[Arm-netbook] Vero Apparatus: Another Arm Laptop Project

Luke Kenneth Casson Leighton lkcl at lkcl.net
Thu Aug 13 13:55:12 BST 2015

On Thu, Aug 13, 2015 at 1:18 PM, Paul Boddie <paul at boddie.org.uk> wrote:

> Is there any reason why they didn't choose to go with EOMA-68 or a related
> modular hardware concept?

 openlunchbox are planning a modular design, for example.

 remember though paul that i've been thinking about what's realistic
and achievable (entirely from scratch) for a significantly long time.
with a $250k budget you could do absolutely anything you wanted -
bring that down to $25k and the options become very very different.

 but if they've budgeted around $40k - and if there's enough people in
the team with enough of a reputation they may have enough income
and/or clout to consider doing that - then the compromises and limits
(such as a 5W absolute max power budget, no fans, single-cell
batteries for dramatic circuit simplification) that i've learned in
order to achieve the goal... many of those compromises do not need to
be made.

 the thing is, paul, that people only see the low power and the "low"
spec of EOMA68 and go "wtf??? that's pathetic! i'm not going to use
*that* in my project exclamation-mark, exclamation-mark"...

 ... they don't realise *why* it was picked... so they set a much,
much higher entry level...

> Also, was the focus on ARM because that's the area
> of most expertise amongst those involved? Sorry if you end up having to ask
> those other people our questions!

 looking at the vera wiki they want PCIe, SATA and more besides,
perhaps even external (PCIe) for an external 3D GPU (MXM or
some-such).  basically that's 30 watt and above territory (and another
30 watts for the external 3D GPU).

 which is an enormous amount of power (and heat).  by contrast, the
libre 15.6in laptop is 15W absolute max [5W absolute max for the CPU
Card but 3.5W nominal; 3.5 for the LCD... i may have to remove one of
the 3 USB2 ports to keep within budget...]

 by the time you've accounted for the LCD (appx 10 watts),
server-class SoC (appx 10 watts), SATA hard drive (appx 5 watts) and
some USB ports (5 watts for 2 USB2s, but 10 watts and above for 2 or
more USB3s).

 in these designs you need to take into account the *max* rated (peak)
power, not the "average" or "scenario design power".  surge current on
LCDs is often 3x its running current.  peak power on these 64-bit ARM
processors is... well, the start-up current for the iMX6 was something
like 4 amps...  you need to assume that the USB ports are all going to
be drained at absolute max power...

 and that means 3-4 cell batteries (take a look at the novena battery
charger circuit - it has cell monitoring - BMS support), you need to
consider thermal management, you need fans, heatsinks, 2oz copper
tracks to handle a 4 to 5A current [meaning: expensive PCBs]...

 in other words i stayed completely away from all of that, for very
good reasons, so that what i am doing stands a realistic chance of

 for the 15.6in libre laptop, literally the only battery charger IC i
could find that was single-cell that could handle 4A charging, and
provide a 3A 5V supply, and could do USB-OTG charging and power
supply, was the Linear LTC4155.

 the LTC4155 is an awesome bit of kit, it does the job of about 50
other discrete components which would cost a heck of a lot more, and i
hope to god it doesn't go end-of-life because if it does i'm buggered

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