Timer/counter/External Interrupts code in ARM assembly

Per Westermark said:

The debate was about the reason for you to ask for help with assembler. If you asked because you thought you needed assembler or if you did know you needed assembler or just wanted to broaden your knowledge.

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

Thanx for your reply

Now from my part for the word "BEST" -> Assembly is best in the case of code optimization which obviously results in memory saving and speed.

Hope "BEST" part is descriptive from the above statement.

Now for why I am asking for the assembler code -> Surely I wanted to broaden my knowledge which results in the better understanding of the system.

Now can any body here guide me with small code( like timer interrupt for LED blinking or something like that).

Per Westermark said:

The debate was about the reason for you to ask for help with assembler. If you asked because you thought you needed assembler or if you did know you needed assembler or just wanted to broaden your knowledge.

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

Thanx for your reply

Now from my part for the word "BEST" -> Assembly is best in the case of code optimization which obviously results in memory saving and speed.

Hope "BEST" part is descriptive from the above statement.

Now for why I am asking for the assembler code -> Surely I wanted to broaden my knowledge which results in the better understanding of the system.

Now can any body here guide me with small code( like timer interrupt for LED blinking or something like that).

Nowadays say that coding in assembly code is best for speed and size is not true, as the compiler may handle many optimization that you would not be able to do (pipeline, cache, ...).
In my opinion, you write assembly code to be deterministic with the code produced (size& speed), or when you no have no high-level language for your device (here the C compiler).

Why don't you specify your needs with speed/code size and try to see if your compiler satisfy these needs ?

If you really want to know how to code this in assembly you can still code it in C and read the listing produced (with the different level optimization).

ohk!

I got the idea what you guys wanted to say about the assembly and HLL and the meaning of BEST also.

As I am on the learning stage so there is no specific need of my application as there is no one presently.

I juz wanted to try my hand on assembly for the interrupt part. As I had already told that I have written the small code for LED blinking in assembly.

I HLL like 'C' I have written the code for interrupt. But I am not getting any concrete idea and hint for writing code in assembly for the interrupts.

I juz know that some changes are to be made in Startup.s file. But what are the changes I am not getting it in a proper manner.

You must READ the above statement clearly.

I juz know that some changes are to be made in Startup.s file. But what are the changes I am not getting it in a proper manner.M

No this is not the case.
if you write you interrupt handler like this:

__irq void DAbt_Handler(void)
{
    handle_failure() ;

    for (;;) ;
}

and then

__asm void handle_failure(void)
{
     // assembly code here
}

you're done.

You can also write the handler itself directly in assembly, like this:

__asm __irq void DAbt_Handler(void)

Let me reiterate:

There is nothing special about assembly code in interrupts handlers. It is just assembly. Nothing magical or mysterious about it. If you need help with the syntax or instruction set - you must have a look in the assembly guide of the tool chain.

One more thing: Use assembly in interrupt handlers does not require any change to the startup file!

I am not asking for inline assembly ... I am asking for the assembly code from scratch that juz include Startup.s file IN KEIL IDE and no .C file at all.

If I am not wrong while making FIQ program in C I had to make changes in Startup.s file ... although this is not the case for Vectored and non-vectored IRQ ... but ya for FIQ i had to make the changes.

Can anybody gimme the code for 1 second timer interrupt!!!

Hi Arvind Shrivastava,

Try to get a book called "The Defi nitive Guide to the ARM Cortex-M3" written by Joseph Yiu.

It has a lot of assembly example code like the following.

LDR R0, =0xE000ED0C    ; Application Interrupt and Reset
                       ; Control Register
LDR R1, =0x05FA0500    ; Priority Group 5 (2/6)
STR R1, [R0]           ; Set Priority Group
...
MOV R4,#8              ; Vector Table in ROM
LDR R5,=(NEW_VECT_TABLE+8)
LDMIA R4!,{R0-R1}      ; Read vectors address for NMI and
; Hard Fault
STMIA R5!,{R0-R1}      ; Copy vectors to new vector table
...
LDR R0,=0xE000ED08     ; Vector Table Offset Register
LDR R1,=NEW_VECT_TABLE
STR R1,[R0]            ; Set vector table to new location
...
LDR R0,=IRQ7_Handler   ; Get starting address of IRQ#7 handler
LDR R1,=0xE000ED08     ; Vector Table Offset Register
LDR R1,[R1]
ADD R1, R1, #(4*(7+16)); Calculate IRQ#7 handler vector
                       ; address
STR R0,[R1] ; Setup vector for IRQ#7
...
LDR R0,=0xE000E400     ; External IRQ priority base
MOV R1, #0xC0
STRB R1,[R0,#7]        ; Set IRQ#7 priority to 0xC0
...
LDR R0,=0xE000E100     ; SETEN register
MOV R1,#(1<<7)         ; IRQ#7 enable bit (value 0x1 shifted
                       ; by 7 bits)
STR R1,[R0]            ; Enable the interrupt

Hi Arvind Shrivastava,

But it is not for your LPC2148.

