[Arm-netbook] Fixing the fex file once and for all

Hendrik Boom hendrik at topoi.pooq.com
Mon Jul 22 15:24:27 BST 2019

On Mon, Jul 22, 2019 at 10:50:24AM +0100, Luke Kenneth Casson Leighton wrote:
> Ok these are the microdesktop with a USBUART attached, you should be able
> to zoom in and see tx rx on the UART PCB.
> You will find quite often that you get a ground loop between the main DC
> PSU and a laptop's PSU which is often not earthed, over the USB, through
> the USBUART, to the MD PCB that will, on powerup of the MD, cause the
> USBUART to either generate random data, or cause it to spike and lock up
> entirely, or in some cases of extremely cheap FT232 USBUARTs, cause them
> permanent irrevocable damage.
> This is just how it is.
> I buy lots of them, and in cases where difficulties occur unplug them
> before powerup. Of course, uboot needs to then be configured not to
> autostart too fast, and also you miss the very early uboot-spl debug
> messages, ho hum.
> You need a 5.5mm jack with a 2.1mm centre hole, middle is +ve (aka "pin
> positive").
> Anywhere between 7 and 21v is fine, minimum 1A, 1 5A is better. One of
> those multi power thingies is fine (5 7.5 9 12v you know the ones I mean,
> get them at RadioShack... or at least you used to), just make absolutely
> sure outer is -ve and inner (pin) is -ve.

Presumably one of these "-ve"s should be "+ve".

-- hendrik

> You almost certainly have a random 12VDC PSU somewhere in the house
> already, for a toy, a charger, or many 12 VGA monitors use a 5.5mm jack,
> it's an extremely common standard, can take up to around 4 to 5A no problem.
> If you have a multimeter for goodness sake use it.
> The Card can be powered up whilst attached to USB OTG however although
> power gets through the PCMCIA connector there is a SY6280 current control
> IC set at 1A which acts as a diode in this case.  You MUST therefore power
> the PCB from the DC Jack in order for the USBUART to work.
> DO NOT wire up the 3.3v supply to the USB UART. ONLY solder on 3 wires: GND
> TX RX. Those work by sinking current to GND and are otherwise floating, I
> believe.
> However they *may* be tied to 3.3v pullups actually on the USBUART and I
> have had this be sufficient in some cases to actually power parts of the
> A20 processor (!!) even when otherwise fully powered down.
> L.
> -- 
> ---
> crowd-funded eco-conscious hardware: https://www.crowdsupply.com/eoma68

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