USB to dual UART adapters for FreeCalypso

[Mychaela Falconia]

Hello FreeCalypso community,

Knowing that some people have been using Sysmocom mv-uart adapters
with our FCDEV3B boards instead of our officially recommended FT2232D
adapter, I have recently bought one of those little mv-uart boards
from Sysmocom webshop for evaluation testing, and I have just completed
my experiments with it.  Based on the results of my testing, the only
recommendation I can make is that FreeCalypso users should NOT use
mv-uart adapters with FreeCalypso boards - until our DUART28 adapter
becomes available (see below), you should use FT2232D adapters such as
these:

http://pldkit.com/other/ft2232d-module

Besides the obvious drawback of CP2105 having poor support for
non-standard high baud rates like 812500 bps (CP2105 supports them on
only one channel, whereas FT2232x support them on both channels),
mv-uart has been found to perform much worse in terms of partial
power-down scenarios as detailed below.

Partial power-down scenario 1: you connect battery-emulating power to
your FCDEV3B (orange power input connector), UART lines are connected
between your FCDEV3B and your USB to dual UART adapter, the latter
adapter has USB power present (ttyUSBx devices appear on your Linux
host), but you haven't pressed PWON or RESET on your FCDEV3B yet -
perhaps you need to run a tool such as fc-loadtool and then press
PWON.  With FT2232D adapters the current flowing from the powered-on
USB adapter into the powered-down Calypso target is about 1.7 mA per
UART line, which is not ideal but certainly quite tolerable.  But with
mv-uart this current has been measured to be as high as 8 mA with the
voltage selection switch set to 2.8V, or almost 12 mA with the switch
set to 3.3V - yikes!  (The difference must be due to CP2105 having
stronger output drivers than FT2232D.)

Partial power-down scenario 2: the Calypso device is fully powered on
and running (green LED lit on FCDEV3B), UART lines are connected
between FCDEV3B and USB UART adapter boards, but there is no USB host
present - the USB adapter board has no power.  Both currently available
options (raw FT2232D adapters and mv-uart) perform badly under this
scenario, with significant current flowing from Calypso output cells
into the powered-down USB board, but mv-uart is worse: with FT2232D
this current has been measured to be about 6 mA per UART line, but
with mv-uart it is somewhere between 10 and 11 mA per UART line.  This
partial power-down scenario can be easily avoided on FCDEV3B (simply
don't do it), but we have another FreeCalypso development board coming
down the pike (another 2-3 months maybe) that will feature an LCD and
a keypad, allowing untethered phone operation - and then this scenario
will begin to matter a lot more.

Thus the current official advice is to use PLDkit FT2232D adapters and
not mv-uart.  But the raw FT2232x option (whether from PLDkit or other
vendors) is not perfect either: it still performs rather badly in
partial power-down scenario 2, and it puts out 3.3V rather than
Calypso-native 2.8V.  I've been wanting it for the past few years now,
ever since we launched FCDEV3B in 2017, but over the past couple of
months I finally bit the bullet and designed my own FreeCalypso-
specific USB to dual UART adapter, called DUART28.

DUART28 PCBs just arrived from China yesterday, I will be taking them
to my assembly shop in the next few days, and then we'll see how long
it takes them to turn around.  Our new DUART28 adapter is based around
the same FT2232D chip as our current PLDkit adapters (sorry Sysmocom
folks, I *really* dislike your CP2105), but this FreeCalypso-specific
adapter principally differs from a generic FT2232D like PLDkit as
follows:

* As reflected in the name, DUART28 will put out 2.8V UART signals
(native for Calypso) instead of 3.3V.  This feat is achieved by
inserting a 74LVC541A buffer after FT2232D outputs, powered by an
on-board 2.8V LDO regulator.

* Another 74LVC541A buffer is inserted in the other direction, between
Calypso outputs and FT2232D inputs.  This buffer is expected to fix
partial power-down scenario 2: LVC buffers are specifically designed
for partial power-down applications and have a datasheet-guaranteed
low Ioff in the uA range.

Assuming that these new DUART28 adapters will work as designed when I
get them assembled, they will become the new standard accessory to be
supplied with FCDEV3B and future FC development boards.  Anyone who
already has an FCDEV3B will be able to get the new DUART28 adapter as
well.  Hopefully we'll have them in another few weeks...

Hasta la Victoria, Siempre,
Mychaela aka The Mother