AVR Freaks

You haven't said which C compiler that is but surely it needs some kind of header for define the special function registers? Anyway I add an include of <avr/io.h> and tried to build it for ATmega328p in AS7 (avr-gcc) and got:

D:\test\test\main.c(7,1): warning: return type defaults to 'int' [-Wimplicit-int]
		 main()
		 ^
		.././main.c: In function 'main':
D:\test\test\main.c(11,2): warning: implicit declaration of function 'USART_Init' [-Wimplicit-function-declaration]
		  USART_Init(UBRR_VAL);
		  ^
D:\test\test\main.c(11,28): warning: integer overflow in expression [-Woverflow]
		.././main.c: At top level:
D:\test\test\main.c(17,6): warning: conflicting types for 'USART_Init'
		 void USART_Init( unsigned int ubrr)
		      ^
D:\test\test\main.c(11,2): info: previous implicit declaration of 'USART_Init' was here
		  USART_Init(UBRR_VAL);
		  ^

Maybe your compiler accepts "main()" as an interface for main() but most would like to see a return type, usually "int" or "void".

But the main problem here is you have called USART_Init(unsigned int) before the compiler "knows" about the function type. Either you have to put the function before main() so the compiler already knows when it encounters it. Or you need to DECLARE the function above main() with the definition below.

Anyway as it stand what I see in the generated code is:

	USART_Init(UBRR_VAL);
  90:	65 ec       	ldi	r22, 0xC5	; 197
  92:	72 e0       	ldi	r23, 0x02	; 2
  94:	80 e0       	ldi	r24, 0x00	; 0
  96:	90 e0       	ldi	r25, 0x00	; 0
  98:	f0 df       	rcall	.-32     	; 0x7a <USART_Init>

In this particular compiler the first parameter to a function is passed in R25:R24 as you see in the function implementation:

0000007a <USART_Init>:

void USART_Init( unsigned int ubrr)

{

	UBRR0H = (unsigned char)(ubrr>>8);
  7a:	90 93 c5 00 	sts	0x00C5, r25	; 0x8000c5 <__TEXT_REGION_LENGTH__+0x7f80c5>

	UBRR0L = (unsigned char)ubrr;
  7e:	80 93 c4 00 	sts	0x00C4, r24	; 0x8000c4 <__TEXT_REGION_LENGTH__+0x7f80c4>

but as you also see the value in R25:R24 is 00:00 and that's because you called the function without declaration and C therefore assumes "int function()" if told nothing else. As such a function does NOT have a parameter then C will not prepare to pass the value. If, instead I used:

#include <avr/io.h>
#define FOSC 16000000
#define BAUD 9600
#define UBRR_VAL ((FOSC/(16 * BAUD)) -1 )

void USART_Init( unsigned int ubrr);

int main()

{
	USART_Init(UBRR_VAL);
	/******************/
}

void USART_Init( unsigned int ubrr)

{
	UBRR0H = (unsigned char)(ubrr>>8);
	UBRR0L = (unsigned char)ubrr;
	UCSR0B = (1<<RXEN0)|(1<<TXEN0);
	UCSR0C= (1<<USBS0)|(1<<UCSZ00)|(1<<UCSZ01);
}

which adds a DECLARATION before the invocation I then get:

		.././main.c: In function 'main':
D:\test\test\main.c(11,28): warning: integer overflow in expression [-Woverflow]

but I can "fix" that warning too by using:

#define FOSC 16000000UL
#define BAUD 9600UL

and then I get:

  90:	87 e6       	ldi	r24, 0x67	; 103
  92:	90 e0       	ldi	r25, 0x00	; 0
  94:	f2 df       	rcall	.-28     	; 0x7a <USART_Init>

0x0067 is 103. QED.

(but if this is avr-gcc have you looked at:  https://www.nongnu.org/avr-libc/user-manual/group__util__setbaud.html ?)