Interrupt-driven Usart not working in Ada, but in C working, why?

["Rego, P." <pvrego@gmail.com>]

I am trying to use interrupt-driven Usart on an Atmel AVR ATmega2560 chip. 

The problem is that when I send anything to the usart, the Ada application restarts (continuously). I simplified the codes and created a C version (or the nearer of it) of the same code (because at first I thought that it could be a hw problem or a datasheet misunderstanding of me, and AVR community would not answer me if I had posted Ada code instead of C code ...). But the C code worked (i.e., the usart echo appears, and the board is not restarted), ao now I became clueless. Maybe the AVR guys could help me? Thanks.

The simplified version of the codes are

-- main.adb
with System.Machine_Code;
with AVR.USART;
with TEST;
pragma Unreferenced (TEST);

procedure Main is
   F_CPU : constant := 16_000_000;
   USART_BAUDRATE : constant := 9600;
begin
   AVR.USART.Reg_USART1.UCSRB.RXEN := True;
   AVR.USART.Reg_USART1.UCSRB.TXEN := True;
   AVR.USART.Reg_USART1.UCSRB.RXCIE := True;
   AVR.USART.Reg_USART1.UCSRC.UCSZ0 := True;
   AVR.USART.Reg_USART1.UCSRC.UCSZ1 := True;

   AVR.USART.Reg_USART1.UBRR (0) := AVR.Byte_Type ((F_CPU / (USART_BAUDRATE * 16)) - 1);
   AVR.USART.Reg_USART1.UBRR (1) := 0;

   System.Machine_Code.Asm ("sei", Volatile => True);

   loop
      null;
   end loop;
end Main;

-- test.ads
package TEST is

   USART1RX : constant String := "__vector_36";
   
   procedure Handle_Interrupt_USART1_RX;
   pragma Machine_Attribute
     (Entity         => Handle_Interrupt_USART1_RX,
      Attribute_Name => "signal");
   pragma Export
     (Convention    => C,
      Entity        => Handle_Interrupt_USART1_RX,
      External_Name => USART1RX);

end TEST;

with AVR;
with AVR.USART;

-- test.adb
package body Test is

   procedure Handle_Interrupt_USART1_RX is
      Curr_Buffer : AVR.Byte_Type;
   begin
      Curr_Buffer := AVR.USART.Reg_USART1.UDR;
      AVR.USART.Reg_USART1.UDR := Curr_Buffer;
   end Handle_Interrupt_USART1_RX;
end Test;


Given that I have mapped the Usart registers correctly on AVR.USART package (because my JtagICE said so).

The C code is
-- simple_isr_usart.c
#include <avr/io.h>
#include <avr/interrupt.h>

#define F_CPU 16000000
#define USART_BAUDRATE 9600
#define BAUD_PRESCALE (((F_CPU / (USART_BAUDRATE * 16UL))) - 1)

int main (void){
	UCSR0B = (1 << RXEN0) | (1 << TXEN0) | (1 << RXCIE0);
	UCSR0C = (1 << UCSZ00) | (1 << UCSZ01);
	UBRR0H = (BAUD_PRESCALE >> 8);
	UBRR0L = BAUD_PRESCALE;
	sei();
	while (1) {}
}

ISR(USART0_RX_vect){
	char ReceivedByte;
	ReceivedByte = UDR0;
	UDR0 = ReceivedByte;
}