HD44780 compatible LED dot matrix help please

[h3ndrix]

Hello,

I am trying to make a HD44780 compatible LED Dot matrix display. Apparently, a French guy made a design that worked, can be found here: **broken link removed**

However, when I replicated his design, from the images that he presents, all I got was a couple of flashing lines on one of the dot matrix displays. The guy does not give any schematics for his designs, only an explanation image that kind of explains it, but doesnt really.

The schematic that I tried to pick up from his PCB board designs is below:



I might have made a mistake or two, I don't know, I am not that good in electronics, I need the display for a project. Any help is appreciated. Thank you.

 

[kripacharya]

There are several points in your schematic labele as DF, or M, or X which i could not see where they go ?

Also - where are you inputing the data/ commands from ?

 

[h3ndrix]

My test setup and the HD44780 pins are below:





What I could understand from Xavier's page is that DF is from the HD44780 chip (which I tried to tap on the LCD), the X is I do not know where it goes, no information is given, the M is supposedly for GND (?) as most of the IC's used in the design specifies those pins that connect to M as GND.

I am using a PIC16F877A to drive the LCD, the crystal was removed to tap the signals. Thanks.

 

[kripacharya]

yeah... very nice. But i think you need to know know wher that X should go. It connects to the OE/ pins and if its floating you'll get strange results.

 

[h3ndrix]

Yes, exactly. One of the OE lines is connected to the E pin on the HD44780, but the other one is not specified, thats why I need the help. I have no idea where it should go...

 

[h3ndrix]

any help here guys?

 

[FvM]

HD44780 outputs multi-level analog voltages to the LCD display. How is it supposed to drive digital logic?

 

[h3ndrix]

Well, according to the authors site, by using NOR gates...Please check the site here

xavier.fenard.free.fr/AffGeant.htm

and here on page 3 by a Polish guy...

https://www.elektroda.pl/rtvforum/topic201946-60.html

I was able to get in touch with Xavier, but I dont think we understand each other; his explaining stuff that I am not asking, and not explaining what I am asking so...I need help

Thanks.

 

[FvM]

Yes, I see that multilevel signal is switched-off by connecting the voltage divider in a non-standard way. So you get pure digital signals at the LCD driver pins that can be used for LED matrix control.

But it's not excactly clear how you connected the HD44780 to your circuit.

 

[h3ndrix]

Exactly!

Thats what I am trying to understand and need help on. Xavier says that DF (or M for HD44780) from HD44780 is connected to the CD4001 and 273 and those are the only connection. However, I can not get that working. I am wondering how crucial the resistors on the V1, V2, V3, V4 and V5 are, but I have no idea how to calculate them...

 

[FvM]

The voltage divider resistors can be zero or infinity in the "digital" operation mode. As said, the Vx pins are jumpered to Vdd and Vss.

The required decoding scheme should be understandable by studying the LCD multiplex part of the HD44780 datasheet.

 

[h3ndrix]

If I can get this working I will be driving a 2x8 character 5x7 LED matrix setup. According to the HD44780 sheet, I should be using 1/5 bias and 1/16 duty, which means all the resistors between V1-V5 should be same, and thats how it is currently with 5 2.2K resistors. So, this is probably not the problem.

The connections, however, are still not clear to me. There must be something that I am missing; Xavier says that one of the OE on HC541 should go to CD4001, but it is not clear where the second OE goes from HC541...or any other connection that I might be missing...Below is his "diagram" for this setup

 

[FvM]

According to the HD44780 sheet, I should be using 1/5 bias and 1/16 duty, which means all the resistors between V1-V5 should be same, and thats how it is currently with 5 2.2K resistors. So, this is probably not the problem.

Seems like you didn't get the point. The author instructs you to remove the resistor between V4 and V5.

La configuration retenue fut V1=V2=V3=V4=Vdd et V5=0.
Il faut donc enlever la résistance du pont entre V4 et V5.

 

[h3ndrix]

Yes, I saw that, but isn't that for one line display, something like 1x16 character?

 

[FvM]

I don't know if you can use the same setup, but very clearly, you can't interface with multi-level signals. So all Vx voltages need to be Vdd or Vss.