According to http://d4.princess.ne.jp/msx/psg/ the logic to generate the noise seems to be X <= X(15 downto 0) & (X(16) xor X(13) xor 1)
According to http://d4.princess.ne.jp/msx/psg/
the logic to generate the noise seems to be X <= X(15 downto 0) & (X(16) xor X(13) xor 1)