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External Digital Clock (MSP430G2553)

Pep Hospe
Intellectual 320 points
Other Parts Discussed in Thread: MSP430G2553

Hello,

For my project I'm planning to use an External Digital Clock of 32768Hz to run the ACKL of the MSP430G2553. The reason is because another component needs also to be clocked at the same frequency. So my idea is to enter the signal to XIN of the MCP430 and connect the XOUT to the other's component XIN.

I know I have to set LFXT1Sx=11 to choose the external clock but I'm not sure about the connection between the MSP430 and the other component. Is it correct my idea?

About the external digital clock, the datasheet says this about the current consumption:

"The current consumption when the output clock is enabled (CLKEN pin at VDD) is a function of the load capacitance on the CLKOUT pin, the output frequency fOUT = 32768Hz and the supply voltage VDD."

Which is going to be the load capacitance seen by the CLKOUT pin when connected to the MSP430? Said in another way, which is the input capacitance in the XIN pin of the MSP430?

Here you can see the Clock datasheet: http://www.emmicroelectronic.com/webfiles/product/other/EM7604_C7_DS.pdf

Thanks

Oriol

  • 0
    Expert 1480 points

    Hello,

    if you need the same clock for both components, why don't you connect the external clock generator also directly to the other component?

    The EM7604 seems to deliver a rectangular and ready to use clocksignal, thus the internal XTAL driver circuitry is not required at all. XOUT of the Basic clock module may only be used in combination with a cristal oscillator as far as I know  (or as GPIO). 

    Regards Marco

  • 0
    Intellectual 715 points

    Yes, I was gonna say you can use an external clock generator that supports more than one device.

    I don't think that you can daisy chain two MSP devices through XIN and XOUT pins.

    If you want to set the capacitance for the crystal then use the XCAP bits in BCSCTL3 register;

    Also you should be able to output ACLK on one of the I/O pins by setting the PxDIR and PxSEL registers right and then you can use the output on that pin to clock the other device.

  • 0
    Expert 2740 points

    Pep Hospe said:
    So my idea is to enter the signal to XIN of the MCP430 and connect the XOUT to the other's component XIN.

    This is a bad idea. XOUT is a very low power output.

    The EM7604  will drive multiple loads.

    Pep Hospe said:
    Which is going to be the load capacitance seen by the CLKOUT pin when connected to the MSP430? Said in another way, which is the input capacitance in the XIN pin of the MSP430?

    Your XIN capacitance can be software adjusted to less then 9pF , see the data sheet. This is insignificant load at 32kHz.

    Peter

  • 0 in reply to Arturs Elksnis
    Guru 227245 points

    Arturs Elksnis said:
    I don't think that you can daisy chain two MSP devices through XIN and XOUT pins.

    Well, it could be possible if the firs tin the chain doesn't set the bypass flag. This increases the power consumption since the XOUT driver is active then, but that's the intention. Maybe you can even stop the clock supply by setting the BYPASS bit (which turns XOUT off). But then the second MSP would probably fallback to DCO.

  • 0 in reply to Jens-Michael Gross
    Guru 58965 points

    You can also use a 32768 Hz crystal (not an oscillator) on the MSP430 XIN & XOUT pins. Program the BCS to generate ACLK using that crystal and route the resulting ACLK to P1.0 as an output. Use that output to supply 32768 Hz clock to your other component.

  • 0 in reply to old_cow_yellow
    Intellectual 320 points

    Thanks all for your suggestions.

    I actually didn't know that you could set the ACLK at pin P1.0.

    So for what you said, if I finally use the external digital clock I should better connect it to the MSP430 and directly to the other component too. But after knowing I can avoid using an external clock I would prefer to use a symple crystal and drive the other component through the P1.0 port as the conspumtion should be lower.

    But here it comes a second problem if I use the P1.0 as ACLK: In my project I need to use the watchdog and I want the microcontroller to wake up only each second from LPM3. So for what I've read, with an input clock of 32768Hz the watchdog can only be 1 second without being reset, so for the tests I made, I had to divide the clock by 2, otherwhise the uC was restaring each second. So then, to drive the other component with the ACLK I would only have a frequency of 16KHz.

    What do you recommend? Is it possible to set the watchdog to count up to 2 seconds with a clock of 32KHz so I can use the ACLK at P1.0?

    Thanks and good weekend to everyone!

  • 0 in reply to Pep Hospe
    Guru 58965 points

    The WDT can either Reset the chip or generates an interrupt. Thus you could you it to wake up from LPM3 in stead of using it as a classical "Watch Dog".

    If you do want to using it as a classical "Watch Dog" and want to keep ACLK=32768 Hz, then you have to wake up from LPM more often than once per second and "kick the dog".

  • 0 in reply to old_cow_yellow
    Intellectual 320 points

    Thanks for all your suggestions. I'll see which is the best solution for my project.

    Have a nice weekend!

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