Atmel CAVR-4 Manual De Usuario

CAVR-4

Part 1. Using the compiler

As noted earlier, you should not modify the file 

cstartup.s90

 if a customized version 

of 

_ _low_level_init

 is enough for your needs. However, if you do need to modify 

the file 

cstartup.s90

, we recommend that you follow the general procedure for 

creating a modified copy of the file and adding it to your project, see Overriding library 
modules, page 61.

There are three standard communication channels (streams)—

stdin

, 

stdout

, and 

stderr

—which are defined in 

stdio.h

. If any of these streams are used by your 

application, for example by the functions 

printf

 and 

scanf

, you need to customize the 

low-level functionality to suit your hardware.

There are primitive I/O functions, which are the fundamental functions through which 
C and C++ performs all character-based I/O. For any character-based I/O to be available, 
you must provide definitions for these functions using whatever facilities the hardware 
environment provides.

To implement low-level functionality of the 

stdin

 and 

stdout

 streams, you must write 

the functions 

_ _read

 and 

_ _write

, respectively. You can find template source code for 

these functions in the 

avr\src

 directory. 

If you intend to rebuild the library, the source files are available in the template library 
project, see Building and using a customized library, page 62. Note that customizing the 
low-level routines for input and output does not require you to rebuild the library.

Note: If you write your own variants of 

_ _read

 or 

_ _write

, special considerations 

for the C-SPY runtime interface are needed, see C-SPY Debugger runtime interface, 
page 76.

The code in the following examples use memory-mapped I/O to write to an LCD 
display:

_ _no_init volatile unsigned char LCD_IO @ address;

size_t _ _write(int Handle, const unsigned char * Buf,

               size_t Bufsize)