[Arm-netbook] 2.7.4 preproduction sample, 1.7 MD, Questions

Luke Kenneth Casson Leighton lkcl at lkcl.net
Fri Jul 10 14:03:23 BST 2020

> On Thu, Jul 09, 2020 at 11:43:58AM +0100, Luke Kenneth Casson Leighton wrote:

> So the next and last step hardware wise is to solder the UART connection
> to the micro desktop? Can I use standard pin headers?

yehyeh.  that's what it's for.  standard 2.54mm pitch.  it's designed
precisely for that.

actually i soldered directly to the PCB.  don't tell no-one :)


there you will see *only* GND, TX and RX connected.  do not *under any
circumstances* connect the VCC_IO line.  this is a reference power
supply *from the USB UART*.  so is the 3.3v supply from the EOMA68

if you connect them together they will destroy each other.

given that USB UART is "pulldown" (floating indicates LOW) it is
perfectly fine not to have a converter IC.  which normally would be
the responsibility of the developer to create an external PCB that did
that.  if they wanted to.  rather than use the pins as GPIO.

when doing it this way (without a converter IC) you will find however
that even just the power coming from the TX and RX pins is enough to
provide 3.3v to the actual A20 processor.

therefore you *will* need to disconnect the USB cable when powering
down.  simply switching off the 12v PSU (15v in your case) is *not

also watch out for GROUND loops between:

* laptop / desktop PSU
* laptop / desktop
* USB UART port connected to laptop
* 12 (15) volt PSU

if you get problems (which will show up as corrupted data and possible
eventual destruction of the FT232 USB UART IC - yes this is just how
it goes) then disconnect the laptop's PSU whilst powering on.

i have in the past hotwired a 5V USB cable (cut the connector off)
directly into the 5V rail of the micro-desktop PCB, bypassing the RT
power converter IC.  if you happen to have one of those "back-to-back
USB cables" around (i had one for a laptop fan base for example) then
you could actually plug that back-to-back USB cable directly into one
of the micro-desktop USB ports.

the USB power is connected straight to the 5.0V rail, which in turn
(through a SY6280 current limiter) powers the EOMA68 Card.

that way you could plug the back-to-back USB cable into the *same
device* as what you also plug in the USB-UART, and you will not end up
with 240v mains-related power fluctuations on the GND cables.

which as i warned you about, in some PSUs might not actually exist
(particularly cheap PSUs - they have a plastic GND pin here in the

when that happens the *only way* that you get a common GND reference
is actually *through the EOMA68 PCB* via the USB cable, and that
really does not go down well when there is a large amount of 240v 50
hz EMI being radiated about.


More information about the arm-netbook mailing list