The Microcontroller Idea Book - Jan Axelson's Lakeview Research

Chapter 8
Listing 8-1. Controls eight LEDs at an output port.
10 A=0E000H: REM address of LEDs
20 L=0FFH: REM initial control value for LEDs
20 XBY(A)=L: REM turn off all LEDs
30 DO
40 INPUT “Select an LED (0-7): ”,B
50 INPUT “Turn on or off (0=off, 1=on)? ”,C
60 IF C=0 THEN L=L.AND.(0FFH-2**B)
70 IF C=1 THEN L=L.OR.2**B
90 XBY(A)=L
80 WHILE 1=1
90 END
outputs of a 74LS374 addressed at E000h. The LEDs are connected as in Figure 8-1B , with
logic-low outputs turning on the LEDs. Listing 8-1 tests the circuit by allowing you to turn
individual LEDs on and off.
Bi-color LEDs
Bi-color LEDs have both a red and a green LED inside a single package. By turning on one,
both, or neither, you can use a single indicator to show as many as four states. Some bicolor
LEDs have two leads, while others have three. Figure 8-2 illustrates.
In the 3-lead, or common-cathode type, the cathodes of both LEDs connect internally (A).
To turn on an LED, you ground the cathodes through a current-limiting resistor and apply
power to the anode of the desired LED. When both LEDs are powered, you get an amber
light. Removing power from both turns the LED off, giving a total of four states that the
device can display. Instead of the one current-limiting resistor shown, you can connect a
resistor to each anode, to set the current through each LED individually.
In a 2-lead, or parallel-connected, bicolor LED, the anode of each LED connects internally
to the other’s cathode (B). To turn on the red LED, you apply +5V to terminal A and ground
terminal B. To turn on the green LED, you do the reverse: terminal A is ground, and terminal
B is +5V. With this type, you can’t turn on both LEDs at once.
With either type, by adding an inverter, you can use a single output to control both LEDs
(C, D).
128 The Microcontroller Idea Book