get_pll_output_frequency
f = abs(get_pll_output_frequency(f0, cur) - clk);
if (abs(get_pll_output_frequency(f0, up) - clk) < f)
else if (abs(get_pll_output_frequency(f0, down) - clk) < f)
f = abs(get_pll_output_frequency(f0, cur) - clk);
if (f < abs(get_pll_output_frequency(f0, best) - clk))