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I am planning on creating a RGB LED POV globe only physically larger then that one.

I am planning on using 64 RGB LEDs, requiring 3x pins per LED for a total of 192 outputs. Obviously I am going to need some shift registers. I have looked in to 74HC595 (8bit) and it would require 24 of these chips to handle all of these LEDs.

I am planning on putting these shift registers in series with 6 per channel from my micro-controller.

The 74HC595 has a switching frequency of 100 MHz.

I been told that I will need to spin the Globe at around 1500 RPM (25 per sec)

The plan is to use a Arduino for the micro controller but I am open to suggestions.

My questions are:

  • Is there a better suited chip then the 74HC595 for this project?
  • Besides the Arduino is there a better suited micro controller for this project?
  • I don't want to power the LEDs off of the micro-controller So I am going to need external power. How would I go about doing this?
  • With 6x 74HC595 in series, How fast could I switch all of the LEDs colors on this channel?
  • With 12x 74HC595 in series, How fast could I switch all of the LEDs colors on this channel?
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+1 cool project. –  davidcary Mar 18 '11 at 15:11
    
@davidcary are you the same David Carrie that I met at Vancouver Hack space (VHS) last year? –  Steven smethurst Mar 18 '11 at 17:05
    
I wish I was -- photos of that space I see on the internet look awesome. I would like to start such a space in Tulsa. Alas, I haven't been in Canada in years. –  davidcary Mar 18 '11 at 21:40
    
@davidcary here is one in Tulsa, it looks more software then hardware but it would be a good start. hackerspaces.org/wiki/Free_Information_Exchange_Foundation –  Steven smethurst Mar 19 '11 at 18:34
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3 Answers 3

up vote 4 down vote accepted

1. A bunch of 74HC595 chips will work great. Other chips that would also work just as well and perhaps slightly better are listed at: Which SIPO chip is better, 74HC4094 or 74HC595 or something else?

2. The Arduino is an excellent choice for prototyping, especially if you are comfortable using gcc. Perhaps it would be quicker to use one for now. Alas, I suspect you will soon write code for this POV display that needs more RAM than the Arduino has available -- at that time, either (a) use one or more of tricks to reduce the RAM needed, or (b) add some external RAM, or (c) port the code to some other microcontroller with more RAM (perhaps the ATMEGA1284 ?).

The Parallax Propeller is an excellent choice for a high resolution POV display -- it has an order of magnitude more internal RAM (32 KB RAM) than the ATmega238 in the Arduino. (Is there anything I can do to support porting gcc to the Propeller?)

Some people prefer "square pixels". I'm sure you already know that the distance around the equator of a sphere is twice as long as the distance from pole to pole (Earth's equator is a little more than twice as long). Since you have 64 pixels from the south pole to the north pole, you might choose to reload a new vertical "line" of pixels 2*64 = 128 times per revolution in order to get 128 "square pixels" at the equator. The simplest way to do that is to store the full frame uncompressed in RAM. That requires 64*128 pixels * 3 bits/pixel = 24 576 bits = 3 072 bytes, plus a few bytes of RAM for other program variables. Alas, the Atmel ATmega328 in the Arduino only has 2 048 bytes of RAM.

Earlier POV displays used microcontrollers with an order of magnitude (!) less RAM than this. So people have developed a variety of tricks you can use to work around this. One trick: Only lighting up the "front" half of the globe turing the time you can see the LEDs, then turning off all the LEDs (or leaving them on some constant color) during the "back" half that you can't directly see the LEDs. That halves the amount of RAM you need, so then it fits in the Arduino. If you don't like that trick, there are other tricks you can use that are less obvious. Another trick: Store the image in the flash program memory. The Arduino has enough flash program memory to store several 3 072 byte frames. Yet another trick: use ASCII text to store the text you want to display, then use flash program memory to store the "character generator ROM" data. I'm pretty sure there are other POV tricks ...

3. If I were building it, I would power the Arduino and the 74HC595 chips from one big power supply, and power the motor that spins the POV from a separate power supply.

