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.
Code:
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
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