The maximum clock frequency of the MSP430F5438IPZ is 18 MHz.
To configure the MSP430F5438IPZ for low power consumption, use the low-power modes (LPM3, LPM4) and adjust the clock frequency, disable unused peripherals, and use the Power Management Module (PMM) to control power consumption.
Yes, the MSP430F5438IPZ can be used with an external crystal oscillator. The device has an internal oscillator, but an external crystal oscillator can be used for more accurate clocking.
The MSP430F5438IPZ can be programmed using the Texas Instruments Code Composer Studio (CCS) integrated development environment (IDE) or other third-party development tools.