Wiring & Controlling Parallax 4x20 LCD display

RobertW

I recently purchased and received a Parallax 4x20 serial LCD display and 3-wire extension cable.· I have a Propeller Prof. Dev. Board and was wondering if I can just plug the extension cable into one of the servo headers at the bottom of the board or is this not recommended and I should make all of my connections within the breadboard area?· Also, can I use the FullDuplexSerial.spin object to send data to the display or is there a better way?· Thank you.
-Rob

Oldbitcollector (Jeff)

Hi Rob.. The PPDB is a fantastic tool!

Attached is some code to get you started with your setup.

Yes, you can use the servo wire to the servo head without issues,
however I would recommend a 10k resistor between the servo connector on the PPDB
and the Propeller.

Set your LCD for 19200 baud and wire the signal line to P0 on the Propeller.

OBC

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RobertW

Hi OBC,

Thanks for the reply. I feel pretty dumb. After I posted my question I realized that simple_serial.spin and serial_lcd.spin objects are already in the Propeller library. I should have looked there first. Thank you for the attachment. I will download it and look it over.

With regards to the 10k resistor, there is already a 150 ohm resistor on the board. Is this enough or should I think about adding a 10k in addition to this? What is the purpose of the 10k resistor you suggest? For current limiting?

Thanks

Oldbitcollector (Jeff)

It's habit for me to use a 10k resistor when dealing with a 5v item connected to the Propeller.
Current limiting the 3.3v Propeller is always good practice when using items like this.

OBC

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RobertW

Ok. Thanks.

Also, FYI, there are updated versions of some of the original library files that you included in your attachment. It looks like they may have been updated around May '09. Comparing the them to the files in your attachment, I believe you will need to modify some of the method names. Method 'Start' is now method 'Init', etc. (If I am understanding everything correctly). Thanks again.
-Rob W.

Oldbitcollector (Jeff)

That wouldn't surprise me.. This is a project from a PPDB book I've been slowly working on since 2008. It figures that by the time I actually get things going forward I'll have to update
all the objects. [noparse]:)[/noparse]

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Drone

Rob,

Why do you need a current limit resistor with the serial LCD? You're driving the 5V LCD serial input from a 3.3V propeller output pin. You would use the current limit resistor when driving a propeller pin from 5V. The LCD is half duplex only, propeller (3.3V) sends to LCD (5V).

I think you will find the "Debug LCD" object very useful with the Parallax 4x20 serial LCD. See here:

obex.parallax.com/objects/40/

Also, you should be aware that the Parallax LCD can draw a significant amount of current from your 5V supply when the back-light is on. I've mentioned this before here. I wish Parallax would mux the back-light to reduce overall power. After all, the LCD's back-pack is just an SX micro-controller and you can already control the back-light on/off via the serial port.

Rgds, David

Oldbitcollector (Jeff)

Drone said...
Rob,

Why do you need a current limit resistor with the serial LCD? You're driving the 5V LCD serial input from a 3.3V propeller output pin. You would use the current limit resistor when driving a propeller pin from 5V. The LCD is half duplex only, propeller (3.3V) sends to LCD (5V).

I think that was actually directed at me, and makes a good point.
I hadn't thought about it being an output pin. Duh.
It doesn't diminish the signal to the LCD, but obviously unnecessary here.

Like I said, force of habit... [noparse]:)[/noparse]

OBC

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