MSP430FR2633: The clock sources of FSM and CapTIvate

Hi Luke,

ok, please let me start at a lower level. The generation of the timings and triggers for the required activities during the operation of the CapTIvate module is consisting of multiple components. This is described in the Technology Guide in the chapter about the "Technology" and the section The "CapTIvate™ Peripheral".

As a base, there is a Timer, similar to the 16-bit Timers of the MSP430, but independent of those, which is responsible for triggering regularly (in the default operating mode) the capacitive sensing activities of the FSM. I am saying default mode, as one could trigger these also by software or by external signals, but this is usually not the case.

This Timer is clocked typically either by the 32kHz crystal oscillator. This allows going down to LPM3. In case where you clock this Timer with the integrated VLO, you could go down to LPM4. This is an exceptional use/implementation option, to allow even lower current consumption, as with non-CapTIvate MSP430 devices in LPM4 all oscillators are off.

The second portion of timings, are the timings for the actual capacitive sensing operations of the FSM. These require higher clock frequencies. This is where the integrated CapTIvate high frequency oscillator comes into play, clocking completely independently of the MSP430 clock system the CapTIvate FSM and logic. This oscillator is not part of the MSP clock system, and exclusively available for the CapTIvate module. That's why the rest of the MSP430 can remain in the selected operating mode, even down to LPM4.

Of course during the operation of it, and the ongoing CapTIvate activities you have to take the additional current into account.

In case a valid capacitive sensing event occurs, the FSM will automatically wake up the CPU by interrupt, where the CPU then wakes up from the LPM and processes the higher level functions, means the activities of the application to be performed on touch events.

So, the answers to your points:

1. The high frequency oscillator is built into the CapTIvate module.

2. Yes, the CapTIvate oscillator is independent of the MSP430 clock system. Thus in principle the rest of the MSP can operate at any frequency within the specification, while CapTIvate is clocked by its own oscillator.

3. As described above, there is a Timer within the CapTIvate module, which triggers regularly the capacitive sensing activities. This Timer is typically clocked with a slow clock (32kHz crystal/REFO/VLO) to save current, as typically here the repetition rate of the measurements is in ms range.

4. As described above, the FSM is using the exclusive high frequency oscillator of the CapTivate module, completely independent of the MSP430 clock system.

Please see also the Technology Guide, which is describing all these aspects and providing even more information, as there are in very detail of course at some points even more options than described here in my brief summary.

Best regards

Peter