Sinking current through the xc2c64a

[tonyennis]

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?

[matseng]

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.

[tonyennis]

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'.

[matseng]

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)...

[megaionstorm]

Look at this document "Driving LEDs with Xilinx CPLDs":
Xillinx xapp805.pdf