CCS C Software and Maintenance Offers
FAQFAQ   FAQForum Help   FAQOfficial CCS Support   SearchSearch  RegisterRegister 

ProfileProfile   Log in to check your private messagesLog in to check your private messages   Log inLog in 

CCS does not monitor this forum on a regular basis.

Please do not post bug Reports on this forum. Send them to support@ccsinfo.com

Trouble getting a DS18B20 to work in parasite power mode...
Goto page Previous  1, 2
 
Post new topic   Reply to topic    CCS Forum Index -> General CCS C Discussion
View previous topic :: View next topic  
Author Message
PCM programmer



Joined: 06 Sep 2003
Posts: 20335

View user's profile Send private message

PostPosted: Tue Sep 15, 2009 6:19 pm     Reply with quote

I was able to make it work in parasite mode. I think the problem was
mainly in the timing of when the DQ pin should be set high. I will post
the list of modifications later, after I test it some more.
Kenny



Joined: 07 Sep 2003
Posts: 173
Location: Australia

View user's profile Send private message

PostPosted: Wed Sep 16, 2009 9:13 am     Reply with quote

Edited:

Deleted post.

Testing showed that the delay at the end of write_bit() wasn't critical within the specified timeslot width.
Had to be a long way outside the specified range before it stopped working.
Guest








PostPosted: Thu Sep 17, 2009 6:00 am     Reply with quote

PCM,

Many thanks, I'm eagerly awaiting your findings!

Joe
PCM programmer



Joined: 06 Sep 2003
Posts: 20335

View user's profile Send private message

PostPosted: Thu Sep 17, 2009 2:23 pm     Reply with quote

This is basically what you need to do. Modify your write_byte() routine
so it has a parameter to tell it to optionally turn on Bus power at the
end of the transmission. Get rid of the separate write_bit() routine.
Code:

void write_byte(int8 val, int8 power_on)
{
int i;

 for (i=0; i<8; i++)
 {
  output_low(DQ);
  delay_us( 2 ); 
  output_bit(DQ, shift_right(&val,1,0)); 

  delay_us(60);
 
  if((i == 7) && (power_on == 1))
    {
     output_high(DQ);
    }
  else
    {
     output_float(DQ);
     delay_us( 2 ); 
    }
 }
 
}


Then modify all the calls to write_byte() in your main code, so they
have the power on parameter. Then set it to turn power on after
the Temp Convert command is sent. Here's the relevant part:
Code:

while(1)
   {
      ow_reset();
      write_byte(0xCC, 0); // Skip Rom command
      write_byte(0x44, 1); // Temperature Convert command

      delay_ms(750);    // Max. time for conversion is 750mS
   
      ow_reset();
      write_byte(0xCC, 0); // Skip Rom command
      write_byte(0xBE, 0); // Read scratch pad command
   
      // Get the data bytes
      for (i=0 ; i<8 ; i++)
      {
          scratch[i] = read_byte(); 
      }
   
      ow_reset();


Then it should work. Putting my finger on the DS18B20 chip causes
the temperature to rise:
Quote:

Temp: 25 C
Temp: 25 C
Temp: 25 C
Temp: 25 C
Temp: 25 C
Temp: 25 C
Temp: 26 C
Temp: 27 C
Temp: 27 C
Temp: 28 C
Temp: 28 C
Temp: 28 C
Temp: 29 C
Temp: 29 C
Temp: 28 C
Temp: 28 C
Temp: 28 C
Temp: 28 C
Temp: 28 C
Temp: 28 C
Temp: 28 C
Temp: 28 C

I didn't completely test this, and I didn't check the timing with a scope
or logic analyzer to see if it fits the data sheet's spec. You'll have to
do that.
Guest








PostPosted: Thu Sep 17, 2009 8:47 pm     Reply with quote

Hi PCM,

That works - many thanks!

Interestingly, I experimented with a similar 'fix' that did not include the 'Power-on' flag. In retrospect that prevented my solution from working.

This experience has convinced me to get a scope, and I've got one on the way off eBay. When I get it, I'll look at the timing and verify that it's 'in spec.' For anyone following the thread, I'll report my findings sometime next week.

Thanks again for your interest, and your effort!

