Answer
$$x= \cos t; y= \sin t; \pi \leq t \leq \dfrac{3 \pi}{2}$$
Work Step by Step
When $t=\pi$ , then the circle is traced clockwise by a point that originated at $(-1,0)$ and terminated at $(0,-1)$ and it completes one full circle of revolution when $t=\dfrac{3 \pi}{2}$.
So, parametric equations for the curve $x^2+y^2=1$ are: $$x= \cos t; y= \sin t; \pi \leq t \leq \dfrac{3 \pi}{2}$$
