Print PDF
Favorite
Email
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.
More information about "Arduino": www.arduino.cc
More information about "Arduino": www.arduino.cc
Remove these ads by
Signing UpStep 1Pulse 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...
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...
| « Previous Step | Download PDFView All Steps | Next Step » |
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.
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?
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.
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!
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
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!!!
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.
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
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.
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.
Also: Are the support boards for sale or can I get some made the same place you did?
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.
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)
drive_LED_array_for_time(mask);
}
The number of different times the LEDs get driven here is proportional to __brightness_bits, not 2 to that power, so in this case 5 (not 32). It scales nicely, linearly instead of exponentially. The remaining problem is simply how to drive the LEDs for a variable amount of time. This might be easy for you, but I need to understand the Arduino interrupt system first, which will take me a while.
Does this make sense?
At first glance I feel you have done this:
a) remove computational needs from the 'decision making' process, better scaling law: O(n)
b) add complexity to the 'driving the LEDs' part
When driving just a single LED (or many in the same way) it would be easy to off-load this task to one of the timers in PWM mode.
For generating the required PWM bit-pattern with shift registers you'd have to run some sort of loop again I think, or use dedicated driver chips with integrated PWM counters (like TI5947, not exactly cheap).
Maybe this is a zero-sum situation, but I can't tell for sure.
Let's see your 'make the LEDs do what I need' code ;-)
I think this is what you meant by "off-load this task to one of the timers in PWM mode". So the right SW architecture here is to use one timer in PWM mode just for display on time. Within each frame, you fire off the PWM timer in __brightness_bits groups * __rows rows/group with a different timer setting for each group, but you don't need to sit and loop inside the interrupts and waste a ton of CPU time. Just load the display bits and the timer and then exit: "Fire and forget." :-) You do of course have to know whether it's finished so you can fire off the next one, but that can either be done off the timer interrupt, or, less precisely, just as part of your main loop where you are polling buttons or whatever else you do in the unused CPU cycles. Assuming the minimum pulse width is one clock cycle, the total driving time is still at least __max_brightness clock cycles plus overhead. So we are constrained by
frame_rate * total_driving_time < CPU_clock_speed
so assuming 16 MHz CPU and 1000 cycles overhead per depth-bit per frame (125 cycles per row display) we get theoretical max frame rates of roughly:
4-bits => 3984 Hz (16000000 / (4*1000 + 16))
5-bits => 3179 Hz (16000000 / (5*1000 + 32))
6-bits => 2638 Hz (16000000 / (6*1000 + 64))
7-bits => 2244 Hz (16000000 / (7*1000 + 128))
8-bits => 1937 Hz (16000000 / (8*1000 + 256))
9-bits => 1682 Hz (16000000 / (9*1000 + 512))
10-bits => 1451 Hz (16000000 / (10*1000 + 1024))
11-bits => 1226 Hz (16000000 / (11*1000 + 2048))
12-bits => 994 Hz (16000000 / (12*1000 + 4096))
13-bits => 755 Hz (16000000 / (13*1000 + 8192))
14-bits => 526 Hz (16000000 / (14*1000 + 16384))
15-bits => 334 Hz (16000000 / (15*1000 + 32768))
16-bits => 196 Hz (16000000 / (16*1000 + 65536))
17-bits => 108 Hz (16000000 / (17*1000 + 131072))
18-bits => 57 Hz (16000000 / (18*1000 + 262144))
19-bits => 29 Hz (16000000 / (19*1000 + 524288))
20-bits => 14 Hz (16000000 / (20*1000 + 1048576))
Note that for under 14 bits, the overhead is most of the time, while for 15 and over, the displaying time dominates. But anyway it seems like 16-bit pixel depth should be achievable at completely flicker-free frame rates (if we have room for the double-size frame buffer). Even assuming the overhead doubles for 9-16 bits, we still get 16-bits => 164 Hz. 8 bits should be easy. One could either stick to a fixed frame rate (using the other timer) or just let this run as fast as it will go for maximum brightness (but with possible variations in brightness if the timing varies due to other loads). With this architecture the CPU overhead at 5-bit depth probably drops from 50% to around 1%. Give me a few days to receive the Arduino Uno I just ordered and a week to play around with it and I'll have code for you.
I don't think it's zero-sum, I think it's a clear win, but the proof would of course be running code. It might be as simple as changing the how-long-to-drive-a-single-row constant to be a variable.
A free PCB would be lovely.
If you require very precise (sub ms) timing though, you may have to expect a few glitches. For interaction with humans it is still plenty fast.
what is the function of P1, P4 and P5?
(I love the project!)
P4: Used for directly programming the chip (once for the bootloader).
P5: Some pins are shared between the programming port (P4) and the connections to the shift register chips. The jumpers can be used to disconnect them while using P4. You could also use it for connecting other things, but you'd lose control over the matrix of course.
I've another question:
your last shift register IC5 needs to source over 200mA/port?
in step 2 of this instructable you said:
Each port can source or sink about 25mA of current. The total current per chip sinked or sourced should not exceed 70mA.
why aren't you using a ULN2803 instead?
(sorry for my bad english (Belgium))
I don't use an additional sink driver, because of chip count. If I had to rebuild the device, I'd replace the sinking 595 chips with tpic6c595 or similar and add p-channel mosfets to source current. I have a prototype with a source driver (udn2981a), but that chip is too slow for PWM.
Its allot better than the previous, Its 30 Frames, Can you do it?