MSP430FR5858: LPM0 and LPM3

Hello Hair,

For your second question, you can also use VLOCLK to source the ACLK in LPM3 mode with much lower power consumption. However, VLO is a low frequency oscillator and not optimal for high time accuracy peripherals. If you really need LFXT I will recommend to go with MSP430FR5848.

Thanks,
Yiding

Hari Muthu said:

Part Number: MSP430FR5858

Tool/software: Code Composer Studio

Hi, 

I am using the MSP430FR5858 microcontroller for my application, where I want to use low power mode. I have done my initial development using MSP430FR5969. My issue is, in 5969, LFXT is there but it is not present in 5858. I cannot clock the ACLK with the HFXT. What is the better clock to source the ACLK? (I am sourcing the Timer A, and ADC using ACLK in LPM3 mode).

I tried using LFMODCLK for sourcing the ACLK, but in that case, I can’t go to the LPM modes (I checked with Energy trace). I tried referring the datasheet, but I can't figure out the issue.

It would be grateful if somebody could suggest a solution to my issue.

"I can’t go to the LPM modes"

It's a common misunderstanding that there are 'LPM modes' in the sense of hardwired operation states that are activated by using the LPMx macros. If you take a closer look at the declaration of these macros, you'll see that they are just setting different combinations of control bits in the status register. Bits with a well-defined behavior. Like switching off an oscillator or stopping MCLK (but not necessarily the oscillator that is driving MCLK). And there are also a some onditions in which case these control bits are overridden. Like having a debugger attached or having a module with an unconditional clock request active etc.

You can't say 'dear MSP, please use less power but else continue with all features'. It is up to you to use only the features you need and disable the others. The MSP offers you to disable what isn't needed but prevents you from disbaling what is still needed. It's like a car where you can switch the engine off but it doesn't let you pull the key (and lock the steering wheel) while it is still in movement. (unfortunately, cars aren't as smart as the MSP here)

The LPMs are just a convenience concept to help you finding useful constellations that allow you power saving, and the macros just help you disabling what you don't use (but not what you might thing you don't use).
In fact, the 4 control bits allow 16 different 'LPMs', but only few of them really make sense (all include stopping MCLK). But sometimes, you would like to manually use one of the othe rcombinations. Like switching ACLK off but not MCLK, for permanent power saving if ACLK is never needed in the code.

On the bottom line, the MSP allows you to disable currently unneeded parts of its hardware and prevents you from disabling needed parts. And the LPM concept is just proposing some common constellations that make much sense (and are more or less hte same on all MSPs regardless of their clock system features) - if your current use of the hardware allows them.