Файл: CodeVision AVR 1.25.7, user manual.pdf

CodeVisionAVR

3.17 Using an External Startup File

In every program the CodeVisionAVR C compiler automatically generates a code sequence to make the following initializations immediately after the AVR chip reset:

1.interrupt vector jump table

2.global interrupt disable

3.EEPROM access disable

4.Watchdog Timer disable

5.external SRAM access and wait state enable if necessary

6.clear registers R2 … R14

7.clear the SRAM

8.initialize the global variables located in SRAM

9.initialize the Data Stack Pointer register Y

10.initialize the Stack Pointer register SP

11.initialize the UBRR register if necessary

The automatic generation of code sequences 2 to 8 can be disabled by checking the Code Generation|Use an External Startup Initialization File check box in the Project|Configure|C Compiler|Code Generation dialog window. The C compiler will then include, in the generated .asm file, the code sequences from an external file that must be named STARTUP.ASM . This file must be located in the directory where your main C source file resides.

You can write your own STARTUP.ASM file to customize or add some features to your program. The code sequences from this file will be immediately executed after the chip reset.

A basic STARTUP.ASM file is supplied with the compiler distribution and is located in the ..\BIN directory.

Here's the content of this file:

;CodeVisionAVR C Compiler

;(C) 1998-2007 Pavel Haiduc, HP InfoTech s.r.l. ;EXAMPLE STARTUP FILE FOR CodeVisionAVR V1.24.1 OR LATER

;DISABLE THE WATCHDOG

LDI R31,0x18

OUT WDTCR,R31

OUT WDTCR,R30

OUT MCUCR,R30 ;MCUCR=0, NO EXTERNAL SRAM ACCESS

;CLEAR R2-R14

CodeVisionAVR

;CLEAR SRAM

LDI R24,LOW(__CLEAR_SIZE) LDI R25,HIGH(__CLEAR_SIZE) LDI R26,LOW(__CLEAR_START) LDI R27,HIGH(__CLEAR_START)

__CLEAR_SRAM: ST X+,R30 SBIW R24,1

BRNE __CLEAR_SRAM

RJMP __GLOBAL_INI_NEXT __GLOBAL_INI_END:

The __CLEAR_START and __CLEAR_SIZE constants can be changed to specify which area of SRAM to clear at program initialization.

The __GLOBAL_INI_TBL label must be located at the start of a table containing the information necessary to initialize the global variables located in SRAM. This table is automatically generated by the compiler.

CodeVisionAVR

3.18 Including Assembly Language in Your Program

You can include assembly language anywhere in your program using the #asm and #endasm directives.

Example:

void delay(unsigned char i) { while (i--) {

/* Assembly language code sequence */ #asm

nop nop #endasm

};

}

Inline assembly may also be used.

Example:

#asm("sei") /* enable interrupts */

The registers R0, R1, R22, R23, R24, R25, R26, R27, R30 and R31 can be freely used in assembly routines.

However when using them in an interrupt service routine the programmer must save, respectively restore, them on entry, respectively on exit, of this routine.

CodeVisionAVR

3.18.1 Calling Assembly Functions from C

The following example shows how to access functions written in assembly language from a C program:

//function in assembler declaration

//this function will return a+b+c

#pragma warn- // this will prevent warnings int sum_abc(int a, int b, unsigned char c) { #asm

#endasm

}

#pragma warn+ // enable warnings

void main(void) { int r;

// now we call the function and store the result in r r=sum_abc(2,4,6);

}

The compiler passes function parameters using the Data Stack.

First it pushes the integer parameter a, then b, and finally the unsigned char parameter c.

On every push the Y register pair decrements by the size of the parameter (4 for long int, 2 for int, 1 for char).

For multiple byte parameters the MSB is pushed first. As it is seen the Data Stack grows downward.

After all the functions parameters were pushed on the Data Stack, the Y register points to the last parameter c, so the function can read its value in R26 using the instruction: ld r26,y.

The b parameter was pushed before c, so it is at a higher address in the Data Stack. The function will read it using: ldd r27,y+2 (MSB) and ldd r26,y+1 (LSB).

The MSB was pushed first, so it is at a higher address.

The a parameter was pushed before b, so it is at a higher address in the Data Stack. The function will read it using: ldd r31,y+4 (MSB) and ldd r30,y+3 (LSB).

The functions return their values in the registers (from LSB to MSB):

•R30 for char and unsigned char

•R30, R31 for int and unsigned int

•R30, R31, R22, R23 for long and unsigned long.

So our function must return its result in the R30, R31 registers.

After the return from the function the compiler automatically generates code to reclaim the Data Stack space used by the function parameters.

The #pragma warn- compiler directive will prevent the compiler from generating a warning that the function does not return a value.

This is needed because the compiler does not know what it is done in the assembler portion of the function.