Digital I/O Wildcard User Guide

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Current Capability of the Digital Output Lines

The inputs and outputs are all provided by a single device, a Xilinx CPLD, XC9536. The digital outputs provide TTL-compatible levels for driving logic devices. But you can also control devices, such as relays and light emitting diodes, that require more current. You can either provide current from a digital output high, or you can sink current into a digital output low. Generally speaking, the output low is capable of sinking much more current than the output high can source, and using it results in lower power dissipation. So if you need substantial current, you should turn on the external device by sinking current from it using an output low.

As an output is loaded, its voltage level, VOL or VOH, rises or falls with the current. It is often useful to know just how much to expect the VOL and VOH levels to degrade with current. For currents of less than 100 mA the voltage change is approximately linear with current; that is, it can be modeled as a voltage source of either zero volts (for an output low) or 3.3 volts (for an output high) and an equivalent series resistance. At greater output currents the resistance becomes nonlinear, but you shouldn’t ever source or sink more than 100 mA.

The following figure shows the degradation of output voltage levels with increasing source or sink current for a typical device.

An output high is typically 3.8 V, but rises to 5 V when the pull-up jumper is installed. As current is drawn by an external device the voltage drops, appearing as a 3.3 volt source and a 30 Ω resistance. That is,

The output impedance for an output low is considerably less, allowing it to sink more current than an output high can source. With no current, the output low is zero volts, and it increases with current as a 12 Ω resistance (12 Ω typically, 21 Ω worst case), up to a limit of approximately 100 mA:

The above graph and equations can be used to choose component values for particular circuits. For example if we wish to use an output low to drive a light-emitting diode we would place the LED in series with a resistor and connect them between an output pin and a +5V supply. The resistor limits the current through the LED and into the output low. From the LED data sheet we note that its forward voltage at a current of 10 mA is specified to be 2.2 V. What should the resistor value be? We calculate it as,