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MSP430F6735 Crystal configuration

Other Parts Discussed in Thread: MSP430F6735

I'm designing my application based on MSP430F6735 and I need to use Real Time Clock Module (RTC) with backup battery, this make me must put a crystal 32.768KHz and when  i search when i put the high frequancy crystal i didn't found a place to it.

it supposed  that this controller can work up to 25 MHz  and when i read the device user guide (MSP430x6xx) i found that i can connect low power frequency X1 and high frequency X2 but in pin description i found only Xin and Xout.

could any one tell me how can i connect the low frequency crystal for RTC and the high frequency for my application in this controller?

  • Ahmed,

    The MSP430F6735 only supports the low frequency crystal.  Our user's guide shows both the XT1 and XT2 options, but only XT1 is available on the part, as described in the device data sheet.

    To use the XT1 module, you connect the crystal to XIN and XOUT, along with any external capacitors you might need to drive it.  In order to get the 25MHz for the part, we have an FLL and DCO which work in tandem.  The frequency from your crystal can be selected as a reference for the FLL to drive the DCO and create the high frequency clock.

    For more information on these modules, please see sections 5.2.4 (XT1 Oscillator), 5.2.6 (Digitally-Controlled Oscillator), and 5.2.7 (Frequency Locked Loop) in the MSP430x5xx and MSP430x6xx Users Guide (http://www.ti.com/lit/ug/slau208l/slau208l.pdf).

    Regards,

    Bart Basile

  • Bart Basile said:
    In order to get the 25MHz for the part, we have an FLL and DCO which work in tandem.

    Due to the way the DCO modulation works, the maximum safe frequency for DCO and FLL is 23.8MHz (else overshoots may exceed 25MHz).

    DCO+FLL only gives a fairly accureate average frequency (over several reference clock periods) but there may be a rather huge jitter between individual clock pulses.

    For  an almost jitter-free, stable 25MHz, you'll need a high frequency 25MHz crystal and use it as direct clock source. However, the F6735 doesn't seem to support this too.
    Experiments on G2xx devices (wiht similar limitation) have revealed that you may feed a high-frequency TTL clock signal into XIN in bypass mode, even if the datasheet only specifies up to 50kHz. Maybe it works on this MSp too.

    You don't need external capacitors for a LF watch crystal. For soem common crystals, internal capacitors are available (XCAPx setting).

    And instead of an external watch crystal, the internal REFO can be used in most situations.

  • hello   / 

    thank you for your answers, 

    i noticed that all new msp430 family support only the low frequency while the older have XT1 and XT2 oscillator.

    my question if i design an application what is more stable using controller which have low and high frequency XT1 and XT2 or choosing a controller like msp430f6735 which have low frequency and use FLL and DCO to get the high frequency ?

    and if i chose the second option will DCO can give me accurate high frequency like 8MHz which i can depend on in communications like UART and I2c which need accurate clock calculations   

  • Ahmed ELlithy said:
    i noticed that all new msp430 family support only the low frequency

    I don't think it is because they are 'new' but rather because they are 'specialized'.

    The 6735 with its AUX supply and RTC is specialized for dual-supply low-power RTC operation. Where HF mode of XT1 doesn' tmake much sense (the RTC requires a LF crystal)
    However, I agree that on 80 or 100 pin package, two pins for an HF XT2 should have been possible. And a TimerB instead of one of the TimerAs.

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