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Reg settings to operate ACLK from external 32,768 clock on XIN with LFXT1 osc off

Jehangir Parvereshi
Intellectual 510 points


On a MSP430F22x, we're using an external 32,768 Hz external clock to drive the LFXT1IN pin.

This being a power sensitive applicaion, we're mostly operating in LPM3 mode with ACLK only active.

Is it possible to operate in LPM3 mode with LFXT1 osc tuned off?

If so, kindly provide the register settings.

Thanks.

JP

  • 0
    Guru 58965 points

    It is not possible to operate in LPM3 mode with the LFXT1 osc turned off. You can do that (and you have to do that) in LPM4.

    By definition, LPM3 means CPU, MCLK, SMCLK, DCO are disabled. DC generator disabled. ACLK is active.

    You can set OSCOFF in SR to turn off the LFXT1 osc.

  • 0 in reply to old_cow_yellow
    Guru 46710 points

    old_cow_yellow said:

    You can set OSCOFF in SR to turn off the LFXT1 osc.

    OSCOFF during LMP3 effectively is just LPM4 mode:

    #define LPM0_bits (CPUOFF)
    #define LPM1_bits (SCG0+CPUOFF)
    #define LPM2_bits (SCG1+CPUOFF)
    #define LPM3_bits (SCG1+SCG0+CPUOFF)
    #define LPM4_bits (SCG1+SCG0+OSCOFF+CPUOFF)

  • 0 in reply to old_cow_yellow
    Guru 227245 points

    old_cow_yellow said:
    It is not possible to operate in LPM3 mode with the LFXT1 osc turned off.

    Well, it is. the LFXT1 oscillator isn't required for ACLK.
    However, to switch it off, LFXT1 has to be switched to bypass mode rather than being disabled.
    I'm pretty sure that's what he was talking about.

    And yes, LPM4 would disable ACLK, with crystal or external clock, it wouldn't matter. But that's not what he was asking. So LPM3 with external clock on XTIN fo ACLK is the described scenario.

    On 2x family, LFXT1Sx in BCSCTL3 needs to be set to 3

  • 0 in reply to Jens-Michael Gross
    Intellectual 510 points

    Thanks for the suggested register settings. 

  • 0 in reply to Jens-Michael Gross
    Intellectual 510 points

    Hi,

    Is it possible to run peripherals with the 32 kHz osc off in LPM3 mode with LFXT1Sx in BCSCTL3 set to 3 ?

    Thanks.

  • 0 in reply to Jehangir Parvereshi
    Guru 227245 points

    Jehangir Parvereshi said:
    Is it possible to run peripherals with the 32 kHz osc off in LPM3 mode with LFXT1Sx in BCSCTL3 set to 3 ?

    You should be able to answer this quesiton yourself by looking at the users guide and understanding how the clock system works.

    Setting LFXT1Sx to 3 in BCLCTL3 means that teh XT1 input is takign an external clock signal.
    XT1 input is routed to ACLK.
    LPM3 does switch all clocks off except ACLK.

    So obviously, running a peripheral from ACLK will operate it with the applied external clock, even if in LPM3.

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