ELM409
ELM409 Operation
The circuit of Figure 4 shows how the ELM409
would typically be connected to debounce the signal of
Figure 1. Other than powering the ELM409, all that is
needed is to connect the options pins (A, B and C) to
VSS (providing low levels on all three), connect the
noisy switch signal to the input pin, and then connect
the ELM409 output pin to the digital circuit.
4
5
3
2
1
8
409
6
7
The output of the ELM409 in Figure 4 would
appear as shown in the ‘scope traces of Figures 2 and
3 (the lower traces). There is a slight delay (25 msec)
due to the debounce period, but otherwise, the signal
appears as you would expect from an ‘ideal’ switch.
Digital
Circuit
The ELM409 is capable of performing several
other functions on the input signal - it can do more that
just debounce the signal. For example, it can provide
a change in output on the signal edges (transitions
from a low to a high level, or from a high to a low
level), and it can alternate (toggle) the output on each
occurrence of an edge. Each of these possibilities are
selected by changing the logic levels at the A, B and C
pins. The eight combinations are described in Table 1
below, and also appear pictorially on the next page.
Figure 4. Debouncing the pushbutton
Function
A
0
0
0
0
1
1
1
1
B
0
0
1
1
0
0
1
1
C
0
1
0
1
0
1
0
1
Debounce Period
25 msec
Notes
Standard contact debounce
For very noisy circuits
Follows
Input
500 msec
25 msec
Output pulse is 50 msec wide
Output pulse is 50 msec wide
Output pulse is 50 msec wide
Output pulse is 50 msec wide
Changes on input H -> L
Changes on input L -> H
Pulse on
Rising Edge
500 msec
25 msec
Pulse on
Falling Edge
500 msec
25 msec
Toggle
on Edge
25 msec
Table 1. ELM409 options
ELM409DSA
Elm Electronics – Circuits for the Hobbyist
5 of 8
< http://www.elmelectronics.com/ >
ELM [ ELM ELECTRONICS ]
