SLAU847E October 2022 – May 2025 MSPM0L1105 , MSPM0L1106 , MSPM0L1116 , MSPM0L1117 , MSPM0L1227 , MSPM0L1227-Q1 , MSPM0L1228 , MSPM0L1228-Q1 , MSPM0L1303 , MSPM0L1304 , MSPM0L1304-Q1 , MSPM0L1305 , MSPM0L1305-Q1 , MSPM0L1306 , MSPM0L1306-Q1 , MSPM0L1343 , MSPM0L1344 , MSPM0L1345 , MSPM0L1346 , MSPM0L2227 , MSPM0L2227-Q1 , MSPM0L2228 , MSPM0L2228-Q1
It is possible to bypass the HFXT circuit and bring in an external digital clock signal into the device to use as the HFCLK source instead of HFXT. To configure HFCLK to use a digital clock input, first configure the IOMUX to enable the HFCLK_IN function on the appropriate pin. When IOMUX is configured correctly and the clock source is outputting a clock to HFCLK_IN, set the USEEXTHFCLK bit in the HSCLKEN register in SYSCTL.
The HFCLK_IN can be used as the reference for SYSPLL. The SYSPLL must be off before changing the configuration of the SYSPLL reference clock source. The user can verify that SYSPLL is off by checking the SYSPLLOFF status bit in the CLKSTATUS register. The user must then select HFCLK_IN as the HFCLK source (by setting the USEEXTHFCLK bit in the HSCLKEN register) before setting the SYSPLLREF bit in the SYSPLLCFG0 register in SYSCTL. Finally, the SYSPLL can be enabled by using the SYSPLLEN bit in the HSCLKEN register.
To source MCLK from HFCLK_IN after selecting HFCLK_IN as the HFCLK source, first set the HSCLKSEL bit in the HSCLKCFG register to select HFCLK as the high-speed clock source. Then, set the USEHSCLK bit in the MCLKCFG register to select the high-speed clock source as the MCLK source. Once USEHSCLK is set, HSCLKCFG must not change and the HFCLK_IN must not be disabled until the MCLK source is switched back to SYSOSC by clearing USEHSCLK and verifying that the HSCLKMUX bit in CLKSTATUS was cleared by hardware.
HFCLK_IN is compatible with digital square wave CMOS clock inputs and should have a typical duty cycle of 50%.