I recently bought a 32x32 RGB LED matrix from aliexpress.com for £17. The only problem I found when I received it was that in order to display anything on it, you have to multiplex each row. You select the address of the row, clock in 32 bits for red, green and blue and then toggle the enable pin. The row lights up and then you do the next row.

The problem is that if your PIC micropocessor has to go off and do some calculations or run some other code, the LED matrix isn't being refreshed so it goes blank.

I decided to make a driver for the matrix. I used a PIC18F26K22 SOIC chip mounted on a very small PCB that I got made at OSHPark.com. The board attaches to the back of the LED matrix display via the 2x8 pin header.

The PIC has been set up as an SPI slave device. It keeps refreshing the display until a master PIC drops the chip enable line and starts writing to it. I have got several commands like brightness etc. and it only takes about 3ms to send a complete new display to the chip. 3ms is pretty quick and you cannot notice the display not being refreshed for that small amount of time.

Currently the colour depth is 1 bit. You can either have the red, the green or the blue in any one pixel either on or off, but that does give you black, white and 6 other colours.







The schematic:

The board:

A zip file with the Eagle files and also the C code for the project.

 

The PIC has been set up as an SPI slave device. It keeps refreshing the display until a master PIC drops the chip enable line and starts writing to it. I have got several commands like brightness etc. and it only takes about 3ms to send a complete new display to the chip. 3ms is pretty quick and you cannot notice the display not being refreshed for that small amount of time.

I would approach this differently. I'd have a timer set a flag (via interrupt) that indicates a refresh is required. A call in the main program loop checks the flag and sends out the next row of data if it is set. If I had lots of intense, time consuming code in the main loop, I'd move the row write code into the ISR itself.

The low-level SPI communications would be done via interrupt, and data packing/unpacking would be done synchronously with the main program loop.

 

joeyd999, I understand what you are saying, but the code to keep the display refreshed is fairly large with large arrays etc. I made it so that even an 8 pin PIC with limited memory could be used to 'talk' to the display. I have built in functions like setting individual pixels, columns, rows etc. if you don't want to send a complete screen full of data over SPI to it.

The schematic:


The board:


A zip file with the Eagle files and also the C code for the project.

 

i am very interest about this project, but i have compilation problems with the xc8 version...
can you send me the HEX file ( direct download ...) ?
thanks ,eric


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@ricou
This builds in XC8 with some warnings that you should resolve. Most of the warnings have to do with the author using *2 to shift. The compiler warns that the result can exceed the 8bit uint. Consider using the shift operator ( << )
I didn't look at the code further to see if it work or what improvements could be made.
XC8 ver 1.36 FREE mode.
.HEX is in the ZIP.

Good luck and welcome to AAC.

 

thanks a lot for your help
i will try to compile with xc8 1.36 ..

ricou