How can unlocking the DAP fix the MOSC clocking into UART block? After POR - launch pad Prints out MOSC 120Mhz, PLL 480 via questionable Tivaware calls below. Removing Pref-flight MOSC checks, the DAP locked MCU and required an unlock device each time mentioned procedure was repeated. The result of unlocking DAP mysteriously fixed UART baud rate clock or PLL frequency, take a pick. Seemingly many posts questioning the PLL frequency has not been corrected for all peripherals, even the ADC sample clock is highly suspect. The simple SysCtrlVCOGet() does not detect partially lock PLL state and clock phasing obviously occurs that effects Stable peripheral operation. Perhaps VCOGet() should check PLL frequency between two points in time rather than 1 quick test reporting GO dog GO.
Obviously the Tivaware MOSC/PLL frequency Get reports what you want to see rather than the true PLL state. If the PLL phases in and out of lock state the UART buad rate clock would be effected in the way previously reported, failure of UART_FR_RXFE, UART_INT_RT flags timing with an external asynchronous device attempting to transfer 32 bytes of data into 16x8 bit FIFO.
MOSC Preflight checks shown below prior to SysCtrlClockFreqSet() must exist or MCU halts and locks the DAP. Review of SysCtrlClockFrqSet() discovers no such HWREG preflight checks of MOSC being asserted. Datasheet initialization procedure for enabling the MOSC block includes preflight checks below. Even with preflight checks the PLL was obviously not fully engaged in a locked frequency state and direcly impacted the UART phasing of the Baudrate clock frequency.