Digital Electronics: Principles, Devices and Applications

Microcontrollers 595+V0BounceFigure 14.27Contact bounce.+VVout(a)+VSchmittTriggerVout(b)Figure 14.28circuit.(a) Hardware debouncing using an RC circuit and (b) hardware debouncing using a Schmitt trigger2. Multiple keys. If more than one key is pressed, then only routines corresponding to valid multiple-keypresses should be executed. Also, the first valid key press pattern is executed.3. Key hold. There are two types of keyboard actuation, namely the two-key lock-out and the N -keyrollover. The two-key lock-out takes into account only one key pressed. An additional key pressedand released does not generate any codes. The system is simple to implement and most often used.The N -key rollover will ignore all keys pressed until only one remains down.Figure 14.29 shows the connection of a 16-key matrix keypad with a microcontroller. Here, eachcolumn and row access line is connected to the microcontroller pin. The columns are generally ata HIGH level. The row lines are configured as output lines and the column lines are used as scanlines. The key actuation is sensed by sending a LOW to each row one at a time through a softwareroutine via the row 1, row 2, row 3 and row 4 lines. The column lines are checked for each row to see