LED cube - basics of timing /frequencies

[atferrari]

Re-posted after seeing that my initial post has vanished.

I've spent many hours in the last days reading about POV and LED cubes.

Given an 8 x 8 x 8 cube and a refreshing frequency of 150, from all my reading I concluded the following:

T (to light them all) = 1/150 = 6667 usec

Time devoted to each LED in the sequence (whether on or off) = 6667 usec / 512 = 13 usec / LED

If I have to introduce some variation in the perceived luminosity of each one I should apply to every LED a PWM signal of suitable frequency during the asigned time slot of 13 usec.

I am not sure if I will build one cube or not (most probably no) but I am interested in the concept.

Please, I am not looking specifically for links but basically anyone telling me if what I say above is correct or not. Comments really appreciated.

Also, isn't a 8 layers cube as proposed, equal to a 2D frame of 64 x 8 LEDs on this regard?

 

[panic mode]

why 6667 usec is divided by 512 and not 8? from what i recall you don't want too low duty cycle or display will be too dim even with huge LED current.

1/8 is fine, doing 1/16 is pushing it, i didn't try it but i would expect that 1/512 is not good idea.

the process is something like:
1. turn on one plane (one of 8 cathodes)
2. load 64 bits (anodes)
3. wait short time
4. increment plane number
5. repeat

how you get 64 outputs for anodes is up to you (octal latches, SIPO registers etc...)

to control brightness of entire display, you could connect MCU PWM output channel to enable input on TTL chips.
to control brightness of individual LED in a plane, you need to control it in software

here is an example using multiplexer to light up one plane at a time (64 LEDs):
https://www.instructables.com/id/Led-Cube-8x8x8/

and here is the part of circuit to load 64 bits:
https://www.instructables.com/id/Led-Cube-8x8x8/step8/IO-port-expansion-more-multiplexing/

they used parallel loading of 8-bit and in only few clocks, all 64 anodes are loaded with corresponding bit-pattern.
cathodes are cycled at 1/8 duty cycle (one plane at a time)

 

[atferrari]

Aha! One 64-LED layer at a time.

Multiplexing with parallel loading, I can see how it works as well.

I cannot figure yet is how to have LEDs in the same layer with different luminosity at the same moment. You say software...(?)

Evidently I need more time to grasp the whole idea.

Gracias pm.