NAME¶
Additional notes from <avr/sfr_defs.h> - The <
avr/sfr_defs.h>
file is included by all of the
<avr/ioXXXX.h>
files, which use macros defined here to make the
special function register definitions look like C variables or simple
constants, depending on the _SFR_ASM_COMPAT
define. Some examples from
<avr/iocanxx.h>
to show how to define such macros:
#define PORTA _SFR_IO8(0x02)
#define EEAR _SFR_IO16(0x21)
#define UDR0 _SFR_MEM8(0xC6)
#define TCNT3 _SFR_MEM16(0x94)
#define CANIDT _SFR_MEM32(0xF0)
If _SFR_ASM_COMPAT is not defined, C programs can use names like PORTA directly
in C expressions (also on the left side of assignment operators) and GCC will
do the right thing (use short I/O instructions if possible). The __SFR_OFFSET
definition is not used in any way in this case.
Define _SFR_ASM_COMPAT as 1 to make these names work as simple constants
(addresses of the I/O registers). This is necessary when included in
preprocessed assembler (*.S) source files, so it is done automatically if
ASSEMBLER is defined. By default, all addresses are defined as if
they were memory addresses (used in lds/sts
instructions). To use
these addresses in in/out
instructions, you must subtract 0x20 from
them.
For more backwards compatibility, insert the following at the start of your old
assembler source file:
#define __SFR_OFFSET 0
This automatically subtracts 0x20 from I/O space addresses, but it's a hack, so
it is recommended to change your source: wrap such addresses in macros defined
here, as shown below. After this is done, the __SFR_OFFSET definition is no
longer necessary and can be removed.
Real example - this code could be used in a boot loader that is portable between
devices with SPMCR at different addresses.
<avr/iom163.h>: #define SPMCR _SFR_IO8(0x37)
<avr/iom128.h>: #define SPMCR _SFR_MEM8(0x68)
#if _SFR_IO_REG_P(SPMCR)
out _SFR_IO_ADDR(SPMCR), r24
#else
sts _SFR_MEM_ADDR(SPMCR), r24
#endif
You can use the in/out/cbi/sbi/sbic/sbis instructions, without the _SFR_IO_REG_P
test, if you know that the register is in the I/O space (as with SREG, for
example). If it isn't, the assembler will complain (I/O address out of range
0...0x3f), so this should be fairly safe.
If you do not define __SFR_OFFSET (so it will be 0x20 by default), all special
register addresses are defined as memory addresses (so SREG is 0x5f), and (if
code size and speed are not important, and you don't like the ugly #if above)
you can always use lds/sts to access them. But, this will not work if
__SFR_OFFSET != 0x20, so use a different macro (defined only if __SFR_OFFSET
== 0x20) for safety:
sts _SFR_ADDR(SPMCR), r24
In C programs, all 3 combinations of _SFR_ASM_COMPAT and __SFR_OFFSET are
supported - the _SFR_ADDR(SPMCR) macro can be used to get the address of the
SPMCR register (0x57 or 0x68 depending on device).
Author¶
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