Draw Circuit for keyboard interfacing with 8086?

For interfacing to the microprocessor system, usually push buttons keys are used. Whenever a key is pressed, there are small mechanical vibrations that cause noise on the input, which can cause the microprocessor to detect several keypresses instead of just one. Bouncing happens because of the tendency of any two metal contacts in an electronic device to generate multiple signals as the contacts close or open. You can solve this problem using software or hardware debouncing.

The hardware approach is shown in figure 8.1. It uses a cross-coupled latch formed with two Nand gates

Simple Keyboard Interface:

The figure 8.3 shows the simple keyboard interface

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When port pin is logic 1,key is open,otherwise key is closed. The software routine to get key code with key debounce is:

In the technique shown in figure 8.3, each key requires separate interface.This means, to interface one key,one input line is required.This is the disdavantage as it requires many line as many keys. This number of lines required can be reduced if keys are put in matrix form.

The figure 8.4 shows the interface of matrix key board

The 16 keys are arranged in four rows and four columns. The connection will be made such that when a key is pressed, it shotrs the corresponding

one row and one column. Two ports are required, input port for connecting rows and and output port for connecting columns. The lines connected to rows are called returned lines and the lines connected to columns are called scan lines

When all a key is pressed it shorts the corresponding row and column. If the output line of this colummn is low, then it makes the corresponding row line low,other wise the status of row line will be high. The pressed key will be identified by the data sent on the oputput port and the input code received from the input port. Figure 8.6 shows the flow chart for interfacing of 8086

As shown in the figure 8.7, the interfacing is done using 8255 PPI. The 8086 is being used in maximum mode and port A of 8255 is used for columns and port B for rows. By making use of the lookup table stored in the memory, the 8086 microprocessor will determine the code of the depressed key, then it will initiate the action. The figure 8.8 shows the flow chart for keyboard interface............