LED Matrix Question

[mekohler]

My grandfather and I are undertaking a project to build a table lit by a 5 x 5 matrix underneath (composed of LED's). The schematic we followed is here:

http://www.injektilo.com/PICHelp/schematic.png

We used an exe to convert the "frames" of a "word" into a string of numbers and then another exe to convert it to a .bin, which is loaded into the EEPROM (24LC64).

For example, to spell TEST, the first frame is T, the 2nd E, etc...and the .txt output is:

1 1 31 1 1
0 31 21 17 0
0 23 21 29 0
1 1 31 1 1

This is then converted to a .bin:

http://www.injektilo.com/PICHelp/sequences.bin

Now we need to control the EEPROM to feed the LED's but we don't know how. We found this picture of someone who did the same project with the same schematic:

http://www.injektilo.com/PICHelp/PICT0003.JPG

Its obviously a microcontroller which connects to the 10 ports in the upperleft of the schematic (next to the 24LC64 EEPROM)...but I can't make out the connections on this circuit and I don't know how I would program it to access the EEPROM bits and "push" them to the Matrix one after the other, and cyle...so the table would read T, then E, then S, then T, then T, then E, etc.

I know this is alot, but we are so close to finishing and this is the first time we have done anything with PICs. Thanks for all the help guys.



Marcos



P.S. - The 4 chips above the expanders (PCF8574A) are http://www.electronelec.co.uk/uln2803a.htm

[SteveB]

Both the EEPROM and the PCF8574A use the I2C communications. At the bottom-right of the schematic you will note two pull up resistors on the lines that connect the I2C bus to the 5 chips.

So, the micro controller will use I2C to address the desired device as needed. EEPROM for storage and retrival of the data. PCF8574A for the actually setting the values for each element of the matrix.

That's the simplified overview.

HTH,
SteveB

[mekohler]

Both the EEPROM and the PCF8574A use the I2C communications. At the bottom-right of the schematic you will note two pull up resistors on the lines that connect the I2C bus to the 5 chips.

So, the micro controller will use I2C to address the desired device as needed. EEPROM for storage and retrival of the data. PCF8574A for the actually setting the values for each element of the matrix.

That's the simplified overview.

HTH,
SteveB

Ok, so I am correct in the assumption that the microcontroller is connected to the left of the EEPROM on the schematic? I've been told that this is an intermediate to advanced programming level for the PIC? Is there anything you recommend me reading so I can program this? Or any sites that offer custom program solutions for $$$?

[SteveB]

Ok, so I am correct in the assumption that the microcontroller is connected to the left of the EEPROM on the schematic?

Yes, this is correct. The pin arrangement is as follows:

1- N/C          10- N/C
2- N/C          9- N/C
3- Data         8- Clock
4- N/C          7- N/C
5- +5V          6- GND

I've been told that this is an intermediate to advanced programming level for the PIC?

Yes, but not insurmountable for a beginner, if approached in a building block manner. The basic communication in the circuit is not too complicated. The algorithms for the display may be a little tougher.

Is there anything you recommend me reading so I can program this?

Well, I can only speak for myself. I read through Myke Predko's book, "Programming and Customizing PICmicro Microcontrollers." (It is a little dated, and deals with assembly, but good initial explanation on the internals of the PIC. He also uses "psuedo code" a lot, so it's not too bad a transition). After that, get into the datasheets, for both the PIC, and other components. For PBP, there are examples all over the place for specific tasks. MeLab's own site has a nice set of example files.

A quick summary:
a) Know the PICmicro controller. (What can it do, and the basics of how)
b) Understand how the components work together in the circuit (Like the I2C comm)
c) Use PBP to get the PIC to do it’s job.

Rather simplistic, and very much my own experience/opinion.

Or any sites that offer custom program solutions for $$$?

I can’t really speak to this. Others may help.

SteveB

[Dave]

mekohler, I might be blind but, I don't see any schematic, only an overlay for a PC board....
I may be able to help as I have used PCF8574A's for many products in the past.

Dave Purola,
N8NTA

[mekohler]

mekohler, I might be blind but, I don't see any schematic, only an overlay for a PC board....
I may be able to help as I have used PCF8574A's for many products in the past.

Dave Purola,
N8NTA

You are correct, I didn't upload the schematic. That is here:

http://www.injektilo.com/PICHelp/schem.bmp


I have a feeling that jumping straight into this form of the project might be too difficult (considering my time constraints). I have come across a more common LED Matrix layout, and I have uploaded the pictures and code here:

http://www.injektilo.com/LEDMATRIX/

Again, I won't be using a 5x7 display, but a handmade 5x5 one. If I use this, it seems as though all the work is done for me (hehe).

My only question is....I want to replace each SINGLE LED with 5 LED's in PARALLEL (since I need to shine this through a diffusive plexi material, so I need the brightness and spread of light). Since they are in parallel they will each receive the same voltage as the single LED...but what about current? Would I need one resistor for each LED in the "parallel cluster" ?

My friend says: the important bit is that each LED should have its own resistor. Otherwise only the LED with the lowest internal resistance will light.

If so, what equation governs the value of each resistor? Argh! I just want to get this project I started over with before I move and then start my serious and gradual study of PIC programming.

Sorry for the change of course (or possible change). Again thanks for all the help guys!