4.3.2 Choosing a Frequency

A wide range of frequencies is possible, because the USB PLL has a very flexible programmable input divider. Each MSP430 header file contains predefined constants for 43 possible frequencies, and many more are possible. (The pre-defined ones are of the format USBPLL_SETCLK_xx_yy, where xx_yy is frequency-specific.)

As shown in Section 4.3.1, each clock source is subject to a frequency range.

There are two considerations in frequency selection:

• Lower-frequency crystals or resonators consume less power than higher ones. (See the device data sheet power consumption values for XT2.)
• If using the USB BSL for factory programming of the MSP430 device, only 4, 8, 12, and 24 MHz are auto-recognized by the BSL. See Section 3.6.

USB-equipped MSP430 boards from TI usually use a 4-MHz ceramic resonator that meets USB precision requirements. Being the lowest-frequency crystal possible, these have the advantage of minimizing power consumption. 4 MHz is also auto-detected by the on-chip BSL.