Dev board 16-segment displays w/ MAX6955 controller?
Nak
Posts: 36
Now that I am on track with the right pinouts the next hurdle is matching up the MAX6955 with the PPDB 16-segment display circuitry. I've already figured out that I will only be able to drive two of the six characters b/c the PPDB wiring is not compatible with the MAX6955's clever multiplexing scheme. 
The next mismatch is the PPDB has limiting resistors already wired inline to each segment, whereas the driver chip expects you to use a single resistor b/w the ISET pin and ground to limit all of the output lines O0-O18. But I assume I can use a low-value resistor there on account of the limiting resistors already hardwired in place?
BTW I have a lead on some blue, 16-segment displays from a manufacturer in China who says they can ship me a special-order lot as small as 100pcs, but I only need like 8 or maybe 16! Have folks typically had success with selling off extra components like this on eBay or elsewhere?
Oops originally cited the wrong part number (MAX6965).
Update: The displays work fine with the extra resistors in line... They're just dimmer than they could be. I've attached a schematic showing how I've wired everything up.
Post Edited (Nak) : 12/21/2009 12:12:45 PM GMT
The next mismatch is the PPDB has limiting resistors already wired inline to each segment, whereas the driver chip expects you to use a single resistor b/w the ISET pin and ground to limit all of the output lines O0-O18. But I assume I can use a low-value resistor there on account of the limiting resistors already hardwired in place?
BTW I have a lead on some blue, 16-segment displays from a manufacturer in China who says they can ship me a special-order lot as small as 100pcs, but I only need like 8 or maybe 16! Have folks typically had success with selling off extra components like this on eBay or elsewhere?
Oops originally cited the wrong part number (MAX6965).
Update: The displays work fine with the extra resistors in line... They're just dimmer than they could be. I've attached a schematic showing how I've wired everything up.
Post Edited (Nak) : 12/21/2009 12:12:45 PM GMT
Comments
The mismatches sound a bit complicated - can you bypass the existing limiting resistors?
Re the max6965 - where are you getting these? I searched a number of suppliers with no luck - digikey does list them but they are a non-stock item.
How much are these chips? I'm just thinking aloud here and wondering, with the new price of the propeller, whether a dedicated propeller driver chip might end up cheaper? 16 segments and 16 common cathodes = 32 lines, so a prop could driver 16 displays directly, and with just a few other chips (possibly a daisy chain of 595s) could drive any number. Or are the dedicated drivers better in some way? This thread is max6565 which seems rare. max6955 is more common... but more $$ than a prop. Are there other alternatives?
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www.smarthome.viviti.com/propeller
Post Edited (Dr_Acula) : 12/20/2009 11:05:08 PM GMT
Plus it seems such a waste to use up all a Propellers IO pins and have maybe 1 cog refreshing the displays! Although I grant you could do some fancy animations or what have you to burn a few of those spare cycles. (Hmm, but you need one or two lines to talk to the Propeller too, right? And of course 16-segment displays really have 17 LED segments if you want the DP.)
I'll keep this thread updated with what I learn along the way, but it'll be awhile as I'm headed out of town in a couple of days...
obex.parallax.com/objects/545/
Steve
Just one thought, your demo photo says "HELLO" which of course you can write with standard 7 segment displays. Maybe you could write "W8AN" as you can't do that on a 7 seg display!
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www.smarthome.viviti.com/propeller
w8an: One cautionary note about the circuit in your demo: The propeller pin sinking the CC pin of the active display can easily be driven over its stated current capacity. FWIW I had a similar demo running a loop for several days with no apparent ill-effects on my Propeller, but a "correct" circuit I came across in Nuts and Volts uses switching transistors to sink those pins to ground so you don't damage your microcontroller.
I hadn't even thought about the word HELLO not using the additional character segments. The photo is the first frame of a HELLO WORLD demo video which also displays the entire alphabet with the famous QUICK BROWN FOX sentence.
Nak-
Very good point about the source current requirements of the display. I calculate that the 3.3 pin output voltage through the 150 ohm resistor on a segment is a whopping 22ma / segment. I don't know what the average number of segments per character is, but I'll guess the typical character uses 30% of the segments or just over 5 segments. That's 117 ma per average character--way over the 40ma sink spec.
The duty cycle with the six characters is under 17% per character. Not sure if that fact has anything to do with the source current for the time a full character is lit.
Fortunately my Propeller is in a socket and I can afford a spare if necessary so I will happily continue my programming having no worries. When the time comes to migrate to a real circuit board, I will certainly add the necessary driver transistor for each character.
Steve
·They are more expensive than a prop,then the prop chip is cheaper than a STAMP,but is more powerfull,But then a sample is FREE! go figure!
·Merry Xmas
··· Dennis
·Go here for more info!
··· http://www.maxim-ic.com/quick_view2.cfm/qv_pk/3540/t/al
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