Joe
Guest








PostPosted: Thu Oct 01, 2009 1:16 pm     Reply with quote

C-coders,

I'd like to use this code to read two different sensors on two separate input pins. To do this, I was thinking of replacing this line:

Code:

#define DQ PIN_A0      // One Wire Bus pin assignment


with something like this

Code:

int8 DQ = 40;


That would allow me to set DQ to any value from the .h file for my PIC and specify a different sensor using the same DS18B20 subroutines..... The problem is that it doesn't seem to work, and I'm not sure why. It compiles, but the temperature that is returned is always zero.

If this is not the proper way to specify different sensors, can someone recommend another method?

Cheers,

Greg
PCM programmer



Joined: 06 Sep 2003
Posts: 20335

View user's profile Send private message

PostPosted: Thu Oct 01, 2009 1:57 pm     Reply with quote

This driver requires careful timing. The ASM code that CCS generates
to do pin i/o functions with a variable argument, is huge. It takes much
longer to execute than if a constant argument is used. You can see this
with a test program:

Code:
#include <16F877.H>
#fuses XT, NOWDT, BROWNOUT, PUT, NOLVP
#use delay(clock=4000000)

//======================================
void main()
{
int8 pin = PIN_A0;

output_low(PIN_A0);

output_low(pin);

while(1);
}


Here's the .LST file. Notice how it takes 4 instructions with a constant,
but a huge number with a variable argument. At 4 MHz, one instruction
takes 1 usec. With a variable argument, all these extra instructions
ruin the careful timing of the existing ds18b20 driver.
Code:
.................... output_low(PIN_A0);
0030:  BSF    03.5
0031:  BCF    05.0
0032:  BCF    03.5
0033:  BCF    05.0
.................... 
.................... output_low(pin);
0034:  MOVF   21,W
0035:  MOVWF  22
0036:  CLRF   23
0037:  CLRF   25
0038:  CLRF   24
0039:  CALL   004
003A:  MOVF   21,W
003B:  MOVWF  22
003C:  CLRF   23
003D:  CLRF   25
003E:  MOVLW  80
003F:  MOVWF  24
0040:  CALL   004
.................... 
// The routine at 0x0004:
0004:  MOVF   22,W
0005:  ANDLW  07
0006:  MOVWF  77
0007:  RRF    22,W
0008:  MOVWF  78
0009:  RRF    78,F
000A:  RRF    78,F
000B:  MOVLW  1F
000C:  ANDWF  78,F
000D:  MOVF   78,W
000E:  ADDWF  24,W
000F:  MOVWF  04
0010:  BCF    03.7
0011:  BTFSC  25.0
0012:  BSF    03.7
0013:  CLRF   78
0014:  INCF   78,F
0015:  INCF   77,F
0016:  GOTO   018
0017:  RLF    78,F
0018:  DECFSZ 77,F
0019:  GOTO   017
001A:  MOVF   23,F
001B:  BTFSC  03.2
001C:  GOTO   020
001D:  MOVF   78,W
001E:  IORWF  00,F
001F:  GOTO   023
0020:  COMF   78,F
0021:  MOVF   78,W
0022:  ANDWF  00,F
0023:  RETLW  00
Guest








PostPosted: Thu Oct 01, 2009 2:10 pm     Reply with quote

PCM,

OK, I understand the timing issues involved.

Do you have a recommendation for how I can read sensors on two separate pins using this code?

Thanks,

Greg
PCM programmer



Joined: 06 Sep 2003
Posts: 20335

View user's profile Send private message

PostPosted: Thu Oct 01, 2009 2:13 pm     Reply with quote

Make a copy of the routines and give them new names. Use your other
pin number in the new routines. Call the new routines for the 2nd chip.
Guest








PostPosted: Thu Oct 01, 2009 2:47 pm     Reply with quote

Hehe, I was afraid you were going to say that Shocked !

I'm going to try a 20 MHz xtal first, and if that doesn't provide sufficient margin, I'll use separate code for each sensor!

Thanks,

Greg
Guest








PostPosted: Fri Oct 02, 2009 1:43 pm     Reply with quote

Hi PCM,

I've been thinking about this issue, and I have a question. I understand that changing the DQ pin designation "on the fly" is time consuming from an instruction point-of-view. Obviously, something happens when the #Define DQ Pin_A0 is executed, and I'm not sure why I can't replicate that functionality to select another pin between readings of my two sensors. Yes, the readings themselves are time critical, but I've got tons of time available between readings. Isn't there some way to get the same effect as the #Define, to switch I/O pins during runtime? I'm not trying to be a pain, just to learn something here about how the compiler works, or its limitations.