Only after I got all that working would I even consider a more complicated separated power supply system. (In principle, if you have a separate "red" switching power supply whose +V is only connected to 74HC595 chips which in turn are only connected to red LEDs, you could independently tune its output voltage to minimize the total power and heat production of the system. But it seems unnecessarily complicated.)

4. and 5. Perhaps the simplest way for the Arduino to drive the POV display is to daisy chain all 24 74HC595 chips in one long single string, and then use the Arduino SPI library. That requires 192 clock pulses to clock in the new column of data, and then a pulse on the RCLK (aka framing pulse, SS, etc.) to start displaying that new data. According to one Arduino to SPI interface tutorial, the fastest SPI clock rate is system speed / 4. So the 16 MHz Arduino can put out a SPI CLK of 4 MHz. If you upgrade it with a 20 MHz crystal, you can get a SPI CLK of 5 MHz. If you can get your program fast enough to keep up with the SPI hardware, you can put out a new column, at best, in 5 MHz / 193 pulses, so the maximum theoretically possible speed is 25 907 columns/second.

At the standard cartoon film refresh rate of 24 frames/second (which flickers noticeably -- you would like something better), and at 128 columns/frame to get "square" pixels at the equator (you might want more to get better resolution), that gives 24 frames/second * 128 columns/frame = 3072 columns/second.

There's nearly an order of magnitude of breathing room between "the speed you want": 3072 columns/second, and "the speed that is completely impossible on Arduino hardware": 25 908 columns/second. Hopefully that gives you enough room.

Some people think you can get a faster column refresh rate by re-arranging the 74HC595 chips into 2 or more chains and loading all chains in parallel. Some people are like that -- they see unused pins on the Arduino, and they are itching to use them for something. But it may be counter-productive -- the SPI hardware is only connected to one set of pins on the Arduino, and so chains hooked to any other pins must be loading with emulated "bit-banging" software, which will invariably be slower and use more CPU time than the built-in SPI hardware.

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Note that to get colors other than just off, white, red, green, blue, yellow, cyan, and fuchsia, you'll need to be doing your own PWM if you're just using a shift register bit to control each LED component. You'll need to be PWMing each bit as you shift each group in, so you'll need to take this into account for your refresh rate, and it will also make your code a little more complex.

Alternatively, you could use an IC like the TI TLC5947, which will do 24 channels of PWM, based on the bits you shift into it. http://focus.ti.com/lit/ds/symlink/tlc5947.pdf

With this solution, you would need to shift in 12 bits (instead of 1) for each channel, but it will take care of the PWM for you, so it may be a worthwhile solution.

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I'll second this. This family of LED drivers is really nice - it eliminates the need for current limiting resistors and makes using separate supplies very easy. The 24 output part would line up nicely with your LEDs. Maybe consider a chip with multiple SPI I/O. –  W5VO Mar 18 '11 at 16:05
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Does PWM work on a spinning POV display? –  davidcary Mar 18 '11 at 21:34
    
@davidcary Yes you can use PWM's on a POV display –  Steven smethurst Mar 19 '11 at 18:26
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All the mechanically spinning displays I've seen so far give at most the 8 colors: the CMYK + RGB + W that you mentioned. The ~ 7000 kbits/second to get full color (rather than ~ 600 kbits/second to get 8 colors) seems difficult for an Arduino, but it might be a cool additional feature. –  davidcary Mar 21 '11 at 16:22
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Let's say you need 500 updates per rotation. That's 500*192*25 'shifts' per second - that's just 2.4Mhz shift out frequency, divided by 6 channels = 400kHz shiftout frequency.

This is achivable with arduino(20Mhz really preferred), but require careful programming & timing.

So for your questions:

1) I guess not. 8-bit registers would require tricky PCB routing

2) If you would have 24 outputs, it would be easier. Also, for such a large display it would be nice to have enough internal memory to hold framebuffer to make programming easier. On the very minimal you would need 16kb memory which is not something you can find on 8-bit AVRs. mid-range ARM's might be better, but harder to develop for.

3) Well, you are not. You are using power from 595, they are not very powerful, but enough in simplest case (check datasheet for specific model you have).

4,5) Faster than needed :-)

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