CPLD CoolRunner-II - IO current limited to 8mA?

Hello,

I want to connect a LED with Uf=1.8V and If=20mA to a CPLD (XC2C512).
But from the datasheet I get that the current is limited to I_OH and
I_OL = 8mA. Is this right? Means this that I couldn't use this LED with
1.8V? Have I to operate the LED with 8mA and the corresponding voltage
(using a resistor in series)?

Regards
Deniz

just use leds with 2mA (for a small price premium)
or use a resistor to limit the curent to <=8mA if your leds have enough 
intensity.
Aurash
denizdikmen@gmail.com wrote:

>Hello,
>
>I want to connect a LED with Uf=1.8V and If=20mA to a CPLD (XC2C512).
>But from the datasheet I get that the current is limited to I_OH and
>I_OL = 8mA. Is this right? Means this that I couldn't use this LED with
>1.8V? Have I to operate the LED with 8mA and the corresponding voltage
>(using a resistor in series)?
>
>Regards
>Deniz
>
>  
>


-- 
 __
/ /\/\ Aurelian Lazarut
\ \  / System Verification Engineer
/ /  \ Xilinx Ireland
\_\/\/
 
phone:	353 01 4032639
fax:	353 01 4640324
    
     

<denizdikmen@gmail.com> schrieb im Newsbeitrag
news:1125575027.735673.124800@g44g2000cwa.googlegroups.com...
> Hello,
>
> I want to connect a LED with Uf=1.8V and If=20mA to a CPLD (XC2C512).
> But from the datasheet I get that the current is limited to I_OH and
> I_OL = 8mA. Is this right? Means this that I couldn't use this LED with
> 1.8V? Have I to operate the LED with 8mA and the corresponding voltage
> (using a resistor in series)?

You can easyly drive a 20mA LED with the Coolrunner-II. The current given in
the datasheet is the guarenteed maximum under certain measurement conditions
(Rise time, output LOW/HIGH level). For a LED this doesnt matter at all.
There is a typical I/O curve in the datasheet, from there you can see that
the output resistance is ~25 ohm. So if you draw 20mA, the output voltage
will be 0.5V away from GND or VCC (depending if you sink or source the
current). Just go ahead, calculate the appropiate current limiting resistor
value and drive the LED with 20mA. Only keep in mind that you should not
drive a logic inputs with this LED in parallel, since the the output
voltages ar not more full specification compliant (but it will work thou)

Regards
Falk

Using a LED with a forward voltage similar to that available is likely to 
give problems. The series resistor normally fitted gives a determination of 
currrent but does need some voltage itself. The smaller the notional voltage 
used across the resistor the wider the span of currents that may result due 
to tolerances. In short using small resistor voltage gives relatively 
un-predicatable currents. No resistor at all is asking for a LED meltdown 
assuming it actually lights at all.

John Adair
Enterpoint Ltd. - Home of Broaddown2. The Ultimate Spartan3 Development 
Board.
http://www.enterpoint.co.uk
<denizdikmen@gmail.com> wrote in message 
news:1125575027.735673.124800@g44g2000cwa.googlegroups.com...
> Hello,
>
> I want to connect a LED with Uf=1.8V and If=20mA to a CPLD (XC2C512).
> But from the datasheet I get that the current is limited to I_OH and
> I_OL = 8mA. Is this right? Means this that I couldn't use this LED with
> 1.8V? Have I to operate the LED with 8mA and the corresponding voltage
> (using a resistor in series)?
>
> Regards
> Deniz
> 

denizdikmen@gmail.com wrote:

> Hello,
> 
> I want to connect a LED with Uf=1.8V and If=20mA to a CPLD (XC2C512).
> But from the datasheet I get that the current is limited to I_OH and
> I_OL = 8mA. Is this right? Means this that I couldn't use this LED with
> 1.8V? Have I to operate the LED with 8mA and the corresponding voltage
> (using a resistor in series)?

Look at the MAX DC voltage specifications, and the Typical I/O curves.
Vol is  < 0.4V @ 8mA, or 50 ohms (Max) RdsON.

Typical is just over 2x beter than the worst case, and Typical current 
is ~30mA, so a worst-case port might deliver only 15mA (and still be
in-spec) - thus 20mA is pushing it, from a single pin, for volume 
production.

You can draw a load-line on the I/O curves, to see the LED current.
eg assume a 3.3Vcc, and 1.8V Vf on LED, your load line will start at
X=1.5V, and will hit the Y axis at 1.5V/Rseries

A Y intercept of 30mA is appx 50 Ohms, and the Vol curve intercepts
the 1.5V<=>30mA load line, at appx 22mA
A 100 Ohms load line, gives ~12.5mA etc.

-jg