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maikon

spartan-6 LVCMOS33 pin but output = 1.8V !

Question

Hi,

I am a new user of FPGAs and  I have the CmodS6 kit and I am having in the IO 1.8 V 

and I need to change it to 3.3V , my ucf shows:

NET "PORTA<0>" LOC = "P5" | IOSTANDARD = LVCMOS33 | PULLUP;

Vcc is 5V from my usb port and I see at ucf CONFIG VCCAUX = "3.3";

I also had a look at ug381 cdg and ug382.

Thanks for your help. 

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Hi maikon,

I'm not sure what the problem is; I know when I look at the master UCF file that Digilent provides from the CmodS6 Resource Center (link here) I don't see the CONFIG VCCAUX setting so maybe that has something to do with it, but in the meantime I have asked some of our applications engineers about this; they'll get back to you here on the forum

Thanks,
JColvin

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tks for ur support / answer jcolvin,

I inserted CONFIG VCCAUX in an attempt to see if I could get 3.3 V in the output.

My project is the one from the website and I didn't change it ... 

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it might be my setup !

I am using a bread board with leds connected directly to the IOs which might be causing an increase 

of current which might be dropping the voltage , when I unplugged the LED , the IO register 3.3 V in the output. 

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I am measuring the PORTS IO , they are configured as inout and they have IO Buffer connected to them,

the exactly same project from the website. When there is no load , the voltage is 3.3 V ! When I connect a LED or a LED plus resistors these INOUT ports have its voltage dropped to 1.8 V. Then I tried to set the pin as an output only and again , open is 3.3 V but when I connect a LED is 2.6 V . 

ARRRRGGHHH

I might be doing some wrong.  

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Hello Maikon,

19 minutes ago, maikon said:

I am measuring the PORTS IO , they are configured as inout and they have IO Buffer connected to them,

the exactly same project from the website. When there is no load , the voltage is 3.3 V ! When I connect a LED or a LED plus resistors these INOUT ports have its voltage dropped to 1.8 V. Then I tried to set the pin as an output only and again , open is 3.3 V but when I connect a LED is 2.6 V . 

ARRRRGGHHH

I might be doing some wrong.  

How big of an LED are you using?  What is the voltage drop and current rating?

It sounds like you are just pulling too much current through the pin and this is causing the line voltage to drop.  

I searched online really quick and found this calculator that might help you.

http://ledcalculator.net/

I would suggest using a power FET or BJT to protect the FPGA if you are using something other than a small LED. 

Best of Luck,

Marshall

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