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SN74LVC3G07: Issue with jtag signals not outputting correctly, mainly NTRST signal.

jonathan khabbaz
Prodigy 90 points

Part Number: SN74LVC3G07

Hello, i  am having some issues with a a jtag connection using the SN74LVC3G07 as a buffer or my signals. i am notice my NTRST signal is very low. If you could take a look at this other E2E post, i was told to create another strictly for the SN74LVC3G07, but the circuit diagram and our board design is shown on here: C2000WARE: Programming with jtag XDS100V2 error on TMS320F28377DGWTEP - C2000 microcontrollers forum - C2000Tm︎ microcontrollers - TI E2E support forums

Small explanation here with a lot more detailed explanation in the above post: 

When reading the output of a our TMS,NTRST,AND TDI from the output of the buffer we have, we do not see any output on NTRST. 
On the developer board, we noticed that the resistor that is connected to ground on the NTRST line was AFTER the buffer. ours is before it. We are unsure if that is the issue if you can provide anything on that. 

  • 0
    TI__Guru* 97316 points

    Hi Jonathan,

    The SN74LVC3G07 is an open-drain buffer. As such, the device can only drive LOW at the output, and a pull-up resistor is required to provide a 'HIGH' logic level.

    Your selected pull-up resistor will affect the timing of the circuit. When the SN74LVC3G07 goes into the high-impedance state, the pull-up resistor will charge the output to VCC over a period of time which can be estimated by:

    t_r = 2.2 * R * C

    With R being the resistor value and C being the total capacitance connected to the SN74LVC3G07 output pin. Typically this will be between 15pF and 50pF for normal CMOS circuits.