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Guest
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 Posted: Thu Oct 01, 2009 1:16 pm |
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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
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with something like this
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 |
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PCM programmer
Joined: 06 Sep 2003
Posts: 21708
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| Posted: Thu Oct 01, 2009 1:57 pm |
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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 |
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Guest
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| Posted: Thu Oct 01, 2009 2:10 pm |
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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 |
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PCM programmer
Joined: 06 Sep 2003
Posts: 21708
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| Posted: Thu Oct 01, 2009 2:13 pm |
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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. |
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Guest
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| Posted: Thu Oct 01, 2009 2:47 pm |
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Hehe, I was afraid you were going to say that  !
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 |
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Guest
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| Posted: Fri Oct 02, 2009 1:43 pm |
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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 |
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Guest
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| Posted: Fri Oct 02, 2009 2:01 pm |
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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 |
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PCM programmer
Joined: 06 Sep 2003
Posts: 21708
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| Posted: Fri Oct 02, 2009 2:17 pm |
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| 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 |
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PCM programmer
Joined: 06 Sep 2003
Posts: 21708
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| Posted: Sun Mar 14, 2010 1:22 pm |
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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);
}
} |
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