrupt anymore.
So :
INTDisableInterrupts() will simply disable the interrupts. This will call "di".
"di" : simply disables the interrupts, in assmebler.
INTEnableSystemMultiVectoredInt() will let tell your PIC32 to use a different function for all your interrupts. If you did not provide interrupt handler functions for each of your interrupts, then it will seem as if they are disabled. Your interrupts are NOT disabled however, and if you write an handler for an Vectored interrupt, your pic will use it.
UPDATE:
#newb7777
To answer your question :
If you have only one interrupt ( not vectored ), then you have one big function that must check all the "Interrupt Flag register" to know what caused the interrupt and process the right code.
If you have 'vectored interrupts', then the PIC behaves like most processors ( they almost all have vectored interrupts ). When something happened that would generate an interrupt then a register changes value. For instance one that would be called "UART_1_Rx_Received". Before executing an instruction, the processor sees that this flag is on and if the 'Interrupt enable register' and the 'global interrupt enable register' are both ON, then the interrupt function will be called. Note that all interrupts also have a priority. If a high-priority interrupt is running then it will never be interrupted by an interrupt with <= priority. If a low priority interrupt is running then a higher priority interrupt could interrupt it.
However, you should not lose interrupts because if a byte comes from the UART that would generate a low-priority interrupt and a higher-priority interrupt is running, then the flag will still be set. When the higher priority interrupt ends, then the lower priority will be executed.
Why do we disable interrupts then ? The main reasons to disable interrupts are:
- the interrupt changes the value of a variable. if the code loops :
for(i=0;i==BufferSize;i++)
and your interrupt changes the value of BufferSize while this loop executes, then the loop could execute forever (if BufferSize changes from 100 to 2 while I has the value 99 then I will not get back to 2 for a long time...). You may want to disable interrupt before doing the loop in that case.
Another reason could be that you want to execute something where timing is important.
another reason is that sometimes, MCU needs you to execute a few instructions in a specific order to unlock something that would be dangerous to execute by error so you don't want an interrupt in the middle of the process.
If you have a circular buffer that received bytes from an interrupt and you pool that buffer from the code then you want to make sure to disable interrupts before removing a variable from the buffer to make sure the variables don't change while you read them.
There are many reasons to disable interrupts, just keep in mind that you can also create a "volatile" variable for global variables that are used in and outside of interrupts.
One last thing to answer your question : if you get an interrupt for every byte that comes in your UART at 115,200 baud, and you have an interrupt function that takes a long time to execute, then it is possible to miss a byte or two. In that case if you are lucky there is a hardware buffer that allows you to get them but it is possible also that there isn't and you would lose byte in your communication port. Interrupts must always be as short as possible. When possible, set a flag in the interrupt and do the processing in your main loop outside of the interrupt. When you have many interrupt levels, always use high priority for interrupts that could trigger often and low priority if an interrupt processes for a long time.
What happens if an ISR is running, and another interrupt occurs? Does the first interrupt get interrupted? Will the second interrupt get ignored? Or will it fire when the first ISR is done?
EDIT
I forgot to include it in the question (but I included it in the tags) that I meant to ask how this worked on Atmel AVR's.
Normally, an interrupt service routine proceeds until it is complete without being interrupted itself in most of the systems. However, If we have a larger system, where several devices may interrupt the microprocessor, a priority problem may arise.
If you set the interrupt enable flag within the current interrupt as well, then you can allow further interrupts that are higher priority than the one being executed. This "interrupt of an interrupt" is called a nested interrupt. It is handled by stopping execution of the original service routine and storing another sequence of registers on the stack. This is similar to nested subroutines. Because of the automatic decrementing of the stack pointer by each interrupt and subsequent incrementing by the RETURN instruction, the first interrupt service routine is resumed after the second interrupt is completed, and the interrupts are serviced in the proper order. Interrupts can be nested to any depth, limited only by the amount of memory available for the stack.
For example, In the following diagram, Thread A is running. Interrupt IRQx causes interrupt handler Intx to run, which is preempted by IRQy and its handler Inty. Inty returns an event causing Thread B to run; Intx returns an event causing Thread C to run.
Image Ref
For hardware interrupts, Priority Interrupt Controller Chips (PIC's) are hardware chips designed to make the task of a device presenting its own address to the CPU simple. The PIC also assesses the priority of the devices connected to it. Modern PIC's can also be programmed to prevent the generation of interrupts which are lower than a desired level.
UPDATE: How Nested Interrupt Works on Atmel AVRs
The AVR hardware clears the global interrupt flag in SREG before entering an interrupt vector. Therefore, normally interrupts remain disabled inside the handler until the handler exits, where the RETI instruction (that is emitted by the compiler as part of the normal function epilogue for an interrupt handler) will eventually re-enable further interrupts. For that reason, interrupt handlers normally do not nest. For most interrupt handlers, this is the desired behaviour, for some it is even required in order to prevent infinitely recursive interrupts (like UART interrupts, or level-triggered external interrupts).
In rare circumstances though nested interrupts might be desired by re-enabling the global interrupt flag as early as possible in the interrupt handler, in order to not defer any other interrupt more than absolutely needed. This could be done using an sei() instruction right at the beginning of the interrupt handler, but this still leaves few instructions inside the compiler-generated function prologue to run with global interrupts disabled. The compiler can be instructed to insert an SEI instruction right at the beginning of an interrupt handler by declaring the handler the following way:
ISR(XXX_vect, ISR_NOBLOCK)
{
...
}
where XXX_vect is the name of a valid interrupt vector for the MCU type.
Also, have a look at this Application Note for more info on interrupts on Atmel AVRs.
The way interrupts work:
The code sets the "Global Interrupt Enable" bit; without it, no interrupts will occur.
When something happens to cause an interrupt, a flag is set.
When the interrupt flag is noticed, the "Global Interrupt Enable" bit is cleared.
The appropriate ISR is run.
The "Global Interrupt Enable" bit is re-set.
Things now go back to step 2, unless an interrupt flag is already set during the ISR; then things go back to step 3.
So to answer the question: When the first ISR is finished, the second ISR will be run.
Hope this helps!