Semaphores in Delphi

I'm trying to implement a simple semaphore to protect critical sections of code with the following procedures.

Question is are these safe, or is it possible that 2 threads could slip through EnterCritical at the same time?

Thanks

procedure EnterCritical;
begin
while semaphore do sleep(30);
semaphore:=true;
end;

procedure ExitCritical;
begin
semaphore:=false;
end;
.