HELP DS18b20 negative reading

[mastero]

'PIC18F2520
Define CLOCK_FREQUENCY = 12
AllDigital
'Define SIMULATION_WAITMS_VALUE = 1

Define LCD_LINES = 2
Define LCD_CHARS = 16
Define LCD_BITS = 4
Define LCD_DREG = PORTB
Define LCD_DBIT = 4
Define LCD_RSREG = PORTB
Define LCD_RSBIT = 2
Define LCD_RWREG = 0
Define LCD_RWBIT = 0
Define LCD_EREG = PORTB
Define LCD_EBIT = 3
Define LCD_READ_BUSY_FLAG = 0
Lcdinit 0

Define 1WIRE_REG = PORTB
Define 1WIRE_BIT = 1

Dim finish As Bit
Dim sign As Bit
Dim temp As Word
Symbol led = PORTC.2

led = 1
WaitMs 1000
led = 0

loop:
1wireInit
1wireSendByte 0xcc, 0x44
WaitMs 1000

loop1:
1wireGetBit finish
If finish = 0 Then Goto loop1

1wireInit
1wireSendByte 0xcc, 0xbe
1wireGetByte temp.LB, temp.HB
Lcdcmdout LcdClear
Lcdout #temp 'raw data readout
WaitMs 2000

sign = temp.15

If sign = 1 Then
led = 1
Lcdout "-"
temp = 65535 - temp
temp = temp + 1
Endif

temp = temp / 16

Lcdout #temp

WaitMs 2000
Goto loop


****************************************************
above is the code i use for reading Ds18b20 it is give proper reading for positive temperature.
The negative temp is not shown i only get to 0 Deg C and raw data is showing 10 in ice cold water.
LED is not getting triggered, tried different DS18B20 from different suppliers.

any ideas ??

cheers
mastero

 

[rjenkinsgb]

What do you get if you display the temp.LB and temp.HB values separately?

The result at that point may help pin down the cause of the problem.

 

[mastero]

temp.lb displays 143
temp.hb displays 1

at room temperature

 

[rjenkinsgb]

OK, what about when below freezing?

 

[mastero]

sign = temp.HB
If sign = 255 Then
'led = 1
Lcdout "-"
temp = 65535 - temp
temp = temp + 1
Endif
temp = temp / 16
Lcdout #temp
SOLVED thanx

 

[rjenkinsgb]

Using 255 would only work to -15'C, if that matters?
You could make it eg.
if sign > 127 then
...
That avoids confusing the bits representing temperature with the sign bits.