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F5529 - can USB clock be independent of system clock?

Todd Anderson78572
Guru 14640 points
Other Parts Discussed in Thread: MSP430F5529

All:

I have a situation where I need to run the MSP430 UART at a non-standard BAUD rate of 1.024 M bps. I have been able to do that, by creating a system clock that will run at 24,576,000 MHz and dividing by 24 to get to my BAUD rate of 1.024 MBaud. By the way System clock is using REFO * (749+1).

I used MSP430F55xx_uxcia0_uart_01.c  and  MSP430F55xx_UCS_10.c  to build code for above.

 

However, if I now want to add USB, does that change the above? OR can the USB clock be independent (48 MHz) of the above-generated System Clock?

Regards,

Todd Anderson

  • 0
    Genius 3770 points

    Hi Todd,

    The USB clock is always independent of the system clock; it's generated by the USB PLL from the XT2 clock. You can of course still source from XT2 for ACLK/SMCLK/MCLK, and you can use it to stabilize the DCO using the FLL, but you don't have to.

    The clock system (UCS) on the MSP430F5529 is quite flexible, ACLK/SMCLK/MCLK can all be sourced independently from XT1/XT2/DCO/REFO, and the FLL can generate almost arbitrary frequencies with the DCO, and within reasonable accuracy if you stabilize it with an accurate source.

    Keep in mind that REFO has a frequency tolerance of +-3.5% over the entire voltage/temperature range, so this would affect the error rate for your UART. I would suggest for your baud-rate (1.024MBd) that you source the FLL from XT1 or XT2 to stabilize the DCO, or source the UART clock straight from XT2.

    Tony

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