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Weird result on iButton search ROM

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Weird result on iButton search ROM
PostPosted: Mon Feb 14, 2005 10:16 am     Reply with quote


I followed Dallas AN 187 about "1-Wire Search Algorithm" to write a Search_ROM function on a PICC 16F877. I finally wrote a working code (or so it seems) and tried to read the ROM code on a 1-Wire Weather Station from AAG : I should find a DS18S20, a DS2423 and a DS2450.

My function returns a 64 bit code for both DS2423 and DS18S20 (according to the family codes), but I only get the beginning of the 64 code for the DS2450. After a random number of bits read by the search ROM algorithm, function returns an error (no device participating in the search).

I doubt the problem is in the code, as it works fine for the two other components, but I'll try have a closer look to the logic anyway. Does anyone know if the DS2450 has particular needs ?! Or I might be interested in a 100% working Search_ROM function for my 16F877...

Douglas Kennedy

Joined: 07 Sep 2003
Posts: 755
Location: Florida

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PostPosted: Mon Feb 14, 2005 5:14 pm     Reply with quote

I used this a few years back maybe it will be useful for you as a comparison to the code you wrote


#define dow_pin pin_b0
#bit    dow_pin_bit=6.0
char dev_id[8];

master_write_bit( short int send_bit)
        if(!dow_pin_bit) return(false);
        if(!dow_pin_bit) return( false);
void set_dev_id_bit(char *dev_id,int rom_bit_index,short int rom_bit)
// sets the bit referenced by rom_bit_index to the value
// on rom_bit
// rom_bit_index 0..63

int this_byte,value,this_bit;
this_byte=rom_bit_index / 8;
this_bit=rom_bit_index % 8;
if (rom_bit==1)

byte master_dow_search_rom(char *dev_id)
// for each bit of the unique 64 bit id that each device has
// the master reads twice and writes once
// on the first read each of the slaves repond with a specific bit
// on the second read  they send the complement
// the master then selects a value for that bit ( 0 or 1)
// slaves with that value stay in the process and the others go offline
// until the next reset
// since  any slave going low takes the line down for all
// the master  can see the following for each bit position in the id
// if slaves have 0 and 1 bits ..... 0 0
// if all slaves have 0        ..... 0 1
// if all slaves have 1        ..... 1 0
//          note:                    1 1 is an error

// master uses the 0 0 state to select among the slaves
// for the 0 1  state it sends 0 to keep all slaves for the next round
// for the 1 0  state it sends 1 to keep all slaves for the next round
// sent_bit_actual is not the bit from a single device
//  ( unless it is the only one still selected )
// but is the wire or value of the conflicting devices
// any device which is pulling low will win

short int sent_bit_actual,sent_bit_complement;
int rom_bit_index,last_discrepancy,marker;


if(done)goto exit;

if( !master_reset()) return (false); // problem with wire
if( !master_read_presence())
     return(false); // no slaves detected

// now inventory the attached slaves


master_write_byte(0x0f); // search rom command sent to slaves


if (sent_bit_actual==1 && sent_bit_complement==1)
    last_discrepancy=1;      // 1 1  something is wrong

if ( sent_bit_actual==0 && sent_bit_complement==0)

           if (get_dev_id_bit(dev_id,rom_bit_index)==0)

else set_dev_id_bit(dev_id,rom_bit_index,sent_bit_actual);

if(rom_bit_index<=63)goto read_dow;
if(last_discrepancy==0)  done=true;


PostPosted: Wed Feb 16, 2005 6:41 am     Reply with quote

Ok, thanks for your code, but I found the answer !!! As I suspected, the problem was not in my code but on the board itself. The DS2450 seems to need more power than the one provided only by the 1-Wire bus for complex operations like Search_ROM : that's why the function worked fine for reading codes of other components, but failed on that specific one. Confused

Added a capacitor to the board, to store some extra power when the line is high, and now I can read the ROM code of all 3 components that share the DATA line. Time for me to try and address each component individually now : wish me luck !!! May the Force be with me... Laughing

search rom for 1 wire device
PostPosted: Thu Mar 17, 2005 10:07 pm     Reply with quote


I would you like your code (.c file full integral) to talking a wheather station ?

I tried but found just ds18s20 on wheather station??????

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