Date: Mon, 20 Jul 2026
From: Michael Rangen
Subject: Re: MIST interface status

Hi Dan,
Thanks for getting back to me,

The connector has changed, it did not meet the vertical clearance
test.  I sent the PCB 3D model to Lazlo and Christian for a fit test
originally with this connector.

Samtec Inc.
EHT-113-01-F-D-RA
Insulation Height: 7.62mm

https://www.digikey.ca/en/products/detail/samtec-inc/EHT-113-01-F-D-RA/6613280

image.png


I have swapped it out for:
Würth Elektronik
62702621721
Insulation Height: 5.00mm

https://www.digikey.ca/en/products/detail/w%C3%BCrth-elektronik/62702621721/21066290?s=N4IgTCBcDaIGxgOwAYwIIyLOkBdAvkA

image.png

Here they are side by side on the PCB for visual reference. There is
also a USB-C connector in the   second image   for vertical scale
reference.

image.png
image.png

With this connector, we should have 1.5mm of vertical clearance from
the shorter black connector to the metal can. I'm waiting on a
response from Lazlo and Cristian to hear if they find this acceptable.

That said, the new connector would still use a IDC (Insulation
Displacement Contact) style cable.  How they will hold up to 20 years
of use I am unsure of.  Thankfully the MIST is a low power device,
the currents travelling on the cable will be very small.  High
current connections would suffer the worst from a degraded
connection, not to say signals wouldn't at all, but I am unsure how
to quantify the possible degradation over 20 years on a cable like
this.  There are many redundant ground connections, only two 5V
connections; perhaps we replace some pins with additional 5V pins?
If you have a reason to veto this style of cable, I will have a hard
time arguing, but I would like help finding a suitable connector
with enough vertical clearance then.

image.png

Carsten suggested we use an outer hole for the cable; I think it
would be good to make the center hole less congested. Could you help
me identify which specific hole?

Another thought about these outer holes, if we continue to run into
connector clearance headaches, what about a connector that lines up
directly with one of the outer holes, such that vertical clearance
is easily guaranteed? Can't crash into a plate where there is no
plate; opens up some options. It wouldn't mess up my single-sided
assembly either.

image.png


Connector Pinout:

*  Grounds on the even pins, which are closer to the board edge.
     +  I came to this same conclusion the second I looked at it in the PCB
        layout editor haha. Much easier with ground on the even pins.

*  SWDIO and SWCLK
     +  DK shouldn't need these for reprogramming the device
     +  They are for debug during development
     +  I will remove them from the DK connector. I'll put them on a more
        standard programming header for developers sitting at a workbench.

*  Config/!Physics
     +  It originated from a brainstorming session we had in my lab during the
        collaboration meeting, but we didn't agree it needed to be a pin. You
        had made a good suggestion that there should be an extra layer of
        lock-in for long-term operation; something that ensured nothing could
        accidentally adjust a setpoint without first deliberately "unlocking"
        the device and switching it out of "Physics" mode and into "Config"
        mode.
     +  My idea with this I/O pin would be
         o  1 - Config Mode (Changes can be made)
         o  0 - Physics Mode (Changes cannot be made) pull-down would be used to
            ensure known/default state
     +  Let's remove this pin and just do mode switching with UART commands

*  Other Control Pins
     +  all are simple GPIO at DK side
     +  !Reset
         o  reset pin for the microcontroller
     +  STM32-BOOT-0
         o  STM32's baked in bootloader, for redundancy
     +  MCUBOOT-BUTTON-0
         o  Zephyr MCUboot bootloader
         o  After reset, the state of this pin determines if the microcontroller
            enters DFU mode for flashing new code.
         o  https://docs.mcuboot.com/readme-zephyr.html

*  Comparator Output signals
     +  By swapping the grounds to even pins, all 4 output signals have a ground
        on either side, for uniform return paths.

*  Summary
     +  Grounds on even pins
     +  Output signals have uniform return paths
     +  Removed:
         o  SWDIO
         o  SWCLK
         o  Config/!Physics
     +  Could be added...
         o  5V
         o  5V
         o  5V
Let me know what you think.

Cheers,
Mike