No open-drain in V5 to drive an external LED?

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HI,

on our PCB there is a LED directly connected to a Virtex5 I/O pad. The
LED is connected through a series resistor to 3.3 V. As it seems
Virtex devices do not support open-drain outputs.

If I drive a zero on that output there will be current flowing from
the 3.3V through the LED into the FPGA. Does the FPGA tolerate that?
Which I/O standard/fashion should I use?

regs, Heiner

Re: No open-drain in V5 to drive an external LED?
Use a tristate output driver.


On Wed, 16 Jul 2008 03:10:12 -0700 (PDT), Heiner Litz

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Re: No open-drain in V5 to drive an external LED?
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Take a look at the Vcco for the bank that drives the LED.  If it is
3.3V
you don't need open-drain, as a CMOS output high will turn off the LED
just as well.  If the Vcco is lower than 3.3v you should use the
equivalent of open-drain, which is to tie the output signal low and
use the tristate function to turn on or off the LED.  In Verilog this
would be like:

assign LED_PIN = drive_led ? 0 : 1'bZ;

If Vcco is much lower than 3.3V, i.e. lower than 3.3v - Vf of the LED,
you may have problems with the LED staying on even if you use open
drain outputs due to the protection diodes on the FPGA I/O pin.

HTH,
Gabor

Re: No open-drain in V5 to drive an external LED?
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That looks like a good way to me.

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A blue or white LED should have a large enough Vf.  Otherwise,
a series diode or so should do it.

-- glen


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