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 > _______________________________________________ > arm-netbook mailing list [email protected] > http://lists.phcomp.co.uk/mailman/listinfo/arm-netbook > Send large attachments to [email protected] _______________________________________________ arm-netbook mailing list [email protected] http://lists.phcomp.co.uk/mailman/listinfo/arm-netbook Send large attachments to [email protected]
