EDE1144 Keypad Encoder IC

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OVERVIEW OF OPERATION

Table One of the previous page illustrates the data that is output upon each keypress. The

following sequence is initiated upon each keypress, and is repeated if the key is held:

1. Keypress is detected

2. One scan cycle is performed

3. Parallel Outputs (D3..D0) are latched with the appropriate value

4. Data Valid signal is activated (pulled low)

5. Beeper (1KHz square wave) is activated for 45mS

6. Serial data is transmitted

This entire cycle takes 50mS (regardless of Baud rate selected). If a key is held longer than the

autorepeat delay of 660 mS, the cycle is repeated again at a rate of 5 times per second. The

following timing diagram illustrates the above sequence:

0ms

10ms

20ms

30ms

40ms

50ms

Keypress

Parallel_Outputs

Data_Valid_Output

Serial_Output

Beeper_Output

D3..D0 latched with appropriate value

45mS

50mS

1uS

(Multiple transitions are due to contact bounce)

(Square wave is not to scale)

Once a key is pressed, any additional closures (bounce) of that contact, or of any other contacts

are ignored for 50mS, during which several signals are output by the EDE1144. First, the parallel

outputs D0..D3 (Pins 6-9) are latched with the appropriate keypress value. One microsecond

later, the Data Valid Output (Pin 17) goes low, indicating that there is valid data on the parallel

output pins. Following this, the Beeper Output (Pin 18) pulse stream is generated for 45mS,

providing auditory confirmation to the keypad operator that a key has been pressed. Finally, the

RS-232 Output (Pin 1) transmits the keypress data at the Baud rate selected by the Baud rate input

select (Pin 2). After this, a variable (determined by Baud rate chosen) delay period is waited to

make the entire process to last precisely 50mS.

If a key is held longer that the autorepeat delay of 660mS, the outputs are cleared and this entire

process is repeated every 204mS until the key is released by the operator.