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ELM401 Datasheet(PDF) 6 Page - ELM Electronics |
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ELM401 Datasheet(HTML) 6 Page - ELM Electronics |
6 / 8 page Elm Electronics – Circuits for the Hobbyist www.elmelectronics.com 6 of 8 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 |
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