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MSP430 XT2 for USB and MCLK?

Orangelynx
Intellectual 430 points
Other Parts Discussed in Thread: MSP430F5510

Hi there!

I'm currently developing a little board for a private cause, mostly to learn more about uC's, uC Protocols and uC Hardware design.

From reading various pdf's regarding my target uC the MSP430F5510, I noticed that i need an external crystal on XT2 of at least 4MHz for proper USB and BSL operation. However as I need a as stable as possible MCLK for a bit banging protocol and the XT1 doesn't support high frequency crystals in the F5510, I wanted to connect a 20MHz crystal with 2 47pF decoupling capacitors to XT2 to feed the MCLK.

I read about the registers that allow XT2 to be divided for the USB PLL but i was unable to find out, whether I could actually use a 20MHz crystal to feed both, USB and MCLK from XT2.

Any comments are appreciated.

Regards,

Max

  • 0
    Intellectual 430 points

    Hi again,

    I managed to find a solution to my problem in another forum. I'm going to leave it here so desperate googlers will be able to find it here.

    Allthough no high frequency crystal can be attached to X1, it can be used to stabilize the DCO's frequency when connecting a standard 32.786kHz crystal to X1 and selecting X1 as reference for the FLL. Have a look at MSP430F55xx_USC_10.c in slac300d.zip (TI examples for the MSP430F55xx) for the required steps to configure the MSP that way.

  • 0 in reply to Orangelynx
    Guru 34453 points

    Don't understand what is your problem. If you want to use MSP430F550x as USB device (bus powered for example), you must have XT2, because USB can't work without it. It is good solution to select frequency supported by USB BSL, so firmware can be updated without any other tool, just MSP430F550x connected to PC by USB connection. It is also good way, to supply MCLK by XT2, and in general 24 MHz is good choice. It is supported by BSL and MCLK is close to max possible frequency (25 MHz).

    I am working with MSP430F5xx family, and never used XT1, till now, only 24 MHz for XT2. Don't understand why you need XT1?

  • 0 in reply to zrno soli
    Intellectual 430 points

    I didn't think running the MSP430 on a frequency other than specified was a good idea.

    20MHz is also a pretty good frequency for my bit-banging protocol (50ns cycle). Maybe i could use a crystal on XT2 that is detected by the BSL and that can also be used as a reference for the FLL to stabilize the OCD (with dividers)?

  • 0 in reply to Orangelynx
    Guru 34453 points

    You don't need to use DCO at all. What MCLK is requested for your protocol? MCLK can be sourced by precise XT2 (using dividers 1,2,4,8,16,32). USB BSL can work with 4, 8,  12, 24 MHz XT2.

  • 0 in reply to zrno soli
    Intellectual 430 points

    I'm planning to control a set of WS2812 LEDs which feature a one wire control protocol without clk, which means that the high/lows have to be timed to a high degree of precision. 20MHz would be nice because its cycle period 5ns is a fraction of each high low, but i guess I can stay within the tolerances using a 24MHz crystal.

    Thanks!

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