TMS320F28377D Timer ISR in assembly - C2000 microcontrollers forum - C2000™︎ microcontrollers

Ajay,

First, I wonder why you want to code in assembly. Are you having a performance issue with the compiler?

To your question, an ISR can be written in assembly. You don't need to put the ISR code in a different section. It can go in .text. If you want to link it somewhere else (e.g., load to flash but run from ram), then yes, use a .sect directive to put it in a different section. The only thing special about an ISR from a regular functions is:

1) You cannot pass an ISR any parameters nor can it return a parameter. I'm not talking about global vars here. I mean passed parameters. An ISR is always of type void MyISR(void).

2) You must context save/restore any registers you modify that are not automatically saved by the CPU. See the CPU Guide SPRU430E Table 3-3 for the list of automatically saved registers.

3) You use an IRET instruction to return from an ISR, whereas for a regular function you use LRET/LRETE/LRETR.

Regards,

David

0 Ajay N Namboothiri over 10 years ago in reply to David M. Alter

Prodigy 210 points

thank u for detailed response

please refer this forum post, i pasted it,i already had a reply from you regarding the performance that i need.

David M. Alter said:

Ajay,

Ajay N Namboothiri

I was planning to use F2837xD in one of our DSP based application, in which we need to implement 15 second order IIR filters,10 extra multiplications,& 10 extra additions(accumulating sum).We need to complete these operations within a sampling period & this period can be be from 1uS to as low as 625nS.

Well, things are a little tight. 625 ns is 125 cycles @ 200 MHz. Each IIR has perhaps 5 MACs in it plus data movement for the history. So, 15*5 = 75 MACs. You've also got 20 more operations (10 mpy, 10 add). In total, you're pushing 95 math operations, plus you've still got pointer setup and other overhead. You've only got 125 cycles to implement this. So, things are very tight.

Now this is just a rough estimate. I don't know if you can parallel any of your operations. If so, you've got the CLA at your disposal as well. That would double your available MIPS here.

I think you'll need to go to hand assembly for this however. You need to squeeze that last bit out of the code. The compiler is not going to do that for you.

Regards,

David

-----------------------------

David M. Alter

Senior Member Technical Staff
Texas Instruments Inc.

So as of now we are OK with the sampling period of 1uS that is we have 1uS time to complete the above said processing.So i thought better move to assembly,i also tried different optimization options in C but i'm not very sure about it ...

I already started Assembly coding for CLA. The CLA is triggered at 1MHz rate.So i thought if i could also share some of the filters in a timer or external interrupt routine which should be somehow in sync with the CLA triggering,
i could get more bandwidth & might be able to complete the processing in time if going by this

I went through the section which you had suggested in the CPU guide.I saw one example which described the regs we need to save to stack.It says a total of 16X2= 32Cycles would be spent in an ISR routine!!
In my case i am available with only 200 Cyces.

David M. Alter said:

To your question, an ISR can be written in assembly. You don't need to put the ISR code in a different section. It can go in .text. If you want to link it somewhere else (e.g., load to flash but run from ram), then yes, use a .sect directive to put it in a different section. The only thing special about an ISR from a regular functions is:

if you could elaborate this a little more,i.e how will we write the assembly instructions in the C ISR routine.
is it by using the asm(" "); function?.You can also direct me to any document...

0 David M. Alter over 10 years ago in reply to Ajay N Namboothiri

TI__Guru 61235 points

Hello Ajay,

Ajay N Namboothiri said:

if you could elaborate this a little more,i.e how will we write the assembly instructions in the C ISR routine.
is it by using the asm(" "); function?.You can also direct me to any document...

You wouldn't use asm(""). That is for special/limited situations where you need to embed an ASM instruction in your C code. What you do is create a source file with a .asm extension. Add it to your project. The compiler will pass this file directly to the assembler stage for building.

There isn't any documentation specific to writing an ISR in assembly. The ASM instructions of course are documented in the CPU guide, SPRU430. What I would do if I were you is this. Write a skeleton for an ISR in C, like this:

extern int x;
interrupt void MyIsr(void) {
x = 5; // Do something
}

Now you can cheat. Look at the generated .asm file for this ISR (check the -k option in the project properties, Build->C2000_Compiler->Advanced_Options->Assembler options, in order to keep the asm file in the build folder). You can copy the basic shell. Might want to add more "Stuff" in the C ISR so you can press the compiler to context save/restore some registers and you can see what it does.

Really, an ISR is not much different from a regular function. You save/restore the used registers on the stack using PUSH/POP. Then, do whatever code you need to do inside the ISR.

Since you are concerned about performance, you will want the ISR to run from RAM. You will need to copy the ISR from flash to RAM at runtime. See appnote SPRA958 for information on how to do this.

http://www.ti.com/mcu/docs/litabsmultiplefilelist.tsp?sectionId=96&tabId=1502&literatureNumber=spra958l&docCategoryId=1&familyId=1414

*** For that matter, you can use one of the projects in that appnote to build a C ISR. I have attached the DefaultISR.asm file here from the F28335 RAM nonBIOS project (just an example). Use the ADC ISR starting at line 1658.

Regards,

David

DefaultIsr_nonBIOS.asm

0 Ajay N Namboothiri over 10 years ago in reply to David M. Alter

Prodigy 210 points

thanks!!

I will check as per your suggestions,It could have been better if we also had some examples in .asm in Controlsuite,like for ISRs.

So in the .asm file can i take the vector addrs of a particular ISR and keep it in the .sect directive,which should tell
the compiler that the following assembly code shall go to this memory location and then i would do all the neccessary push/pop operations in extra to my actual assembly code,

so my doubt is can we give a flash location address in the .sect directive?,is it neccessary to load the ISR code from flash to ram,it would improve access speed, but is it mandatory??

0 David M. Alter over 10 years ago in reply to Ajay N Namboothiri

TI__Guru 61235 points

Ajay,

Ajay N Namboothiri said:

So in the .asm file can i take the vector addrs of a particular ISR and keep it in the .sect directive,which should tell
the compiler that the following assembly code shall go to this memory location and then i would do all the neccessary push/pop operations in extra to my actual assembly code,

Not sure what you mean here. You can use .sect directive to place the ISR in it's own section. You can then link the section however you want (i.e., load to flash, run from RAM) using the linker .cmd file. You can also use .global to make the ISR label visible outside the file, and use the label in your interrupt vector table.

Ajay N Namboothiri said:

so my doubt is can we give a flash location address in the .sect directive?,is it neccessary to load the ISR code from flash to ram,it would improve access speed, but is it mandatory??

Again, you use the linker .cmd file to link the section to memory. You do not specify a hard address in the .sect directive.

It is not necessary to run the ISR from RAM, but you will get better performance executing from RAM. If you're going through the trouble of writing the ISR in assembly code, you may as well run it from RAM too. If you don't want to move it to RAM, at least align the ISR on a 128-bit flash fetch boundary using the linker command file. This will make sure you fetch a full packet of instructions on your first packet fetch for the ISR.

Regards,

David