The presence pulse comes from DS1821. It is about 0.5msec Active Low pulse and indicating to the master (here your 8051) that it is ready to operate given an appropriate function command. So, after pushing low DQ line to Low (about 0.5msec-This is your reset pulse) you have to set your R0 to HiZ (High Impedance) and have to wait Presence Pulse will come from DS1821.. That's all..
Here, HiZ means set the R0 as an input pin. In other words, set the output driver of R0 to HiZ mode.. Why you should do it? Because, in next step you need the read the level of DQ line.. If it is continuously High, it indicates that something is wrong.. But, If you can read DQ as Low about during 500msec(or maybe shorter), it is Presence Pulse that coming from DS1821. It indicates that DS1821 is alive and waits your function commands...
make the coresponding pin output and put it in low state for 60us. then make it input again.
just read very careful the timing diagrams in the ds1821 datasheet. you will get the answers. remenber that in 1Wire bus is a bidirectional line and the timing is critical.