There might be several reasons to write code for AVR microcontrollers using plain assembler source code. Among them are:
Code for devices that do not have RAM and are thus not supported by the C compiler.
Code for very time-critical applications.
Special tweaks that cannot be done in C.
Usually, all but the first could probably be done easily using the inline assembler facility of the compiler.
Although avr-libc is primarily targeted to support programming AVR microcontrollers using the C (and C++) language, there's limited support for direct assembler usage as well. The benefits of it are:
Use of the C preprocessor and thus the ability to use the same symbolic constants that are available to C programs, as well as a flexible macro concept that can use any valid C identifier as a macro (whereas the assembler's macro concept is basically targeted to use a macro in place of an assembler instruction).
Use of the runtime framework like automatically assigning interrupt vectors. For devices that have RAM, initializing the RAM variables can also be utilized.
For the purpose described in this document, the assembler and linker are usually not
invoked manually, but rather using the C compiler frontend
(avr-gcc
) that in turn will call the assembler and
linker as required.
This approach has the following advantages:
There is basically only one program to be called directly,
avr-gcc
, regardless of the actual source language
used.
The invokation of the C preprocessor will be automatic, and will include the appropriate options to locate required include files in the filesystem.
The invokation of the linker will be automatic, and will include the appropriate
options to locate additional libraries as well as the application start-up code
(crt
XXX.o
)
and linker script.
Note that the invokation of the C preprocessor will be automatic when the filename
provided for the assembler file ends in .S (the capital letter "s"). This
would even apply to operating systems that use case-insensitive filesystems since the
actual decision is made based on the case of the filename suffix given on the
command-line, not based on the actual filename from the file system.
Alternatively, the language can explicitly be specified using the -x
assembler-with-cpp
option.
The following annotated example features a simple 100 kHz square wave generator using an AT90S1200 clocked with a 10.7 MHz crystal. Pin PD6 will be used for the square wave output.
#include <avr/io.h> ; Note [1] work = 16 ; Note [2] tmp = 17 inttmp = 19 intsav = 0 SQUARE = PD6 ; Note [3] ; Note [4]: tmconst= 10700000 / 200000 ; 100 kHz => 200000 edges/s fuzz= 8 ; # clocks in ISR until TCNT0 is set .section .text .global main ; Note [5] main: rcall ioinit 1: rjmp 1b ; Note [6] .global TIMER0_OVF_vect ; Note [7] TIMER0_OVF_vect: ldi inttmp, 256 - tmconst + fuzz out _SFR_IO_ADDR(TCNT0), inttmp ; Note [8] in intsav, _SFR_IO_ADDR(SREG) ; Note [9] sbic _SFR_IO_ADDR(PORTD), SQUARE rjmp 1f sbi _SFR_IO_ADDR(PORTD), SQUARE rjmp 2f 1: cbi _SFR_IO_ADDR(PORTD), SQUARE 2: out _SFR_IO_ADDR(SREG), intsav reti ioinit: sbi _SFR_IO_ADDR(DDRD), SQUARE ldi work, _BV(TOIE0) out _SFR_IO_ADDR(TIMSK), work ldi work, _BV(CS00) ; tmr0: CK/1 out _SFR_IO_ADDR(TCCR0), work ldi work, 256 - tmconst out _SFR_IO_ADDR(TCNT0), work sei ret .global __vector_default ; Note [10] __vector_default: reti .end
#define work 16
int
by default in order to calculate constant integer
expressions. In order to get a 100 kHz output, we need to toggle the PD6 line
200000 times per second. Since we use timer 0 without any prescaling options in order to
get the desired frequency and accuracy, we already run into serious timing considerations:
while accepting and processing the timer overflow interrupt, the timer already continues
to count. When pre-loading the TCCNT0
register, we
therefore have to account for the number of clock cycles required for interrupt
acknowledge and for the instructions to reload TCCNT0
(4
clock cycles for interrupt acknowledge, 2 cycles for the jump from the interrupt vector, 2
cycles for the 2 instructions that reload TCCNT0
). This
is what the constant fuzz
is for.
.global. main
is the application entry point that will be jumped to from the ininitalization routine in
crts1200.o
.sleep
instruction (using idle mode) could be used as well, but probably would not conserve much
energy anyway since the interrupt service is executed quite frequently.
.global in order to be acceptable for this purpose. This will only work
if <avr/io.h>
has been included. Note that the assembler or
linker have no chance to check the correct spelling of an interrupt function, so it should
be double-checked. (When analyzing the resulting object file using
avr-objdump
or avr-nm
,
a name like __vector_N
should
appear, with N being a small integer number.)_SFR_IO_ADDR
. (The AT90S1200 does not have RAM thus the
memory-mapped approach to access the IO registers is not available. It would be slower
than using in
/ out
instructions anyway.) Since the operation to reload
TCCNT0
is time-critical, it is even performed before
saving SREG
. Obviously, this requires that the
instructions involved would not change any of the flag bits in
SREG
.SREG
. (Note that this serves as an
example here only since actually, all the following instructions would not modify
SREG
either, but that's not commonly the case.)
Also, it must be made sure that registers used inside the interrupt routine
do not conflict with those used outside. In the case of a RAM-less device like the
AT90S1200, this can only be done by agreeing on a set of registers to be used exclusively
inside the interrupt routine; there would not be any other chance to "save" a register
anywhere. If the interrupt routine is to be linked together with C modules,
care must be taken to follow the register usage guidelines imposed by the C compiler. Also, any register modified inside the
interrupt sevice needs to be saved, usually on the stack.__vector_default
. This must be
.global, and obviously, should end in a
reti
instruction. (By default, a jump to location 0
would be implied instead.)The available pseudo-ops in the assembler are described in the GNU assembler (gas) manual. The manual can be found online as part of the current binutils release under http://sources.redhat.com/binutils/.
As gas comes from a Unix origin, its pseudo-op and overall assembler syntax is
slightly different than the one being used by other assemblers. Numeric constants follow
the C notation (prefix 0x
for hexadecimal constants),
expressions use a C-like syntax.
Some common pseudo-ops include:
.byte allocates single byte constants
.ascii allocates a non-terminated string of characters
.asciz allocates a \0-terminated string of characters (C
string)
.data switches to the .data section (initialized RAM
variables)
.text switches to the .text section (code and ROM
constants)
.set declares a symbol as a constant expression (identical to
.equ)
.global (or
.globl) declares a public symbol
that is visible to the linker (e. g. function entry point, global variable)
.extern declares a symbol to be externally defined; this is
effectively a comment only, as gas treats all undefined symbols it encounters as
globally undefined anyway
Note that .org is available in gas as well, but is a fairly pointless
pseudo-op in an assembler environment that uses relocatable object files, as it is the
linker that determines the final position of some object in ROM or RAM.
Along with the architecture-independent standard operators, there are some AVR-specific operators available which are unfortunately not yet described in the official documentation. The most notable operators are:
lo8
Takes the least significant 8 bits of a
16-bit integer
hi8
Takes the most significant 8 bits of a
16-bit integer
pm
Takes a program-memory (ROM) address, and
converts it into a RAM address. This implies a division by 2 as the AVR handles ROM
addresses as 16-bit words (e.g. in an IJMP
or
ICALL
instruction), and can also handle
relocatable symbols on the right-hand side.
ldi r24, lo8(pm(somefunc)) ldi r25, hi8(pm(somefunc)) call something
This passes the address of function somefunc
as the
first parameter to function something
.