Sir,

Thanx a lot for your kind reply and valuable guidance.

No problem if the code is not for LPC2148 ... i juz wanna 2 know the concept.

thanx once again

Hi Arvind Shrivastava,

It is embarrassing that, for the people who were involved in this thread, I am afraid that, you and me, might be the only two persons, who don't know how to code in assembly.

I think the situation is, for those who know how to code in assembly, they don't need things like tutorial or example code, they just know how to do that; for those who don't know how to code in assembly, the resource is very limited.

And I believe that you already know this, just to remind you once again,

Assembly is very platform dependent, though the general concept should be very similar.

No Problem if I dont know how to code in assembly, atleast I can try my hand.

For the rest of the guys in this forum ... might I am not been able to give them the clear picture of what I wanted to do.

So again I am briefing ...

I wanted to write a simple code in assembly for TIMER Interrupt ... lets say for 1 second and toggle some GPIO thats it.

Now for the concept ... where I am confused or having no idea is ...

how to write the ISR?
how to assign the address of the ISR?

Above task is done successfully in c by me ... coz many tutorials are available on net.

But for the assembly ... I am bit confused ... this juz may be the result of lack of my information and concrete knowledge.

Hi Arvind Shrivastava,

In your case (about writing an ISR), I will recommend you to get an Evaluation Board/Kit equipped with a LPC1768, and get the "The Definitive Guide to the ARM Cortex-M3". It would be a good beginning to you.

If it is not possible, then you can try to use the KEIL IDE's Simulator with "The Definitive Guide to the ARM Cortex-M3"

If it is still not possible to you, and you decide to work with the LPC2148, then you will need to read the ARM7TDMI xxx Technical Reference Manual first.

From ARM7TDMI xxx Technical Reference Manual:

2.9.2 Entering an exception
When handling an exception the ARM7TDMI-S core:
1. Preserves the address of the next instruction in the appropriate LR. When the
exception entry is from:
• ARM state, the ARM7TDMI-S copies the address of the next instruction
into the LR (current PC + 4, or PC + 8 depending on the exception)
• Thumb state, the ARM7TDMI-S writes the value of the PC into the LR,
offset by a value (current PC + 4, or PC + 8 depending on the exception).
The exception handler does not have to determine the state when entering an
exception. For example, in the case of a SWI, MOVS PC, r14_svc always returns to
the next instruction regardless of whether the SWI was executed in ARM or
Thumb state.
2. Copies the CPSR into the appropriate SPSR.
3. Forces the CPSR mode bits to a value which depends on the exception.
4. Forces the PC to fetch the next instruction from the relevant exception vector.
The ARM7TDMI-S core also sets the interrupt disable flags on interrupt exceptions to
prevent otherwise unmanageable nestings of exceptions.

2.9.3 Leaving an exception
When an exception is completed, the exception handler must:
1. Move the LR, minus an offset to the PC. The offset varies according to the type
of exception, as shown in Table 2-3 on page 2-19.
2. Copy the SPSR back to the CPSR.
3. Clear the interrupt disable flags that were set on entry.

And you need to be familar with the KEIL toolchain also.

If I am not wrong while making FIQ program in C I had to make changes in Startup.s file ... although this is not the case for Vectored and non-vectored IRQ ... but ya for FIQ i had to make the changes.

how to write the ISR?
how to assign the address of the ISR?
Above task is done successfully in c by me ... coz many tutorials are available on net.

Cortex-M3 is very different from ARM7, especially about the Interrupt (IRQ mode).

For ARM7, the Interrupt Controller is not one part of the ARM core. So the Interrupt Controller varies from manufacturer to manufacturer.

In your case, the LPC2148 has a Vectored Interrupt Controller (a peripheral). In the startup file of LPC2148,

Vectors:        LDR     PC,Reset_Addr
                LDR     PC,Undef_Addr
                LDR     PC,SWI_Addr
                LDR     PC,PAbt_Addr
                LDR     PC,DAbt_Addr
                NOP                            /* Reserved Vector */
;               LDR     PC,IRQ_Addr
                LDR     PC,[PC, #-0x0FF0]      /* Vector from VicVectAddr */
                LDR     PC,FIQ_Addr

{ LDR PC,[PC, #-0x0FF0] } actually points to the address of the Vectored Interrupt Controller. And you need to register your ISR to the VIC.

When you write an ISR in C with the KEIL toolchain, actually the KEIL toolchain does a lot of work for you, it is KEIL's magic. When you write an ISR in assembly, you need to do all that work by yourself.

But that magic can be seen by writing an ISR in C, and then look at the produced assembler output. So Keil can produce a stub that you can then extend with own assembler code to perform the required task.

can any body gimme a simple code example.

The problem I am facing is ... whether the Interrupt handling by VIC variables are needed to be modied or CPSR values are to be set or reset for the interrupt.

Secondly how will i give the address of the ISR to any of the VICVectAddr register.

If some body has the code ... for KEIL then plz post it here!!!