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CCS/MSP-FET430UIF: MSP430FR5728

Part Number: MSP-FET430UIF

Tool/software: Code Composer Studio

Hi,

I have question concerning the recommended connection of the /RST/NMI input of the MSP430FR5728. Refering page 40 of SLAU272C there is a pullup resistor of 47K to Vcc and a pull-down capacitor in the range of 10 nF downto 2.2 nF recommended. We are using in our design a combination of 47K to Vcc and 2.2 nF to Ground. On some boards the 2-wire JTAG communication runs and other boards it does not run. The MSP430 Hardware User Guide (SLAU278AE) recommend under conditions which are refering to some EVMs a capacitior of 1.1nF.  If the JTAG is not connected this capacitor determines the RESET timing of the MSP.  A variation of the capacitor in range from 1.1nF upto 10nF infuences the time constant by a factor of 10.

What is the recommended value for this capacitor?

Thanks in advance for a quick answer.

Ulrich

  • Ulrich,

    As the MSP430 Hardware User Guide states, the upper limit for the pull-down cap is 2.2 nF when using current TI tools. It is hard to give a concrete recommended capacitor value for that signal because the proper capacitor value needed will be a function of the trace capacitance on the PCB and the rise and fall times required for the SBW communication you are using.

    As Dennis stated in the linked E2E post, the 2.2nF capacitor might be too large of a capacitance for your board causing programming issues with some of the boards. I would recommend replacing the 2.2nF cap with a 1.1nF cap to see if that fixes the issue.

    Best regards,

    Matt

  • Matt,

    Thanks for the quick response. We will replace the capacitor and see, if the debugging runs.

    However, the influence of the capacitor on the RESET timing of the MSP remains when the JTAG is not used. Are there any known issues concerning decreasing the capacitor value and the RESET timing of the MSP.

    Bets regards.

    Ulrich

  • Ulrich,

    There are no known issues  that I know of.

    Best regards,

    Matt

  • Matt,

    Thanks for your answer.

    I have discussed the capacitor issue with my colleagues. In this dicussion the following questions came up:

    Is this capacitor a must or not? What will happen if no capacitor is connected to the RESET pin regarding to the Power-on RESET of the MSP. Is there any influence of this capacitor concerning the power-on RESET of the MSP? The JTAG communication runs without this capacitor.

    The background for these questions is the following contradiction in the MSP430FR57xxFamilyUser's Guide (SLAU272D):

    Please refer point  "1.7 Reset Pin Configuration" on page 40:

    ...If the RST/NMI pin is unused, it is required to have either the internal pullup selected and enabled or an external resistor connected to the RST/NMI pin as shown in Table 1-4.

    Please refer table 1-4 "Connection of Unused Pins" on page 40:

    RST/NMI     47-kΩ pullup or internal pullup selected with 10-nF (2.2 nF(2)) pulldown

    Best regards,

     Ulrich

  • Ulrich,

    Please refer this E2E post where Mike and others explain in detail the purpose of the pull-down capacitor and pull-up resistor and when each are needed.

    Best regards,

    Matt