A very cute hack for a 1-wire LCD panel interface, including power!

[WB4JFI]

My only concern was the current needed for the LEDs in the display 
backlight - if any.

While 2400 baud seems quite slow, that's almost 240 characters per second, 
and most of the time you need to slow the data flow to LCD displays anyway, 
so the slower serial speed may not be as bad as it seems.

Using 2400 baud also means that it matches up much better to a software 
serial UART on the Arduino.  The software UARTs  don't work too well much 
above 4800 to 9600 baud on the Arduinos.  Using a software UART frees up the 
Arduino hardware serial port for debugging, etc

Technically, this is actually a TWO-WIRE solution, as a GROUND connection is 
also required!

There are other solutions available as well.  Several different I2C LCD 
drivers are available, and the number of wires is only doubled to four 
required.  I2C is also a shared bus, so one can argue that you aren't 
actually losing ANY pins or connections on any micro that has I2C.  I have 
some projects that have four different devices sharing the one I2C bus on 
Arduinos, without issues.

Hardware I/O expander ICs are another way to drive LCD displays using fewer 
wires.  Hooking one I2C to 16-bit I/O device up to an Arduino is also 
already supported with libraries for the Arduino.  This is a slight variant 
of the I2C mentioned above, primarily because a dedicated springhead is not 
used, just a fancy hardware I2C UART.

If you planned to use a multi-color backlight, the shown solution's hardware 
will need to be modified to add the other two color LEDs, plus additional 
software would be required.  Several I2C LCD drivers already have the 
multi-color backlight built-in.

Happy New Year!
73, Terry, WB4JFI


-----Original Message----- 
From: Michael O'Dell
Sent: Friday, January 02, 2015 4:17 PM
To: Rob Seastrom
Cc: Tacos AMRAD
Subject: Re: A very cute hack for a 1-wire LCD panel interface, including 
power!

The Dallas one-wire interface is pretty feeble in terms of current 
available.
To me, the cutest part of this implementation is that there is no voodoo 
required
to talk to the panel - it’s just 8N1 at 2400 baud which damn near anything 
can bang-out
with little to no grief. That means I can hang it on a second surreal port 
as a mini-console
on a one-board-wonder as easily as I can drive it from an Arduino or other 
digital goober.
everything either has a hardware surreal port or there is bit-banger code 
that will do it,
and even the most sluggish spring-head can make 2400 baud.

while it would be nice to make the panel auto-baud, the cute hack of looking 
for
the ASCII NUL with an R/C circuit gets major props for most-est with the 
least-est
in my book. changing the baud rate would mean the time constant would have 
to change, too.

-mo



On Jan 2, 2015, at 3:03 PM, Rob Seastrom <rs at seastrom.com> wrote:

>
> Michael O'Dell <mo at ccr.org> writes:
>
>> http://www.edn.com/design/systems-design/4427215/One-wire-brings-power---data-to-LCD-module
>
> That's a bit interesting, mainly for the cute trick of parasitic
> powering the LCD off the comm line, but 2400 baud is kinda slow.
> Non-overdriven Dallas 1-Wire is 16.3 kbit/sec.
>
> There are plenty of circuits out there that utilize the Maxim DS2408
> and an HD44780 together, and pre-debugged libraries for writing to
> that combination.
>
> I see that the cap they use for parasitic poweer is 0.47uF.  Dallas
> 1-Wire stuff typically is powered by an 800pF cap.  I wonder if a
> 1-Wire interface (obviously with fewer stations on the "microlan")
> would source enough power to keep the 44780 and the backlight happy.
>
> -r
>

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