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The presentation will now take a deeper look into the capabilities of the clock system and provide some meaning behind each of the clock options that are possible. LFXT1 is the most common clock source within the MSP430 clock system and is used to provide a 32.768 kHz clock source to the ACLK branch of the tree, sourcing at a minimum a timer, perhaps an RTC. This is in every MSP430 device and typically is in conjunction with an external 32.768kHz watch crystal; note that there are integrated load capacitors built into LFXT1 on the MSP430 when using it with a 32.768kHz crystal. It is possible in TI’s newer parts to use LFXT1 to source a low frequency digital clock from off-chip, check the specific device datasheet or family user’s guide for more details. Also it is worth noting that LFXT1 has a high frequency mode as well, often referred to as HFXT1 or just XT1 that we will discuss shortly. The VLO, or very low frequency, very low power oscillator is built into our 2xx, 5xx and 6xx devices. This clock source is fully integrated and provides a clock frequency of ~10 kHz to the system. It consumes less than 1 µA and when used as a wake up timer source it provides the lowest power LPM3 operation possible. However, it is not designed to be accurate and varies quite a bit device to device and over temperature and voltage. Think of it as a good enough wake-up clock that is reliable, cheap and ultra-low power. REFO is yet another option for a fully integrated clock source and is built into our 5xx/6xx devices. REFO is also designed to be low power but does consume more than the VLO at ~3 µA. The added current also means a more stable frequency; REFO is factory calibrated to 32.768 kHz and offers +/-3.5% tolerance over temperature and voltage. It is not as accurate as a watch crystal but does provide another option to designers looking for a smaller BOM, lower cost and reasonable clock accuracy.
