Confused regarding how much current PSOC 5LP GPIO pins can drive

Question

I'm confused regarding how much current 5LP GPIO pins can drive; any help is appreciated.

Here's where I get confused:

1. The CY8CKIT-059's "User LED BLUE" is an LTST-C170TBKT from Lite-On Inc.
2. This LED has a DC Forward Current of 20 mA, according to its data sheet.
3. On the CY8CKIT-059, this LED is driven by a single pin P2_1 (via a 820 ohm resister), according to the CY8CKIT-059 schematics.
4. Cypress AN72382, § 2.4 states "In general, [PSOC 5LP] GPIO pins can source 4 mA."

My confusion: If the PSOC 5LP GPIO pins can only source 4 mA (unless ganged), then how can pin P2_1 drive enough current to light "User LED BLUE" if that LED has a DC Forward Current of 20 mA?

What am I missing here?

I'm sure it is something really obvious.

Thanks in advance,

-Chris

Answer 1

The key distinction is source vs. sink, the pins can source less current than they can sink. Note: The concepts should be considered in terms of conventional current flow (positive to negative.)

Sourcing current involves connecting the I/O pin to VCCIOx (positive). Sinking current involves connecting the I/O pin to GROUND.

When driving a LED we will typically connect the LED to a positive voltage source, then to the I/O pin through a resistor. When we want the LED to turn on, the pin must sink current to ground. The pins can control more current this way.

(Image from CircuitsToday.com)