The OP is trying to figure out how to set the firing angle (phase control) of a triac controlling a load at mains 230V AC. The schematic as posted has major flaws and needs redesign and the code part too. The most siple way to start is to achive the ON/OFF of a resistive load using a read ZDC as MOC3041 as mentioned here.

Another idea is to use the Analog Comparator inside the AT89C2051. Ideas for this are presented in the mentioned application notes from NXP. Ideas to be considered as examples and guides in HW and SW part. The actual circuit can be implemented either following every line or by opening the mind of the OP with new posibilites and knowledge.

The AT89C2051 does not support In System Programming, so the OP has to pull out the chip from the actual circuit, place it on the programmer, load the hex file, and place back the chip to the circuit. So to program the chip even if the power directly from mains had chosen (carefully designed) does not threat the system in operation.

The schematic posted is designed using a transformer to provide power to the circuit but the zero crossing part has to be corrected to a lot of things.

Even the in the summary of schematics a flaw exists in figure 18. Can you spot it ?