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Simple 7-segment (LCD) displays are in many respects a simple form of decoder, where an input code of 4 bits is used to specify one of 16 different hexadecimal outputs to be displayed on an LCD module. The LCD module itself usually consists of 7 individual LEDs or lights (hence the alternative name “7-segment display”), which are driven to deliver a particular character as shown in Figure 27.1. Obviously the display can show any combination of the 7 individual bits, with 2 7 combinations; however, in practice they are mostly used to display alphanumeric characters 0-9 and A-F.
In addition to the basic character display, there is also usually a decimal point light, and this has its own control. Therefore, the LCD can be controlled using 8 individual bits; however, to make the designer’s job easier, a decoder which uses the correct hexadecimal character as the input (e.g., 0000 for the character 0) is implemented. This also has the benefit that, for counters, the input character can be determined using arithmetic and then the decoder will handle the translation to the individual segments to be shown. This is a typical design reuse task, where once this has been done once, then the designer can reuse the code on any application that uses this type of display.
Finally, depending on the type of module or drive circuit, the logic for the LCD segment being on may be high or low. This will vary from device to device, and in the case of this chapter, the particular development kit being used defined the logic levels as being Low for ON and High for OFF. A typical LCD display has anode and cathode connections to each light emitting diode (LED) in the display and depending on the type of device will have either common cathodes or anodes. This information will be found on the device or development kit data sheet. Using this approach, the coding for a hexadecimal character input to the LCD output is defined in the following table:
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