# 64 pixel RGB LED Display - Another Arduino Clone

This display is based on an 8x8 RGB LED Matrix. For testing purposes it was connected to a standard Arduino board (Diecimila) using 4 shift registers. After getting it to work I permatized it on a fabbed PCB. The shift registers are 8-bit wide and are easily interfaced with the SPI protocol. Pulse width modulation is used to mix the colors, more on that later. Part of the MCU`s RAM is used as a framebuffer to hold the image. The video RAM is parsed by an interrupt routine in the background, so the user can do other useful things like talking to a PC, read buttons and potentiometers.

## Step 1: Pulse width modulation for mixing colors

Pulse width modu - WHAT ?

Pulse width modulation essentially is turning the power fed to an electrical device ON and OFF pretty quickly. The usable power results from the mathematical average of the square-wave function taken over the interval of one period. The longer the function stays in the ON position, the more power you get. PWM has the same effect on the brightness of LEDs as a dimmer on AC lights.

The task ahead is to individually control the brightness of 64 RGB LEDS ( = 192 single LEDs ! ) in a cheap and easy way, so one can get the whole spectrum of colors. Preferably there should be no flickering or other disturbing effects. The nonlinear perception of brightness exhibited by the human eye will not be taken into account here ( e.g. the difference between 10% and 20% brightness seems "bigger" than between 90% and 100% ).

Image (1) illustrates the working principle of the PWM algorithm. Say the code is given a value of 7 for the brightness of LED(0,0). Furthermore it knows there is a maximum of N steps in brightness. The code runs N loops for all possible levels of brightness and all necessary loops to service every single LED in all rows. In case the loop counter x in the brightness loop is smaller than 7, the LED is turned on. If it's larger than 7, the LED is turned off. Doing this very quickly for all LEDs, brightness levels and base colors (RGB), each LED can be individually adjusted to show the desired color.

Measurements with an oscilloscope have show that the display refresh code takes about 50% CPU time. The rest can be used to do serial communication with a PC, read buttons, talk to an RFID reader, send I2C data to other modules...
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skandinaff4 months ago

Hi Madworm, great tutorial you've got here. So I've been working on implementing your circuit, but I have only arduino mega, and apparently it doesn't work there. I presume, it's something in functions that are working with SPI and timer register, that must be different for Mega.

Would you be able to point out what exactly has to be changed? I relatively new to arduino, and coding myself, so yeah.

I've tried simulation with arduino uno, and it worked fine. Mega doesn't work in simulation either..

madworm (author)  skandinaff4 months ago

First make sure you've connected the right pins. According to the datasheet [atmega1280] that chip only has ONE hardware SPI device. See where the pins MOSI, MISO, SCK and CS end up on the Arduino Mega.

Some error messages would be helpful, if there are any.

4 months ago

Yeah, I've connected data pin to 51st of arduino mega, clock to 52nd latch to 53rd. (according to second table here - http://www.arduino.cc/en/Reference/SPI)

In proteus simulation I'm getting "Writing to UDR3 while transmission is not enabled"

As for real circuit, I was testing just with one (or four) led(s), not entire matrix, and if I remember correctly, LED(s) just didn't do anything, and shift registers maintained constant levels on their outputs.

madworm (author)  skandinaff4 months ago

That is odd. UDR indicates involvement of an UART.

Try this: http://pastebin.com/PcjfytxE

That should "blink" all 4 shift registers on/off every second.

If that works, we know SPI is good.

madworm (author)  skandinaff4 months ago
I need to look at the ATmega1280 datasheet first. I assume you've got this board (http://arduino.cc/en/Main/arduinoBoardMega).

That will take some time. It is not radically different, but I need to find the right bits & pieces.
rfrantzen5 months ago

Hi Madworm, I'm currently playing with this setup and do wonder one thing, Was there a specific reason to put the resistors on the cathode side? As they are all the same value technically they can be on the anode side as well, reducing the amount to only 8 instead of 24. I know it isn't the cost but it does makes the board a bit cleaner. Or am I overseeing something here?

madworm (author)  rfrantzen5 months ago

Yes, there is an issue.

Putting the resistors on the anode side leads to noticeable and unpleasant brightness variations when more than one colour is turned on.

1 resistor + 1 LED --> normal brightness

1 resistor + 2 LEDs --> roughly half the current for each --> dimmer

5 months ago

Clear, Thnx

stefa1686 months ago

Hi Madworm, thanks a lot for your instructable!

Right now, I'm trying to implement a control with a second arduino, so that the one connected to the matrix can just do that stuff (it won't get interferences from other parts of the code)

I was going to use SPI because with interrupts I can transmit all the 192 Bytes (I will reduce them when I will manage to do this). To do so, I have to use the SS pin of the Processor, which is pin 10. Now, here's the problem: I tried to modify correctly all the sketch so that it doesn't use pin 10 but pin 8. However the matrix filckers and then completely turns off. I don't know what I can do, as I need that pin to be free so that I can use it with the SPI_STC_vect interrupt..

How can I free that pin, so that I can use it for slave SPI?

Thanks

madworm (author)  stefa1686 months ago

The AVR already uses hardware SPI in master mode to talk to the shift registers. This is quite time-critical interrupt code. I'm not quite sure how you would have it listen as a slave at the same time without disabling the matrix stuff, or if that is possible / advisable at all. Sounds like trouble to me.

6 months ago
But isn't possible to use the the shiftOut function for the matrix instead of the hardware SPI? or it is slower?
madworm (author)  stefa1685 months ago

Yes and no. You can use it, but it is waaaaaaaaaaaay too slow.

MarcM67 months ago

Thank you for posting this example. It gave me inspiration to make a more complete version and write a full driver for tri color matrices while talking to them through a higher level library to draw pixmaps, letters, shapes and get scrolling text in multiple colors:

http://marc.merlins.org/perso/blog/cat/arduino/post_2015-01-06_Driver-for-direct-driving-single-to-3-color-LED-Matrices-with-software-PWM.html

https://github.com/marcmerlin/LED-Matrix

Would you mind adding a link for folks who might be looking for such a driver?

Cheers, Marc

madworm (author)  MarcM65 months ago

Sorry for answering so late. I didn't get notifications from Instructables for a while, today I got lots.

Luisdlahuerta made it!12 months ago

Very good project!!! thanks you very much, its running perfect in my protoboard.

congratulations ;)

nice blog

best regards

madworm (author)  Luisdlahuerta12 months ago

:-)

sirkevin271 year ago

I spent all day learning about interrupts and now understand nearly all the interrupt business in your code.

I just don't understand TIMER1_CNT = 0x0130 corresponds to 32 levels of brightness... that number is 304 no?

because you have TCNT1 = TIMER1_MAX - TIMER1_CNT
so TCNT1 = 0xFFFF - 0x0130 = 65536 - 304 ? Does that mean TCNT1 is initialized to 65232, increments on every prescaler cycle till it hits 65536, starts the interrupt where TCNT1 is re-initialized to 0xFFFF-0x0130 and does the interrupt business?

I understand this TCNTn business with the CTC mode, but I'm confused how you used it here in normal mode and how you find the value corresponding to the amount of brightness levels. Also was there a benefit to using an overflow interrupt than a CTC one? Once I understand this, I'll finally feel like I truly understand your code (After staring at it all summer).

Thanks!! Really want to build this.
madworm (author)  sirkevin271 year ago
Let me look at that code... it's been a while :-)

Ah... I see.

OK, the purpose of preloading the timer is to have control over the frequency at which it is called. The plain overflow interrupt would take 2^16 cycles to complete, which in this case is too long.

You see, the crude PWM generation relies on persistence of vision and uses a lot of loops in the interrupt code. These loops take some time to finish.

If the ISR frequency is too high, all CPU time is eaten up in the interrupt, up to the point that the system appears to be frozen.

If the frequency is too low, the display will flicker.

The TIMER1_CNT value was determined by trial and error. The more colour-levels are to be displayed, the more time the ISR takes --> TIMER1_CNT must get larger.

Using this manual preloading vs. CTC mode... it should be equivalent. I think in later versions of the code (github) I used CTC mode. Later I switched to binary weighted PWM generation (much less CPU utilization, but it has other issues).
1 year ago
Ahh ok! I was thinking there was a formula or something for determining that, but when I calculated the frequency of the interrupts it was 51.23Hz, so I was thinking it was a trial and error kind of value. Now that you've confirmed that, I think I'm ready to build this thing!

Thanks a ton!
madworm (author) 2 years ago
It has been done, but it comes at a cost.

Generating the PWM in software for true-color (sort of) takes up a big slice of CPU time. At some point, there's nothing left for anything else. If I had to do it again for a 128px display, I'd at least use dedicated LED-drivers with integrated PWM generation. A faster micro would help as well.
BAKSTEEN2 years ago
How about combining 2 of these, for like a game of tetris or just a larger game/screen is is possible to just hook up more ic`s and a second screen ?
devilmaycry4 years ago
Hi Im currently doing GCSE ELECTRONICS KS4 AND i WANT make a project like your one but the problem is I want to make a Arduino Clone if u guys know any step by step instructable for beginners cos im new to micro controllers.MANY THANKS
2 years ago
http://www.theparsley.com/arduino/diy/
madworm (author)  devilmaycry4 years ago
Well, the good news is that there is nothing special about making an arduino clone, no magic involved. A minimal working board is just a few parts (no on-board usb).

I don't know if you're shooting at making your own pcb with smd parts or through hole components. There's also nothing special about that, except that it costs serious money to have pcbs fabbed (more than just a few) and getting a non-functional board is quite upsetting. So you'll want to make sure you have a functional prototype of your schematic working on breadboard or vero/perf/strip-board before you shell out big money.

As you've mentioned GCSE, I take it you're in the UK. For small or one-off prototypes, there's a UK site that offers a nice service for just that ( http://www.badnetwork.co.uk/ )

As far as minimal arduino clones go, the 'boarduino' seems like a good starting point to me ( http://www.ladyada.net/make/boarduino/download.html ). Easy to build with self-sourced parts.

As far as using PCB layout software, this can be a bit tedious at first, probably even quite annoying. Personally I use KiCAD ( http://store.curiousinventor.com/guides/kicad ), which is open source and does not have any constraints like the 'free' version of EAGLE.

Also I'd like to invite you to join the arduino forum on www.arduino.cc - quite a lot of UK folk hang out there as well.
angler3 years ago
How is the total shift register (per chip) current kept below 70mA? Is this why you chose 270ohm?
3 years ago
Oops, just read the 1:8 duty cycle comment below.
acotton14 years ago

I have one week to make an RGB LED matrix for school. How did you wire your matrix up before you transferred it all to PCB? Due to time and money constraints, I can't have a PCB manufactured. Do you have photos, schematics, or diagrams?
madworm (author)  acotton14 years ago
One full week... snigger. SCNR.

Before it was transfered to a PCB, I uses a breadboard to test the circuit. Suffice to say that it was no fun at all. So many wires...

If you go to my blog (the link is here somewhere), you will find schematics and photos (flickr) and some code as well. The best entry point is the projects page. Other posts may have outdated content. If you intend to use any of it, make sure to get the latest versions of both from the git repositories, otherwise it may have unpredictable effects.

wmtt4 years ago

Good news, I have success in having a working RGB matrix after following your codes and instruction.

May I know how do i modify the code if I want to run it on a stand alone ATMEGA168/328 but NOT from a Arduino?

Thank you again!
madworm (author)  wmtt4 years ago
Well, once you have compiled the code (for an 168 or 328) into the .hex-files, you can just take your favourite ISP programmer and flash the chips. No need to change the source code. On linux systems the .hex files are temporerily created in '/tmp/build.xxxx', on windows I frankly don't know.

Just make sure the FUSE settings of the chips are correct. For an 168 these would be:

a) no bootloader, 16MHz quartz, 16kb usable:

lock: 0x3F
lfuse: 0xFF
hfuse: 0xDD
efuse: 0x01

b) with bootloader, 16MHz quartz, 14kb usable:

lfuse: 0xFF
hfuse: 0xDD
efuse: 0x00
wmtt4 years ago
Dear Madworm, you are great! I saw your works elsewhere in the net and I think you are the one to answer my question.

I want to build a 5x5x5 LED matrix using RGB LED, I don't think I will have problem controlling each layer but I have 5 layers to control. I want to

1. mix colour (not just the 7 colours) of the RGB and
2. contol the brightness of each and single LED

so I think PWM (software by 74HC595 or by hardware TLC5940) is my answer.

Do you recommend

1. use the 74HC595 and mutliplex them for each layer (5 layers) or
2. Use TLC5940 and multiplex them for each layers?

I worry that by multiplexing the chips, I do not get enough refresh rate and give rise to LED flickering......

Looking forward to your kind assistance. Any information would be much appreciated!!!
madworm (author)  wmtt4 years ago
I'd use TLC5947 if possible. Very similar to 5940, but simpler to use. Needs no external grayscale clock, which keeps the microcontroller busy all the time. Just send the data and forget about it. With these you'll need a few extra transistors to drive the layers. Maybe something like UDN2981A if you can get that one.

The multiplexing makes it dimmer, but with professional driver chips you can compensate to some degree by adjusting the external current reference resistor. Just make sure that you never stop multiplexing...

But for color mixing anything is better than 595 chips. It may be doable, but at some point you'll wish you hadn't gone that way ;-) A whole lot of time is wasted just for generating the pwm signals.
4 years ago

Thanks for your kind reply. I have already received the 5947 samples from Ti and will give it a try. But exactly do i multiplex it, can you give me some direction or exact way / scehmatic that I can follow.....

One more question, is your 64-pixel RGB matrix true color and not just the 7-color....?

Thank you once again.

WM Tang
madworm (author)  wmtt4 years ago
For starters, you can find an example for a single line of 8 RGB LEDs driven with a 5947 there:

http://docs.macetech.com/doku.php/octobrite

For multiplexing, you need to have a look at the BLANK input. It should be used when a new row is addressed.

A project based on the 5940 is this one:

http://www.thebox.myzen.co.uk/Hardware/Mini_Monome.html

It should convey the principle of what you need to do.

What the 5947 needs is similar. You need to compensate for the 24 vs 16 channels and throw everything out that deals with GSCLK, as this is handled internally.

My doodad can do more than 7 colors ;-) Technically it supports 32768 shades, but not all of them look different to the eye.
dunnos4 years ago
When I read the last step I laughed, so hard. Seriously man, piggyback! awesome
wilhelmmaybach4 years ago
Hey Madworm, great Instructable, just one question: I understood most of the project, but I dont get the PWM part. I understand you use th 74 595 for selecting or controlling each led and each lead of the LEDs, (1 for red, 1 for green, 1 for blue, 1 for the anode or cathode) but, I dont get how you "connect" the PWM to each RGB, I understood very well what PWM is, but, for example, you have in your PWM a frecuency of 1 Hz (only example), and a PWM resolution of 8 bits, and duty cycle of 25%, ok, you want to connect the output of this PWM to any lead of any led, how do you achieve that?? how do you refresh or shift data into the shift registers to achieve that?? thanks for your response, and please be a little detailed, thanks a lot...:)
madworm (author)  wilhelmmaybach4 years ago
There is no single PWM source that needs "connecting" to the LEDs. The PWM is created by sending a bit-stream to the shift registers at high speed.

If it helps you understand think of it as many virtual PWM sources that are sequentially sampled, which created the bit-stream. All of this is done in software. The result of this is then sent to the shift registers, which just reproduce it. Each SR pin is a PWM source that way.
VJHAL4 years ago
I think I know how to get it to work with 8-bit pixel depth ... or maybe even 16-bit if you can handle a frame buffer that large. Can I get access to the driver code to try out my idea?

Also: Are the support boards for sale or can I get some made the same place you did?
madworm (author)  VJHAL4 years ago
You can grab the code and _all_ design files (KiCAD + schematic + gerber files) on my blog ;-) The link should be on my page somewhere here.

Using the gerber files, you can have the boards made for you anywhere. If you just need one board (no parts), this shouldn't be a problem as well ;-) I will find one in my many boxes.
4 years ago
(Trying again, code got garbled the first time.)

OK I have the code. I am new to Arduino so I am having trouble seeing where the duration of a light pulse is set, but let me describe the idea here in words and pseudocode.

Instead of a countdown loop with "<" comparisons, we look at each bit of the brightness from MSB to LSB. (I will here assume 5 bit depth but the generalization should be obvious.)

In pseudocode (borrowing from the actual code):
#define __brightness_bits 5
#define __brightness_levels (1<< __brightness_bits)
...
for (bit=(__brightness_bits - 1);bit>=0;bit--)
{
mask = (1 << bit);
for (led = 0; led <= __max_led; led++) {
if (mask & brightness_red[row][led]) {
red &= ~(1< }
if (mask & brightness_green[row][led]) {
green &= ~(1< }
if(mask & brightness_blue[row][led]) {
blue &= ~(1< }
}
// Now here comes the key idea:
// Drive the LEDs for time proportional to (1<< bits)