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Is 1ms period LMP3 sleep-then-wake-up necessary?

Matt Shira
Expert 2180 points
Other Parts Discussed in Thread: MSP430F2274

Hi,

Is it recommended to put CPU into LPM3 and to wake it up every 1ms?

In MSP430F2274 datasheet SLAS504F, table “Wake-Up From Lower-Power Modes (LPM3/4)” on page 39, DCO clock wake-up time is around 1us, but for CPU wake-up time, I found the parameter tClock,LPM3/4 is undefined in the document. How long is it?

And that the datasheet gives partially the information regarding wake-up time. In SLAU144I MSP430x2xx Family User's Guide, “2.3 Operating Modes” describes the speed for SR bits to take effect to be:

“When setting any of the mode-control bits, the selected operating mode takes effect immediately”

May I understand “immediately” as clock cycle time? Then for 1MHz DCO one clock cycle is 1us, so that “immediately” also means 1us, and is roughly the same as the DCO wake up time defined in the MSP430F2274’s datasheet (not including tClock,LPM3/4 for which information is lacking).

Although 1ms timer interrupt period is much longer than the total several μs’s sleep-and-wake-up time, it seems too frequently if judging by common sense. Moreover, even in 1MHz active mode the CPU consumes only around 0.3mA of current which if multiplied by 1ms is almost negligible. So is it really necessary to do such a frequent 1ms interrupt wake-up?

An alternative methods I found on the forum is by using _delay_cycles(unsigned long) compiler intrinsics. This would of course put the CPU in active mode. Which of the two methods is recommended?

 

Matt

  • 0
    Guru 58965 points

    It depends on what you want to do and what limitations you have. Some applications do not to wake up periodically at all, and wake up by external events only. Some battery operated weather stations only need to wake up every half an hour. Some main powered application never need to go to sleep mode.

  • 0
    Guru 227245 points

    Matt Shira said:
    DCO clock wake-up time is around 1us, but for CPU wake-up time, I found the parameter tClock,LPM3/4 is undefined in the document. How long is it?

    The same. When the crystals are down, the CPU rusn from DCO and its wakeup tiem is the same for any LPM.

    Actually, there is no such thing as 'The LPM'. The different LPMs are just a cascading set of power-saving actions. Starting with disablign the CPU (LPM0), disablign DCO (LPM1) and so on.

    To wakeup, the CPU requires oen clock source, and if no othe ris up and running, the DCO is used, so the DCO wakeup time applies.
    It doe snot mean that when the CPU is starting, al disabled crystals are also running. THis will take as much time as it took to get them initially running. So if you need the crystals, don't disable them.

    On most MSPs, however, the LPMs will only stop the internal clocks and not stop the crystals. So LPMs>1 wil just stop additional peripherals but are the same as LPM1 for the CPU itself.

  • 0 in reply to Jens-Michael Gross
    Expert 2180 points

    old_cow_yellow and Jens,

    We have tested that both the sleeping and wake-up time is very short, basically unnoticeable when the period of LPM3 sleep-then-wake-up is at 1ms. So it seems that such a "frequent" sleep and wake up is fine in the MSP430 world.

    Jens,

    Thanks for the explanation on the wake-up clock, this helps a lot for us in making decision on the clocking scheme.

    Matt

     

     

     

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