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What is the SWD driver impedance of SWDIO line?

godmode

As in topic I'am concerned about the impedance of SWD interface drivers.

I'am building and SWD interface multiplexer that will be used with 48 devices using J-Link Pro debugger both for production programming and debugging.
The cables are going to be about 100 cm long (yes, I know they are long :) ).
The SWCLK and SWDIO signals will be passed by multiplexer's FETs and impedance matched by external resistor plus variable resistor.

I need to know what is the impedance of SWD drivers in nRF52832 microcontroller?

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  • 0

    Hi, the input impedance is very high, in the Gohm range. If you need to design circuitry you can assume the leakage current is <0.1uA in worst case, though they are likely in the few nA range.

  • 0 in reply to Kenneth

    Actually I need to know what is the output impedance of SWDIO to do impedance matching for long cables (suppress signal reflection)?

  • 0 in reply to godmode

    We have not seen long cables to be a problem, and our own lab we have similar setup (very long cables in some cases). The input is very high impedance, the output is very low impedance. You can find some data in our datasheet in terms of rise and fall times with a regular GPIO (SWDIO can be considered a GPIO configured in low drive mode in those tables):
    http://infocenter.nordicsemi.com/topic/com.nordic.infocenter.nrf52832.ps.v1.1/gpio.html?cp=2_1_0_19_3#unique_1820404704 

    Note that we do not recommend a setup where you try to program many chips in parallel, and only readback/verify on possible 1 chip. We have seen similar attempts before, and in those cases there have been various issues, for instance the timing will be controlled by the 1 you actually verify.

  • 0 in reply to Kenneth

    I do not want to program many chips in parallel - I want to multiplex one debugger to many devices. It will program one at a time but without disconnecting cable from Device(n) to Device(n+1) and so on.
    After one device is done mutiplexers are switched are switched to another device. Always only one device is connected to debugger.

    Thank you for your help!

  • 0 in reply to godmode

    Thanks for the clarification, I just wanted to make sure.

    Best regards,
    Kenneth

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