Maximum current that can be drawn from FPGA pin

[xenix]

I am confused about the maximum pin currents (IoH & IoL) of Xilinx xc3s1500.

Below, there is the datasheet, at page 62 & 63 these current values are specified. There are current values from 2mA to 24mA for LV type input-outputs, which values i can use?

https://www.xilinx.com/support/documentation/data_sheets/ds099.pdf

 

[radix]

What don't you understand?

The IoL parameter specifies the maximum amount of current that your load can draw before the VoL parameter of the FPGA driver is no longer guaranteed.

What this means is that if you're using a 4ma driver and your load draws more current than that when the driver is low, the driver is no longer going to drop below 0.4 volts.

The same goes for the IoH. If your load draws more than the IoH value then the driver is tying to drive a logic one then the output of the driver is not going to reach the Vcc - 0.4 level specified in VoH.

Hope this helps.

Radix

 

[xenix]

Thanks,

I know the definition of Iol & Ioh. What i dont understand is "what is the max current for a pin". They dont specify the max current at the datasheet, i wonder if i can drive a 5mA led direcly.

Do i have the option to chose 4ma driver or 16mA driver for a pin?

 

[amitjagtap]

hi
As I know, you dont have any progrmmable option as such.
In this case maximum current is 24mA. U can easily drive an LED with FPGA output pin with some resistor in series..

 

[radix]

xenix,

I don't mean to be rude, but I'm not sure that you understand the definition of IoH and IoL. They are the "effective max" currents. If you draw more than that amount you won't get the digital logic voltages (VoH and VoL) you expect.

Normally you put a resistor in series with the LED to limit the current flowing through the driver and the LED. LEDs are semiconductor devices and you usually want to limit the voltage across the cathode and anode in order to minimize the current through the device.

If you need 5ma, select a 6ma or 8ma driver and size your resistor so that

R = (VoH - Vled) / 5ma

If you selected a 4ma driver your VoH would be lower than the specified Vcc - 0.4V.

Cheers,
Radix

 

[SweetMusic]

Hi
Those 5ma 6ma... drivers, I see they are giving u the current levels on fpga pins, but what are exactly and how are they selected(software or...?)?
Thank you and sorry for my newbie question, please continue

 

[radix]

Sweetmusic,

Most FPGA vendor tools let you select the driver type (logic levels and drive strength) through a gui when you do your pin assignments.

Pin assignments are when you select the actual pins on your fpga device that are going to carry the various signals of your design.

Radix