ELM401 Rotary Encoder Debounce Circuit Description Features The ELM401 is an 8 pin integrated circuit that is used to ‘debounce’ the signals from a mechanical rotary (quadrature) encoder. The low power CMOS technology used ensures that only a very small current is required over the entire 2.0 to 5.5 volt operating range. There is no need for external filtering or debounce circuits with the ELM401, as this is all performed within the integrated circuit. The ‘A’ and ‘B’ motion sensing encoder signals are both passed through a filter then a comparator circuit, and finally a timer circuit to remove noise and contact bounce, while the switch input receives standard debounce processing. The result are three virtually noise-free signals that are suitable for direct use by an electronic circuit. Note that no inversion or manipulation of any kind is performed on the input signals, other than the debouncing. If your applications requires decoding of the signals, you may wish to look at our ELM402 to ELM408 family of products. • • • • • • • Low power CMOS design Wide supply range – 2.0 to 5.5 volts Complete debouncing of the encoder signals No external filtering needed Includes switch debouncing circuit Startup delay timer High current drive outputs Connection Diagram PDIP and SOIC (top view) VDD 1 8 VSS A 2 7 A out B 3 6 B out Sw 4 5 Sw out Applications • Microcontroller Interfaces • Monitoring of Encoder Signals • General Switch Debouncing Block Diagram VDD A 2 VDD 7 A out Debounce Circuit 6 B out Debounce Circuit 5 Sw out B 3 VDD Debounce Circuit Sw 4 Rotary Encoder ELM401DSB Elm Electronics – Circuits for the Hobbyist www.elmelectronics.com 1 of 8 ELM401 Pin Descriptions VDD (pin 1) This pin is the positive supply pin, and should always be the most positive point in the circuit. Internal circuitry connected to this pin is used to provide power on reset of the internal logic, so an external reset signal is not required. Refer to the Electrical Characteristics section for more information. A (pin 2) This input is usually connected to what is normally known as the ‘A’ signal from a rotary (quadrature) encoder. An external pullup resistor is required for the encoder (a typical value is 10 KΩ), but no external capacitors are needed (as the internal debounce circuitry provides the filtering). Sw out (pin 5) This output is the debounced representation of the signal that is at pin 4. B out (pin 6) This output is the debounced representation of the signal that is at pin 3. A out (pin 7) This output is the debounced representation of the signal that is at pin 2. VSS (pin 8) Circuit common is connected to this pin. This is the most negative point in the circuit. B (pin 3) This input is usually connected to what is normally known as the ‘B’ signal from a rotary (quadrature) encoder. An external pullup resistor is required for the encoder (a typical value is 10 KΩ), but no external capacitors are needed (as the internal debounce circuitry provides the filtering). Sw (pin 4) This input may be used to debounce any standard mechanical contact (from a switch or relay). The circuitry uses simple set/reset logic to control a timer, so is not suitable for use with the A or B signal from a rotary encoder (as those signals continue to generate sliding noise while the contact is closed). All rights reserved. Copyright 2011 Elm Electronics. Every effort is made to verify the accuracy of information provided in this document, but no representation or warranty can be given and no liability assumed by Elm Electronics with respect to the accuracy and/or use of any products or information described in this document. Elm Electronics will not be responsible for any patent infringements arising from the use of these products or information, and does not authorize or warrant the use of any Elm Electronics product in life support devices and/or systems. Elm Electronics reserves the right to make changes to the device(s) described in this document in order to improve reliability, function, or design. ELM401DSB Elm Electronics – Circuits for the Hobbyist www.elmelectronics.com 2 of 8 ELM401 Ordering Information These integrated circuits are only available in either a 300 mil plastic DIP format, or in a 150 mil SOIC surface mount type of package. To order, add the appropriate suffix to the part number: 300 mil Plastic DIP............................... ELM401P 150 mil SOIC..................................... ELM401SM Outline Diagrams The diagrams at the right show the two package styles that the ELM401 is available in. The first shows our ELM401P product, which is an ELM401 in a 300 mil DIP package. This is a standard through hole type dual inline package. The ELM401SM is our surface mount version of the ELM401. The device package has a 3.90 mm wide body, and is commonly called a 150 mil SOIC package. The drawings shown here provide the basic dimensions for these ICs only. Please refer to the following Microchip Technology Inc. documentation for more detailed information: • Microchip Packaging Specification, document name en012702.pdf (7.5MB). At the www.microchip.com home page, click on Packaging Specifications, or go to www.microchip.com/packaging ELM401P 2.54 6.35 max 10.92 ELM401SM 1.27 3.90 6.00 • PIC12F508/509/16F505 Data Sheet, document 41236E.pdf (1.5 MB). At the www.microchip.com home page, click on Data Sheets, then search for 12F508. ELM401DSB Note: all dimensions shown are in mm. Elm Electronics – Circuits for the Hobbyist www.elmelectronics.com 3 of 8 ELM401 Absolute Maximum Ratings Storage Temperature....................... -65°C to +150°C Ambient Temperature with Voltage Applied..................................-40°C to +85°C Voltage on VDD with respect to VSS............ 0 to +6.5V Note: Stresses beyond those listed here will likely damage this device. These values are given as a design guideline only. The ability to operate to these levels is neither inferred nor recommended. Voltage on any other pin with respect to VSS........................... -0.3V to (VDD + 0.3V) Electrical Characteristics All values are for operation at 25°C and a 5V supply, unless otherwise noted. For further information, refer to note 1 below. Characteristic Minimum Typical Supply voltage, VDD 2.0 5.0 VDD rate of rise 0.05 Power on reset time 9 Average supply current, IDD Maximum Units 5.5 Conditions V V/msec see note 2 18 30 msec see note 3 0.6 1.1 mA VDD = 5.0V 0.2 0.3 mA VDD = 2.0V VDD = 5.0V 10 mA VOL = 0.25V VDD = 3.0V 5.0 mA VOL = 0.25V VDD = 5.0V 2.5 mA VOH = 4.75V VDD = 3.0V 1.7 mA VOH = 2.75V A & B debounce period 5.5 msec Startup time delay 50 msec Internal timing variation 1 Output low current (sink) Output high current (source) 4 % see note 4 see note 5 Notes: 1. This integrated circuit is based on a Microchip Technology Inc. PIC12F5XX device. For more detailed specifications, please refer to the Microchip documentation (www.microchip.com). 2. This spec must be met in order to ensure that a correct power on reset occurs. It is quite easily achieved using most common types of supplies, but may be violated if one uses a slowly varying supply voltage, as may be obtained through direct connection to solar cells, or some charge pump circuits. 3. The internal reset circuitry stops the ELM401 from doing anything during this period, so that the power supplies and oscillators have time to stabilize. During this time, all pins behave like inputs. 4. Typical only - the actual period varies with the amount of noise present in the input signal. 5. All filtering, delay, and output timing is based on an internal master oscillator. The frequency of this oscillator will vary with voltage and temperature. Values shown are typical maximums for 2.0V ≤ VDD ≤ 5.5V, and temperatures of -40°C to +85°C ELM401DSB Elm Electronics – Circuits for the Hobbyist www.elmelectronics.com 4 of 8 ELM401 Rotary Encoders A rotary encoder (sometimes referred to as a quadrature encoder) is a device that produces digital (on/off) outputs in response to rotary, or circular, motion. It is often constructed such that it looks very much like a potentiometer, or audio volume control (see the picture of a typical device, at the right). As the encoder shaft is turned, internal contacts open and close, creating two waveforms that are ideally separated in phase by 90 degrees (ie ‘in quadrature’). Actually, you need to provide external ‘pullup’ resistors and a power supply to create these waveforms, as the contacts themselves can not do this. An ideal waveform from a rotary encoder would look like this: A B Figure 1. Quadrature Waveforms Due to the 90 degree phase difference, when one waveform changes, the other is always stable. By noting the direction of the change and the level of the other input at that time, you can determine the direction of motion of the shaft. Rotary encoders are not ideal, however. Due to their construction, and variations in shaft speed, the A typical rotary encoder waveforms are not perfectly square with the 50% duty cycles shown. Figure 2 shows a captured trace from a real rotary encoder that is more representative of what you will typically find. Note that the two ‘scope channels (1 and 2) represent the encoder outputs A and B, respectively. The ch 1 (A) waveform leads the ch 2 (B) waveform, which usually means that the shaft is turning in a clockwise direction. The first rising edge of the channel 2 waveform shows another problem that occurs with moving mechanical contacts - multiple pulses due to bounce. When two contacts meet, the moving one will tend to bounce, like a ball does when it is dropped on the floor. Each bounce results in an electrical connection being made, then broken, which will look like multiple inputs to a fast electronic circuit. Various mechanical means are used to reduce the amount of bounce, but it can never really be eliminated. The following section discusses how the ELM401 uses electronic means to remove the bounce. Figure 2. Actual Rotary Encoder waveform ELM401DSB Elm Electronics – Circuits for the Hobbyist www.elmelectronics.com 5 of 8 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. rotary encoder input 3 msec timer to output logic tc = 1.7 msec filter timer Figure 3. Internal Debouncing Logic ELM401DSB Elm Electronics – Circuits for the Hobbyist www.elmelectronics.com 6 of 8 ELM401 Design Considerations There are a few details to consider when using the ELM401 to interface to a rotary encoder. The first is the fact that the signals available from the encoder are usually only dry contacts closing and opening. You will need to provide pullup resistors for these signals in order to use them in a circuit. The size of the pullup resistor is chosen based on the encoder specifications. The main concern is the maximum current carrying capacity, which sets a lower limit for the pullup resistance. An upper limit for the resistance is set by the minimum current required for contact wetting. If you do not provide enough current through mechanical contacts when they are closed, they will tend to go open with time. A maximum current specification is usually in the range of 1 to 10 mA, while the minimum wetting current would be in the range of 1 mA. This means that with a 5V supply, a pullup resistor of 5 to 10 KΩ is typically required. The second concern is the use of capacitors on the ‘A’ and ‘B’ signal lines. Many encoder circuits show these as a way to provide some pre-filtering of the signal. That is fine, as long as you realize that the ELM401 inputs are CMOS and do not have Schmitt trigger waveshaping. This means that you should keep the rate of change of the input signal as high as possible to avoid problems (we usually try to maintain at least 1V/µsec). Typically, with a 5V supply, a 10 KΩ pullup, and TTL thresholds, this means capacitor values of no more than about 330 pF, while with a 2.0V supply, the limit would be about 100 pF. One other issue to consider is that during the initial circuit startup, there is a period (of about 20 msec) when the ELM401 is being held in a reset state, and ELM401DSB the outputs are in a tristate condition. During this time, the outputs will sit at the level they were at before power up (0V) due to stray capacitance having discharged through the protection diodes. If you require that the outputs be at a high level as soon as possible after powerup, you may want to install a pullup resistor (of about 10 KΩ) on each output to charge the capacitance quickly. After the initial 20 msec period, the ELM401 sets all pins to their quiescent levels, but does not change any outputs for an additional 50 msec. This ensures that the external circuits have had adequate time to initialize, before being presented with signals to process. Elm Electronics – Circuits for the Hobbyist www.elmelectronics.com 7 of 8 ELM401 Example Application Figure 4 below shows how the ELM401 might be used with a microprocessor. Power for the ELM401 has been obtained from the microprocessor circuit, and a small capacitor has been added to ensure that the supply is filtered. The positive supply has been shown simply as VDD, since the ELM401 will operate with any voltage from 2.0 to 5.5 volts. The circuit uses 10 KΩ pullup resistors with the rotary encoder signals in order to convert the signal from the dry contacts into one that swings from VDD to VSS. The momentary pushbutton contact from the encoder is treated no differently than the ones from the encoder wheel. Since both ICs share the same supply, they can be directly connected as shown. That’s about all there is to using the ELM401 in your circuit. Connect pullup resistors to your rotary encoder signals, connect the encoder signals to the ELM401, then connect the ELM401 to the controlled circuit. Use a common supply for all the devices, and add a small bypass capacitor across the supply line. The controlled devices can be digital pots, microprocessors, or almost any circuit that will accept CMOS signals. Enjoy! VDD VDD VDD your microprocessor VDD 0.01µF 10 KΩ A 10 KΩ 10 KΩ B port pins 1 8 2 7 3 6 4 5 Sw rotary encoder C Figure 4. Connecting the ELM401 to a microprocessor ELM401DSB Elm Electronics – Circuits for the Hobbyist www.elmelectronics.com 8 of 8