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ELM408 Datasheet(PDF) 7 Page - ELM Electronics |
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ELM408 Datasheet(HTML) 7 Page - ELM Electronics |
7 / 10 page 7 of 10 ELM408DSA Elm Electronics – Circuits for the Hobbyist www.elmelectronics.com ELM408 Output Waveforms Once the ELM408 has some bounce-free signals to work with, it can generate outputs based on them. The output sequences that the ELM408 generates depends on the direction of shaft rotation, and on the level at the Write Enable input (pin 4). The direction logic always assumes that the encoder is a standard one, where the ‘A’ signal leads the ‘B’ for a clockwise or ‘up’ rotation. Note that the level at pin 5 only determines when the sequences are output, and does not affect the waveshape or timing of them. Figures 4 to 7 below show the sequences that the ELM408 can generate. In all cases, the basic time interval is 200 µsec (if it does not seem apparent). Figures 4 and 5 show the two types of sequences that would occur if the Write Enable is low (ie disabled). The level at the U/D output when the CS signal goes low determines whether the the controlled device will count up (level is high), or down (level is low) when the U/D pin next goes from low to high. For many digital potentiometers, the level at the U/D pin when the CS output returns high determines whether the current setting of the digital potentiometer is to be stored in non-volatile memory or not. If the ELM408’s Write Enable input is high, then the U/D output will be held high during the transition, causing a write for many digital potentiometers. The resulting waveforms are shown in Figures 6 and 7. The logic to decode the motion of an encoder shaft, and so decide when to provide output sequences is not as simple as it would first appear. Some authorities recommend simply monitoring an input and when it changes, provide an output based on the level of the other input. This does not always work, as the encoder can output multiple signals from only the ‘A’ or or only the ‘B’ contact if the shaft is moved ever so slightly when at the detent or at the mid-point position (between detents). Simply seeing one input change is not sufficient to say that there is any significant shaft rotation. The ELM408 monitors both ‘A’ and ‘B’ transitions, and determines the outputs based on the sequence in which the transitions have occurred. This is a better way to guarantee that the output signals are generated properly. The internal logic also performs some self- checking, and monitors for problems such as an output pulse being initiated before the previous one had completed, which might occur for some very fast inputs (the second one will be ignored in this case). The output of the ELM408 is a series of pulses, as shown in Figures 8 and 9. The first figure shows 2x Figure 4. Up (Clockwise) with Write Enable = Low CS U/D 200 µsec Figure 5. Down (Counterclockwise) with Write Enable = Low CS U/D 200 µsec Figure 6. Up (Clockwise) with Write Enable = High CS U/D 200 µsec CS U/D 400 µsec Figure 7. Down (Counterclockwise) with Write Enable = High |
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