The idea behind the bidirectional output can be carried further as a way of multiplexing LEDs, as shown in Figure 7.10. In general, N(N −1) LEDs can be driven from N pins by extending this circuit. It relies on the one-way characteristic of a diode and its nonlinear relation between current and voltage. A single LED is selected by driving one pin high,
one low, and configuring all other pins as inputs. For example, suppose that P1.0 is high, P1.2 is low, and P1.1 is an input and therefore effectively disconnected. Current flows through D5 and two of the series resistors, which limit the current in the usual way.
A parallel path lies through D3 and D1 but this has two LEDs in series. Each receives only half the voltage, so very little current flows. The remaining LEDs are reverse biased.
We could light D6 instead by driving P1.0 low and P1.2 high, and similarly for other LEDs
Only one LED can be addressed at a time, which may seem a serious disadvantage. This is resolved by repeatedly addressing each LED in turn. The eye does not detect that the LEDs are flashing rather than continuously illuminated, provided the frequency is above 100 Hz or so. In fact LEDs are often more efficient when operated in this way, in the sense that they need a smaller average current to produce the same apparent brightness. As usual this feature comes at a price: The current must be higher during each pulse, and this may exceed the limit of the MSP430’s pins.
A more conventional form of multiplexing is often used with seven-segment LED displays. These tend to consume a large current, which clashes with the low-power ethos of the MSP430, but are simpler than liquid crystal displays. The layout and circuit of a single digit are shown in Figure 7.11(a). Note that there are usually eight segments, despite the name; the eighth segment is a decimal point. In the circuit shown, the cathodes of all the LEDs are connected to give a common cathode display. (The cathode is the negative terminal when the diode is forward biased, shown by the bar on the symbol.) Common anode displays are equally common. The usual resistors should be connected in series with each segment to limit the current. A higher value may be needed for the decimal point because it draws a lower current. Sometimes the segments comprise two or more LEDs in series, but this requires a higher voltage than an MSP430 can provide.
Suppose that two digits are needed. This would require 16 pins of the microcontroller if the displays were connected separately, which is excessive. They are usually multiplexed instead, as shown in Figure 7.11(b). These are again common-cathode devices. The corresponding segment pins of the displays are connected in parallel and the common cathodes are used to select a particular digit. In principle each cathode could be connected to a pin of the microcontroller but the current would exceed the rating of the MSP430 so I use an n-MOSFET as a switch instead; this is described fully in the section “Driving Heavier Loads” on page 247.
The two digits are driven alternately. To select digit 1, the gate of its FET is driven to VCC, which turns it on, while the gate of the FET for digit 2 is driven to VSS to turn it off. Individual segments of digit 1 can then be lit by bringing the corresponding pins high or turned off by pulling them low. Both displays feel the voltages on the segment lines but only digit 1 is able to respond to them. The voltages on the FETs are then reversed to make digit 2 active and the segment lines are changed to give the desired pattern. The details of the software, such as using lookup tables to get the correct patterns on the segment linesA large, seven-segment display needs more current than the MSP430 can provide itself.
This is not a problem because plenty of special ICs are available to drive LEDs. The LEDs are usually driven from constant-current sources to give better control of illumination than a simple resistor. Many have serial interfaces such as SPI or I²C (see Chapter 10), which save pins on the microcontroller.