------------------------------------------------
                        CKSEL0 = [.]  1   1 MHz  default
                        CKSEL1 = [v]  0
                        CKSEL2 = [v]  0
                        CKSEL3 = [v]  0
------------------------------------------------
                        CKSEL0 = [v]  0   2 MHz
                        CKSEL1 = [.]  1
                        CKSEL2 = [v]  0
                        CKSEL3 = [v]  0
------------------------------------------------
                        CKSEL0 = [.]  1   4 MHz
                        CKSEL1 = [.]  1
                        CKSEL2 = [v]  0
                        CKSEL3 = [v]  0
------------------------------------------------
                        CKSEL0 = [v]  0   8 MHz
                        CKSEL1 = [v]  0
                        CKSEL2 = [.]  1
                        CKSEL3 = [v]  0
------------------------------------------------