Greg
Guest








PostPosted: Fri Oct 02, 2009 2:01 pm     Reply with quote

Another thought is that there is some code written by Peter Anderson that is floating around that supports up to four sensors each on a separate pin. The only issue is that it's written for Port B. I looked at the code and it looks like I could change the PORTB calls to PORTA, and the TRISB to TRISA, and get the same functionality on Port A?

Greg
PCM programmer



Joined: 06 Sep 2003
Posts: 20335

View user's profile Send private message

PostPosted: Fri Oct 02, 2009 2:17 pm     Reply with quote

Quote:
something happens when the #Define DQ Pin_A0 is executed,

It happens at compile-time, not at run-time. At compile-time, the
compiler creates the machine code for the Hex file. It uses that
#define statement to put the correct numbers into the BSF and BCF
instructions in the machine code.

You could possibly use a switch-case statement. Re-write the routines
to accept a channel number (0 to 3), and inside each case, you could
do the i/o operation on the appropriate pin.

This is the Peter Anderson code. It might give you some ideas.
http://www.phanderson.com/PIC/PICC/CCS_PCM/ds1820.html
PCM programmer



Joined: 06 Sep 2003
Posts: 20335

View user's profile Send private message

PostPosted: Sun Mar 14, 2010 1:22 pm     Reply with quote

Here's the complete test program for the ds18B20 in parasite-power
mode. This is the original poster's program, with the modifications
added, that I recommended in a post above. I'm posting the integrated
program because of a request.
Code:

#include <16F877.H>
#fuses XT, NOWDT, BROWNOUT, PUT, NOLVP
#use delay(clock=4000000)
#use rs232(baud=9600, xmit=PIN_C6, rcv=PIN_C7, ERRORS)

#define DQ   PIN_C2   

//-------------------------------------
void ow_reset(void)
{
output_low(DQ);
delay_us(500);          // Min. 480uS
output_float(DQ);
delay_us(500);          // Wait for end of timeslot
}

//-------------------------------------
// Read bit on one wire bus

int8 read_bit(void)
{
output_low(DQ);
delay_us(1);         
output_float(DQ);
delay_us(12);        // Read within 15uS from start of time slot
return(input(DQ));   
}                       

//-------------------------------------
int8 read_byte(void)
{
int8 i;
int8 val = 0;

for(i=0 ; i<8 ; i++)
   {
    if(read_bit()) val |= (0x01 << i);
       delay_us(120);  // To finish time slot
   }

return val;
}

//-------------------------------------
void write_byte(int8 val, int8 power_on)
{
int i;

for(i=0; i<8; i++)
   {
    output_low(DQ);
    delay_us( 2 ); 
    output_bit(DQ, shift_right(&val,1,0)); 

    delay_us(60);
 
    if((i == 7) && (power_on == 1))
      {
       output_high(DQ);
      }
    else
      {
       output_float(DQ);
       delay_us( 2 ); 
      }
   }

}

//==========================================
void main(void)
{
int8 i;
signed int16 temperature;
int8 scratch[9];       
   
output_float(DQ);     
   
while(1)
  {
   ow_reset();
   write_byte(0xCC, 0); // Skip Rom command
   write_byte(0x44, 1); // Temperature Convert command

   delay_ms(750);    // Max. time for conversion is 750mS
   
   ow_reset();
   write_byte(0xCC, 0); // Skip Rom command
   write_byte(0xBE, 0); // Read scratch pad command
   
   // Get the data bytes
   for(i=0; i < 8; i++)
      {
       scratch[i] = read_byte(); 
      }
   
   ow_reset();
     
   temperature = (signed int16) make16(scratch[1],scratch[0]);
         
   if(temperature >= 0)
      temperature = (temperature + 8)/16;
   else
      temperature = (temperature - 8)/16;
           
   printf("Temp: %4Ld C \n\r",temperature);
  }

}
Display posts from previous:   
Post new topic   Reply to topic    CCS Forum Index -> General CCS C Discussion All times are GMT - 6 Hours
Goto page Previous  1, 2
Page 2 of 2

 
Jump to:  
You cannot post new topics in this forum
You cannot reply to topics in this forum
You cannot edit your posts in this forum
You cannot delete your posts in this forum
You cannot vote in polls in this forum


Powered by phpBB © 2001, 2005 phpBB Group