Controlling HD44780 as a character device? - Page 2

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Re: Controlling HD44780 as a character device?
On 20/09/13 10:33, Miguel Giménez wrote:
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The LCD I thought was driven by the HD44780, with whatever voltage it  
uses as supply rails.

So I cant see the releavance of your (entirely correct) comment.



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Re: Controlling HD44780 as a character device?
El 20/09/2013 14:59, The Natural Philosopher escribió:
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The LCD needs a minimum voltage to change status. A module designed for  
5 volts (the most usual) has an LCD with a threshold higher than 3.3 V,  
so if you supply the module (and hence the HD44780) with 3.3 V the LCD  
will stay blank.


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Miguel Giménez

Re: Controlling HD44780 as a character device?
On 20/09/13 16:31, Miguel Giménez wrote:
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I don't remember anyone ever suggesting that you supplied the module  
with 3.3v.as a POWER SUPPLY.

The argument was whether the 3.3v *logic output* of the GPIO would be  
enough to drive the chip when the chip was being supplied with 5v.

The answer is, on the spec, yes.





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Re: Controlling HD44780 as a character device?
El 20/09/2013 18:24, The Natural Philosopher escribió:
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Are R-Pi GPIOs 5-volt tolerant?. If not, you can connect the module only  
if you don't read from it. This means using a delay instead of checking  
the BF flag before writing.


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Saludos
Miguel Giménez

Re: Controlling HD44780 as a character device?

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No, they are not

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Exactly. We need to use ist either in readonly mode or add an
additional level converter chip.

http://learn.adafruit.com/drive-a-16x2-lcd-directly-with-a-raspberry-pi/wiring
http://pmb.neongrit.net/blog/?p64%

Sven

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Re: Controlling HD44780 as a character device?
El 20/09/2013 18:50, Sven Geggus escribió:
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In 4-bit mode you can do it with just eight resistors.

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Saludos
Miguel Giménez

Re: Controlling HD44780 as a character device?

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No need for resistors in readonly mode.

See schematic at adafruit:
http://learn.adafruit.com/drive-a-16x2-lcd-directly-with-a-raspberry-pi/overview

Sven

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Re: Controlling HD44780 as a character device?

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They're not 5v tolerant - well - they might be for a few nanoseconds ;-)

Most drivers I've seen on most microcontrollers don't bother with reading
the status bit anyway - when I put mine together I tied the r/w pin to 0v
forcing the display into read only mode. The timing for the long-time
operations is published anyway (e.g. clear, home) so trivial to impement.
Only 6 wires needed for 4-bit mode to the display. I also worked out that
you can connect multiple displays to the Pi too - just need one more wire
per display and off you go (that was a specific request from a client).

The question in-hand is whether they ought to be part of the kernel
or have some other driver that you can use to use "standard" curses,
etc. libraries, but hey, they Pi is all about programming and interfacing
(to a lesser degree), so just get on & program... And who's to say
(e.g.) my C driver code isn't "standard" anyway - it's my standard ;-)

Gordon

Re: Controlling HD44780 as a character device?
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What I don't know is if there's a sufficiently generic GPIO framework that
we can say:

Take hd44780lcd.ko
Plug in GPIO 73 into R/w
Plug in GPIO 34 into D0
GPIO 45 is an interrupt
Now create /dev/hd44780lcd0

and make it work, given that the 'plumbing' between the LCD driver module
and the SoC is something the user controls.  From what I've seen, OpenWRT
makes a meal of this because every router, even using the same SoC, has a
different pin to enable wifi power/talk to the SPI flash/drive the LEDs, and
every router has a different C file in the kernel to describe this.  But it
doesn't work if the user can change that mapping.

Theo

Re: Controlling HD44780 as a character device?
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I've tried to run 5v displays at 3.3v Vcc - they don't work, but all
the ones I've tried run fine at 5v Vcc, driven by the Pi's 3.3v signals.
(And judging by the feedback I get over my Pi software for them, there
are 100s of others out there doing it like that too)

There are ones designed to run directy at 3.3v though, but they're
more expensive.

Gordon

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