74C922 PDF

MM74C Key Encoder? The keyboard scan can be implemented by either an external clock or external capacitor. These encoders also have on-chip pull-up devices which permit switches with up to 50 k? No diodes in the switch array are needed to eliminate ghost switches. The internal debounce circuit needs only a single external capacitor and can be defeated by omitting the capacitor.

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Will you be reading more than one key at a time? Will these combination key presses have meaning to the main program? Will the program code be doing anything else while the key is being pressed? Are you going to use the key release in the main code? How fast are you going to use the keypresses? How are multiple presses of the same key handled.

How is holding one key down and pressing another one be handled? A key press is one of the slowest activities that the AVR encounters. Assuming that it takes about 80 or so instructions to do a basic decode of a keypress, and that the AVR is processing instructions at 8 million per second, then even a simple AVR can process keypresses per second. Is the program going to be in a do-nothing loop for during the keypresses?

With a PC or verbal instruction set of a process, a lot of details can be omitted. But with a microcontroller program, every You can never just simply read a keypress, because on the AVR lever, there is nothing simple about reading a key press. Oh, yes, debouncing. There is always a burst of pulses when the metal contacts are pressed or released.

If your program is operating in human sensory perception time, then you can just do nothing during this wait-and-recheck period. And they are cheap. A little too big for many uses, but still worth considering. Even if the user will only be pressing the keypad number keys, it still might be easier to use a full sized keyboard. Consider expanding your program to use all the other keys.

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Embedded Lab

MM74C Key Encoder? The keyboard scan can be implemented by either an external clock or external capacitor. These encoders also have on-chip pull-up devices which permit switches with up to 50 k? No diodes in the switch array are needed to eliminate ghost switches. The internal debounce circuit needs only a single external capacitor and can be defeated by omitting the capacitor. A Data Available output goes to a high level when a valid keyboard entry has been made. The Data Available output returns to a low level when the entered key is released, even if another key is depressed.

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74C922 - 74C922 16-Key Encoder Datasheet

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