DS1820 again

[paxmowa]

Hi,

I'm trying to use code I found here to get a reading from a DS1820 device. Whatever I try, this always gives me a 'not present' error. But the brand new 1820 is right there and connected to PORTC.0

I'm using a PIC16F876A for this project.



DEFINE OSC 4

' Allocate variables
command var byte ' Storage for command
i var byte ' Storage for loop counter
temp var word ' Storage for temperature
DQ var PORTC.0 ' Alias DS1820 data pin
DQ_DIR var TRISC.0 ' Alias DS1820 data direction pin

Pause 500 ' Wait for LCD to start

Lcdout $fe, 1, "Temp in degrees C" ' Display sign-on message


' Mainloop to read the temperature and display on LCD
mainloop:
Gosub init1820 ' Init the DS1820

command = $cc ' Issue Skip ROM command
Gosub write1820

command = $44 ' Start temperature conversion
Gosub write1820

Pause 2000 ' Wait 2 seconds for conversion to complete

Gosub init1820 ' Do another init

command = $cc ' Issue Skip ROM command
Gosub write1820

command = $be ' Read the temperature
Gosub write1820
Gosub read1820

' Display the decimal temperature
Lcdout $fe, 1, dec (temp >> 1), ".", dec (temp.0 * 5), " degrees C"

Goto mainloop ' Do it forever


' Initialize DS1820 and check for presence
init1820:
Low DQ ' Set the data pin low to init
Pauseus 500 ' Wait > 480us
DQ_DIR = 1 ' Release data pin (set to input for high)

Pauseus 100 ' Wait > 60us
If DQ = 1 Then
Lcdout $fe, 1, "DS1820 not present"
Pause 500
Goto mainloop ' Try again
Endif
Pauseus 400 ' Wait for end of presence pulse
Return


' Write "command" byte to the DS1820
write1820:
For i = 1 to 8 ' 8 bits to a byte
If command.0 = 0 Then
Gosub write0 ' Write a 0 bit
Else
Gosub write1 ' Write a 1 bit
Endif
command = command >> 1 ' Shift to next bit
Next i
Return

' Write a 0 bit to the DS1820
write0:
Low DQ
Pauseus 60 ' Low for > 60us for 0
DQ_DIR = 1 ' Release data pin (set to input for high)
Return

' Write a 1 bit to the DS1820
write1:
Low DQ ' Low for < 15us for 1
@ nop ' Delay 1us at 4MHz
DQ_DIR = 1 ' Release data pin (set to input for high)
Pauseus 60 ' Use up rest of time slot
Return


' Read temperature from the DS1820
read1820:
For i = 1 to 16 ' 16 bits to a word
temp = temp >> 1 ' Shift down bits
Gosub readbit ' Get the bit to the top of temp
Next i
Return

' Read a bit from the DS1820
readbit:
temp.15 = 1 ' Preset read bit to 1
Low DQ ' Start the time slot
@ nop ' Delay 1us at 4MHz
DQ_DIR = 1 ' Release data pin (set to input for high)
If DQ = 0 Then
temp.15 = 0 ' Set bit to 0
Endif
Pauseus 60 ' Wait out rest of time slot
Return

End


What am i doing wrong here? The LCD works OK, it's just the reading from the 1820 that won't work, i don't understand why, because it's so simple to connect.

Thanks in advance,
Steve

[paxmowa]

Don't bother everyone, I didn't notice the line saying (BOTTOM VIEW) at the base of the IC lay-out, thus GND and VDD where connected the wrong way. Thank god it still lives and functions now.

Steve

[DynamoBen]

Don't feel bad I did the exact same thing a week ago. However I wasn't so lucky I need to purchase a new sensor.

[paxmowa]

Bummer...

It's always these rather stupid mistakes you have to watch out for. I do recall i did exactly the same thing some time ago with a LM35 sensor, so it aint the first time this happened to me ;)

One of these days i'm gonna manage to wriggle a brandnew athlon64 into it's socket the wrong way around and see it go up in flames a little later, it's bound to happen one of these days ;)

Steve