clk monitoring & switching

respected sir
anyone can tell me i m having clk input of 80MHz. now what i want is i want monitor this signal & whenever it goes below 1MHz than i want to switch new clk that is 1MHz.now what circuit is required plz help me

How about route the 80 MHz clock via a PLL, and configure the PLL to 'bottom out' at 1 MHz, use the PLL's lock detector as a Flag