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RTC questions series2 and series5

Pes
Prodigy 40 points

Hi,

There are things I don' t understand about RTC.

I want to do a design with RTC to save time.
I hesitate between a series2 and a series5 with RTC.
Series2 has no RTC hardware, it' s written 0.7µA for real-time clock mode and 0.3µA for standby mode, while 2.5µA for real time clock mode of series5.

What is the difference betwwen standby mode and Real Time Clock mode?
What is the Real Time Clock mode for series2 whereas it hasn' t RTC?
Series2 is more energy efficient than Series5?
What current can I expect with RTC hardware in séries5?
And if I do a RTC with a software timer, what current can I expect in Series 2 and Series 5?

Thank you.

  • 0
    Guru 227245 points

    Pes said:
    What is the Real Time Clock mode for series2 whereas it hasn' t RTC?

    I guess this means the 32kHz watch crystal is running, and a timer that counts the crystal ticks, so the CPU can wake up every second and increment the time in software.
    Pes said:
    Series2 is more energy efficient than Series5?
    The series5 usually has much more ram that requires some power to keep its content (up to 18k compared to sometimes just 128 bytes).
    However, Series5 sometimes (depending on device) supports alow power mode 3.5, which means the crystal is active and the RTC module, but everything else (!) is shut off, even the ram retention. The MSP can only be reactivated by a power cycle (some MSPs have a separate backup voltage for the RTC, else of course the RTC content is lost), an RTC alarm or a port pin interrupt. In this mode, current should be quite low.
    The 563x is specified with 1.1µA during LPM3.5, with VCC=3V still applied. (less on lower backup). Apparently the crystal takes ~1µA on its own (this is the difference between LPM3 and LPM4). Maybe due to the backup system. I don't know offhand any MSP that has LPM3.5 but not backup system.

    I must admit that I don't really understand the datasheet here. I thought if the device is in LP;3.5, VCC couls be off (that's the purpose of a backup voltage supply). Yet the table implies that in LPM3.5, 0.5µA are drawn from VCC. Does it mean the RTC will fail if I shutdown VCC or does it mean that if I shutdown VCC, the current goes further down by 0.5µA without any further impact? I didn't work with the backup system yet, so I don't know what's going on.

    On 5438A, standby mode (LPM4.5) is specified with 0.1µA. So there's no general conclusion about efficiency.

  • 0 in reply to Jens-Michael Gross
    Prodigy 40 points

    Thank you. It' s more clear.

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