Cyclone V, current of pins to drive LED

1

Hello,

I have the DEnano SOC fpga which is a Cyclone V. On the development board, there are 2 GPIO headers. I would like to drive LEDs with these headers. I noticed in the pin assignments tool (inside of quartus) that I can adjust the output current to 16ma Default and ‘maximum current’ for the 3.3-V-LVTLL.

I have 2 questions.

  1. what is the actual value of ‘maximum current’ ? I see that the default is 16 mA, but what is actually the maximum that the pin can safely put out? I would like to drive a an LED with one of these bins (in series with a resistor of course).

  2. Suppose I have 2 of the 3.3-V-LVTLL pins of my fpga connected to either side of the LED (in series with a resistor). If I keep the one pin as an input and I drive the other pin high, will this turn the LED on? I think that it would because this would drive the one side HI (3.3v) and the other side would be sinking the input current which would mean that about 3v would be dropped across my LED. Here is a picture of what I mean:

3.3-V-LVTLL output pin @ max current ===> LED ===> Resistor ===> 3.3-V-LVTLL input pin

My concern is that the short circuit protection and ESD circuitry input pin may cause the impedance to be very high which will restrict the overall current of my LED such that it will never ‘turn on’

3.3-V-LVTLL output pin @ max current ===> LED ===> Resistor ===> Hi-input impedance 3.3-V-LVTLL input pin

Thank you you for taking time to read this

2

These 3.3-V-LVTLL pins claim to support JESD8-B which implies that the maximum would be 20 mA of current. 16 to 20 mA is enough to drive my led.

Ref:

3

I found a way to test this out. I can blink an LED using the below setup:

3.3-V-LVTLL output pin @ max current (toggles HI and LOW) ===> LED ===> Resistor ===> 3.3-V-LVTLL output (driving LOW).

Seems to be working out well. :slight_smile: