#define DS1990_FAMILY 1

typedef unsigned char byte;
//|8-bit family|48-bit serial number|8-bit crc|
typedef struct {
byte family;
byte sn[6];
byte crc;
}t1wds;

t1wds ibutton;

//calc crc 7 bytes, to 8 byte crc
#pragma warn-
void ds_crc(byte *b) {
#asm
push r16
push r17
push r18
push r26
push r27
clr r31
ldi r18,0x18
ld r26,y
ldd r27,y+1
ldi r17,7
_do_crc:
ld r30,x++
push r30
ldi r16,8
_crc_loop:
eor r30,r31
ror r30
mov r30,r31
brcc _zero
eor r30,r18
_zero:
ror r30
mov r31,r30
pop r30
lsr r30
push r30
dec r16
brne _crc_loop
pop r30
dec r17
brne _do_crc
st x,r31
pop r27
pop r26
pop r18
pop r17
pop r16
#endasm
}
#pragma warn+

..........................

ibutton.family=DS1990_FAMILY;
ibutton.sn[0]=0;
ibutton.sn[1]=0;
ibutton.sn[2]=0;
ibutton.sn[3]=0;
ibutton.sn[4]=0;
ibutton.sn[5]=0;

ds_crc((byte*)&ibutton); //ibutton.crc=0x3d;