2 wire sensor problem

[precision]

I attempt to read ds1621 with pic16f877A, but always reading 00 value;
Where is mistake ? Please .....

' pic16f877a

DEFINE OSC 4
INCLUDE "MYLCD.BAS" ' lcd on portb
CMCON = 7
ADCON1 = 7

SCL VAR PORTD.2
SDA VAR PORTD.3

;( A0,A1,A2 = gnd )

W0 var word
ds var byte
Startconvert var byte

Startconvert = $EE ' Initiates temperature conversion.
ds = %10100000 ' device address


start:
i2cwrite SDA,SCL,ds,$AC,[0] ' Access Config
pause 100
i2cwrite SDA,SCL,ds,Startconvert ' Start Conversion
pause 750
gosub gettemp
lcdout $fe,1
lcdout "Temperature = ",dec2 W0

pause 800
goto start



gettemp:
i2cread SDA,SCL,ds,$AA,[W0] ' Read last converted temperature value from temperature register.
W0 = W0 >> 8
return