[dorkbotpdx-blabber] another LED array question

[Donald Delmar Davis]

Greg, The joy of the 8 bit parts is that they are standard and cheap.

look at hte q25 for these parts and add to the group order.




On Dec 15, 2008, at 12:24 PM, Greg Grunest wrote:

> Paul,
> That's an awesome chip!  I'd ask you where you found such a great IC  
> but you
> might not want to divulge too many of your sources. :)
>
> I was looking at using a LS 674 or 675 for a similar purpose because  
> it
> comes in a 24 pin dip package and is a 16-bit shift register instead  
> of
> 8-bit but it will only source 20ma.  Have you seen a similar part to  
> the one
> you mentioned but in a 16-bit register?
>
> Also, many years ago I worked on a project in which we built a serial
> loadable shift register with ram.  Its purpose was to load up  
> approximately
> 8k x 40bits of SRAM with code to be injected onto a processor bus.   
> The SRAM
> was loaded serially, and then dumped onto the 40-bit-wide processor- 
> data-bus
> in parallel.  It worked like a stack in a processor where every time  
> the
> "chip" was clocked (an equivalent POP instruction), it would pump  
> out onto
> the data bus the next 40-bit word in the sram.  It was kind of cool  
> because
> when you exceeded the memory of the sram it would cycle around  
> starting at
> the beginning again.  It was really just 5 8-bit serial loadable  
> sram chips
> on a single module but the problem is I can't remember what the heck  
> type of
> chips they were or who made them.
>
> I was thinking that 15 of those connected in serial to an AVR chip  
> could be
> loaded with the "PWM"-type pattern you described below via serial  
> and then
> just "clocked" to drive the LEDs, thus freeing up the processor  
> until a
> pattern change needed to occur.  I just can't remember what the heck  
> the
> chips were.  ARGH!
>
> - Greg
>
>
> -----Original Message-----
> From: dorkbotpdx-blabber-bounces at dorkbot.org
> [mailto:dorkbotpdx-blabber-bounces at dorkbot.org] On Behalf Of Paul  
> Stoffregen
> Sent: Monday, December 15, 2008 10:20 AM
> To: A discussion list for dorkbot-pdx (portland, or)
> Subject: Re: [dorkbotpdx-blabber] another LED array question
>
> I would try a big 120 bit shift register, probably built out of 15
> TPIC6C596N chips, which costs about $16.
>
> The AVR can shift out 8 bits in 17 cycles, so running at 16 MHz in
> theory the whole shift register could be updated 62754 times per  
> second,
> if you didn't have to do anything else.  In practice, I'd imagine
> getting between 1/4 to 1/3 of that if you're writing in C.  If you're
> willing to do a bit of assembly (or copy mine from the recent LCD
> project) you can write to the SPI data register exactly every 17
> cycles.  Just copy the code 15 times.
>
> I'd probably go for 32 levels of light (not necessarily linearly
> distributed).  If you're reading bit patterns from a table and putting
> them out to the shift register, you can do a lot better than PWM.  For
> example, if you want 50% brightness, shift out 1,0,1,0,1...  if you  
> want
> 40%, shift out 1,0,1,1,0.  The patterns could go in a lookup table
> (which can be any distribution of intensities that "look" even to the
> human eye even if they're not linear) that you index by the desired
> brightness, so each time you build another 15 bytes to shift out,  
> you do
> 120 lookups and shifts and probably some other simple stuff.  Even if
> you only manage to get all 15 bytes computed and shifted out 1000  
> times
> per second (only 1/60th of the theoretical max) if you scatter the  
> bits
> like that there shouldn't be any visible flicker, except maybe at very
> low brightness where it's only 1 or 2 on and many off in the pattern.
>
> The other (much harder) idea that comes to mind is actually building  
> 120
> PWM timers inside a FPGA!
>
>
> -Paul
>
>
>
>>
>> i'm sizing up a project for LIGHTBAR that involves an array
>> of 40 RGB LEDs, each of which i want to control in complete
>> gradations of brightness, and i want them to be arrayed
>> on an 8x5 matrix inscribed on a sphere, approximately.
>> so that's 120 variable-brightness or PWM pins i need to
>> be able to control.
>>
>> i'm considering using blinkMs, since the wiring and programming
>> would both be significantly simpler.  but the cost goes through
>> the roof if i do that. (i'm asking thingM to give me the blinkMs
>> for free, but that trick almost never works.  =)
>>
>> is there something like an analog shift register that i can
>> interface to a dorkboard, that will let me set these 120 different
>> pin voltages individually, rapidly, easily?  is there some
>> clever textbook solution to such a problem that i should know about?
>>
>> and will i ever figure out anything myself, or will i just keep
>> bothering all of you busy people?
>>
>> seasons blinkings,
>> -m-
>> ---
>> HELP!  A Bear is Eating Me!
>> http://helpabeariseatingme.com
>>
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>
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