s921_writereg
rc = s921_writereg(state, 0xf2, (f_offset >> 8) & 0xff);
rc = s921_writereg(state, 0xf3, f_offset & 0xff);
rc = s921_writereg(state, 0xf4, f_switch);
rc = s921_writereg(state, 0x01, 0x40);