SLAU847D October 2022 – May 2024 MSPM0L1105 , MSPM0L1106 , MSPM0L1227 , MSPM0L1228 , MSPM0L1228-Q1 , MSPM0L1303 , MSPM0L1304 , MSPM0L1304-Q1 , MSPM0L1305 , MSPM0L1305-Q1 , MSPM0L1306 , MSPM0L1306-Q1 , MSPM0L1343 , MSPM0L1344 , MSPM0L1345 , MSPM0L1346 , MSPM0L2227 , MSPM0L2228 , MSPM0L2228-Q1
The LFSS has its own reset generation circuit that is implemented in the VBAT domain. Two subcircuits are related to the reset of the LFSS. The power-on-reset (POR) circuit is Vth based supply voltage detector on the VBAT domain. The POR circuit is used to reset the PMU and the startup of the cold boot sequence. The brown-out-reset (BOR) circuit is a reference-based voltage monitor and enables the LDO when the VBAT supply is large enough to operate the LDO safely. On initial power up of the VBAT supply and the enable of the VRTC LDO, the VRTC domain sees initial reset. When the reset is de-asserted, the VRTC domain does not see another reset unless the power supply on VBAT drops below the BOR level. The POR or BOR reset from the device PMU does not reset the VRTC domain.
In addition, the LFSS supports an LFSS-POR level software-reset request. This reset supports software development and from user point of view it looks like the VBAT supply was temporary disconnected and reconnected. The reset disables the VBAT PMU and reenables the VBAT cold boot sequence. This also means the VRTC domain is brought down and back up and all flops have a full reset. The LFSS software POR request also triggers the VBAT power-down and power-up interrupts indicating the loss on the VRTC domain.
After the LFXT and RTC are configured and initialized by software, a persistent status bit in the VRTC domain indicates the “running / RTC active” status to the software. On device power-up, the software reads the reset cause table and initializes the device accordingly (for example, initial power up or wake from SHUTDOWN mode). Before initializing the RTC, the software should check the LFXT and RTC good/active status and skip the initialization when already running and active.