Sinking current through the xc2c64a

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Sinking current through the xc2c64a

Postby tonyennis » Sat Nov 17, 2012 9:22 am

This is something I just noticed, from the description of DP's xc2c64a CoolRunner II board:

LED D1 and D2 source current from the CPLD. Generally, designers sink current from CPLDs and FPGAs - the chip connects the LED to ground instead of power - because many parts can sink 20mA but only source a few. On this chip, however, the maximum current is a paltry 4mA for both source and sink. The LEDs have 2K resistors (R2,R3) that keep the current at an acceptable 2mA.


I intend to sink LEDs using the CPLD. Does this mean any pin can sink 4mA, or the total of all sinkage can be 4mA, or do I misunderstand completely?
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Re: Sinking current through the xc2c64a

Postby matseng » Sat Nov 17, 2012 9:57 am

From what I can see in the datasheet http://www.xilinx.com/support/documenta ... /ds311.pdf at page 4 the XC2C64A can source or sink up to 8 mA if you can accept a voltage droop of up to 0.4 volts in reference to gnd or vccio.

Normally this is per io-pin and there's a separate mention of max current out/in on the gnd- or vcc-pins, but in this datasheet there's no data of max total current. But between 100mA per supply pin (there are several of them, 3 gnd and 2 vccio) is not unreasonable.
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Re: Sinking current through the xc2c64a

Postby tonyennis » Sat Nov 17, 2012 10:00 pm

Thanks, I searched for the word "sink" but it did not appear. I never would have realized that a -8ma 'high level output voltage' meant 'sink'.
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Re: Sinking current through the xc2c64a

Postby matseng » Sun Nov 18, 2012 8:53 am

Yea, the negative milliamps are a strange notation for sink capabilities, but when thinking of it a bit it actually is rather logical (pun intended)...
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Re: Sinking current through the xc2c64a

Postby megaionstorm » Fri Jun 07, 2013 3:55 pm

Look at this document "Driving LEDs with Xilinx CPLDs":
http://www.xilinx.com/support/documenta ... app805.pdf
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Re: Sinking current through the xc2c64a

Postby PurpleSquirrel » Thu Jul 17, 2014 12:51 pm

Im familiar with that appnote and if you want to invest in a LED driver that you can control via the CPLD that is a good solution however I much rather prefer to use a logic FET or a NPN transistor as my LED controller. Does it require a few extra parts? Yes. Does it keep you from killing your CPLD from inadvertently sinking too much current? Yes it does.
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