LED displays
Basically, an LED display is nothing more than several LEDs moulded in the same plastic case. There are many types of displays composed of several dozens of built in diodes which can display different symbols.
Most commonly used is a so called 7-segment display. It is composed of 8 LEDs, 7 segments are arranged as a rectangle for symbol displaying and there is an additional segment for decimal point displaying. In order to simplify connecting, anodes and catodes of all diodes are connected to the common pin so that there are common anode displays and common catode displays, respectively. Segments are marked with the latters from A to G, plus dp, as shown in the figure on the left. On connecting, each diode is treated separtely, which means that each must have its own current limiting resistor.
Displays connected to the microcontroller usually occupy a large number of valuable I/O pins, which can be a big problem especially if it is needed to display multy digit numbers. The problem is more than obvious if, for example, it is needed to display two 6-digit numbers (a simple calculation shows that 96 output pins are needed in this case). The solution to this problem is called MULTIPLEXING. This is how an optical illusion based on the same operating principle as a film camera is made. Only one digit is active at a time, but they change their state so quickly making impression that all digits of a number are simultaneously active.
Here is an explanation on the figure above. First a byte representing units is applied on a microcontroller port and a transistor T1 is activated at the same time. After a while, the transistor T1 is turned off, a byte representing tens is applied on a port and a transistor T2 is activated. This process is being cyclically repeated at high speed for all digits and corresponding transistors.
The fact that the microcontroller is just a kind of miniature computer designed to understand only the language of zeros and ones is fully expressed when displaying any digit. Namely, the microcontroller doesn't know what units, tens or hundreds are, nor what ten digits we are used to look like. Therefore, each number to be displayed must be prepared in the following way:
First of all, a multy digit number must be split into units, tens etc. in a particular subroutine. Then each of these digits must be stored in special bytes. Digits get familiar format by performing “masking”. In other words, a binary format of each digit is replaced by a different combination of bits in a simple subroutine. For example, the digit 8 (0000 1000) is replaced by the binary number 0111 111 in order to activate all LEDs displaying digit 8. The only diode remaining inactive in this case is reserved for the decimal point. If
| DIGITS TO DISPLAY | DISPLAY SEGMENTS | |||||||
| | dp | a | b | c | d | e | f | g |
| 0 | 1 | 0 | 0 | 0 | 0 | 0 | 0 | 1 |
| 1 | 1 | 0 | 0 | 1 | 1 | 1 | 1 | 1 |
| 2 | 1 | 0 | 0 | 1 | 0 | 0 | 1 | 0 |
| 3 | 1 | 0 | 0 | 0 | 0 | 1 | 1 | 0 |
| 4 | 1 | 1 | 0 | 0 | 1 | 1 | 0 | 0 |
| 5 | 1 | 0 | 1 | 0 | 0 | 1 | 0 | 0 |
| 6 | 1 | 0 | 1 | 0 | 0 | 0 | 0 | 0 |
| 7 | 1 | 0 | 0 | 0 | 1 | 1 | 1 | 1 |
| 8 | 1 | 0 | 0 | 0 | 0 | 0 | 0 | 0 |
| 9 | 1 | 0 | 0 | 0 | 0 | 1 | 0 | 0 |
a microcontroller port is connected to the display in such a way that bit 0 activates segment “a”, bit 1
activates segment “b”, bit 2 segment “c” etc., then the table below shows the “mask” for each digit.
In addition to digits from 0 to 9, some letters of alphabet - A, C, E, J, F, U, H, L, b, c, d, o, r, t - can also be displayed by performing appropriate masking.
If the event that common chatode displays are used all units in the table should be replaced by zeros and vice versa. Additionally, NPN transistors should be used as drivers as well.
If the event that common chatode displays are used all units in the table should be replaced by zeros and vice versa. Additionally, NPN transistors should be used as drivers as well.
Using LED display
The following examples describe the use of LED displays. Common chatode displays are used here, which means that all built-in LEDs are polarized in such a way that their anodes are connected to the microcontroller pins. Since the common way of thinking is that logic one (1) turns something on and logic zero (0) turns something of, Low Current displays (low power consumption) and their diodes (segments) are connected serially to resistors of relatively high resistance.
In order to save I/O pins, four LED displays are connected to operate in multiplex mode. It means that all segments having the same name are connected to one output port each and only one display is active at a time.
Tranzistors and segmenats on displays are quickly activated, thus making impression that all digits are active simultaneously.
