Elm Electronics – Circuits for the Hobbyist

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ELM401DSB

ELM401

Debouncing Circuits

Any time that two metal surfaces meet, as they do

inside a rotary encoder, there will be a tendency for

the moving one to bounce, which causes the electrical

connection to make and break. The duration of this

bouncing action may be very short, but it is usually fast

enough to cause multiple counts to be recorded by

connected electronic circuits. As the number of

bounces can not be predicted, a means of removing

them is necessary. Circuits that remove the bounce

are usually called ‘debouncing’ circuits.

Many debouncing circuits employ a simple timer to

determine if an input is stable. This generally works

well if the two contacts meet and then remain still. With

a rotary encoder however, one of the contacts meets

the other then usually continues sliding over the

surface of the stationary contact. This will produce

noise while the contact is sliding, occasionally enough

to make it look like there are more inputs.

The ELM401 employs a two stage system to

remove the bounce and the sliding noise from the

encoder signal. A block diagram of the stages are

shown in figure 3. The first stage is a digital filter circuit

that is used to determine the average value of the

waveform over a time. If a long enough time is chosen,

short duration pulses will have little effect on the

overall average. If the time chosen is too long,

however, the circuit will be slow to respond, and may

in fact average out some legitimate inputs. Choosing

the time period (or time constant of the circuit) is thus

very important in determining how effective the filtering

function will be. We have found that with typical rotary

encoder specifications (usually 3.0 msec of bounce or

noise, maximum), the ELM401 debounce circuit works

quite well.

After the signal has been filtered, it is compared to

some reference levels, and the output of these

comparators are used to control a simple timer. The

timer is used to ensure that the output of the filter is

stable, and not just a momentary transient, while the

use of two comparator levels provides hysteresis, so

that some variation in the filter output can be tolerated.

The Sw input uses a very simple debounce circuit

that employs a timer only (and not the filter section

shown in Figure 3). This is very similar to the logic

used by our ELM409, ELM410, and ELM411 circuits.

Once the signals from the rotary encoder have

been debounced, they may be used by your circuit to

decode direction, etc.- the following sections provide

some tips on this.

Figure 3. Internal Debouncing Logic

tc = 1.7 msec

3 msec

timer

timer

filter

rotary

encoder

input

to

output

logic