1) A 1-bit field that specifies the source of the frame synchronisation transmit (FSX) pulse. At reset, TXM = 0. This bit is stored in the serial port control register (SPC) and TDM serial port control register (TSPC). 2) Bit 3 of the synchronous serial port control register (SSPCR); determines whether the source signal for frame synchronisation is external or internal.