Hi
I want to use spartan 3AN devices to build a led sign. I have a question. can i use the fpga ports to derive the leds directly.i want to turn 128 leds on with 128 outputs of the device simultaneously each capable of driving 24ma to each led. is that possible? is there any limitations
thanks for you replies.
Led sign design
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Led sign design
by ali_dehbidi » Fri Dec 11, 2009 10:29 am
- ali_dehbidi
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by NickH » Mon Dec 21, 2009 11:53 am
I don't know if it's possible (better read the data sheet carefully), but I wouldn't recommend it: that's a lot of current through the FPGA!
It's more likely to work if you reduce the current per LED. You might find that 6mA is bright enough.
Also:
* I think there's a limit on total current per VCCO bank.
* There's a limit on the number of simultaneously changing outputs per VCCO bank.
* Remember to set the right I/O constraints, e.g. "SLOW | DRIVE=12".
* Don't rely on the FPGA to limit the current. Use resistors.
* Make sure the power supplies are well decoupled.
Alternatively, consider a matrix arrangement, e.g. 16x8 (depending on the shape of your display). Only drive one row at once. You'll need 8 transistors, but fewer resistors than before. Again, take care with power supplies and watch out for FPGA's limits for simultaneous current switching. [Advantages: fewer pins, fewer components, simpler wiring. Disadvantages: the sign won't be as bright, and may flicker.]
If you're making a really big sign, use power PNP transistors for the rows; and constant-current LED driver chips with serial input (shift registers) for the columns (and maybe also use them to drive the row transistors' base). You can then have separate power supplies for the LEDs and the logic.
Regards,
Nick
It's more likely to work if you reduce the current per LED. You might find that 6mA is bright enough.
Also:
* I think there's a limit on total current per VCCO bank.
* There's a limit on the number of simultaneously changing outputs per VCCO bank.
* Remember to set the right I/O constraints, e.g. "SLOW | DRIVE=12".
* Don't rely on the FPGA to limit the current. Use resistors.
* Make sure the power supplies are well decoupled.
Alternatively, consider a matrix arrangement, e.g. 16x8 (depending on the shape of your display). Only drive one row at once. You'll need 8 transistors, but fewer resistors than before. Again, take care with power supplies and watch out for FPGA's limits for simultaneous current switching. [Advantages: fewer pins, fewer components, simpler wiring. Disadvantages: the sign won't be as bright, and may flicker.]
If you're making a really big sign, use power PNP transistors for the rows; and constant-current LED driver chips with serial input (shift registers) for the columns (and maybe also use them to drive the row transistors' base). You can then have separate power supplies for the LEDs and the logic.
Regards,
Nick
- NickH
- Posts: 88
- Joined: Tue Sep 02, 2008 1:53 pm
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