ELM ELM402SM

ELM402
Rotary Decoder - 2 wire 2x Interface
Description
Features
The ELM402 is an 8 pin integrated circuit that is
used to convert the signals from a rotary encoder
into a series of up and down output pulses. 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 ELM402, as this is all
performed within the integrated circuit. After
debouncing the encoder signals, the ELM402 then
determines the direction of shaft rotation, and
generates the appropriate pulses at the Up or Down
output pins. The duration of these pulses can be
selected for either 0.2 or 2.0 msec.
The ELM402 provides 2x decoding of a rotary
encoder signal. If your application requires 4x
decoding, please see the ELM403 (it is identical to
the ELM402 in every respect, except that it provides
4x decoding).
•
•
•
•
•
•
•
Low power CMOS design
Wide supply range – 2.0 to 5.5 volts
Complete debouncing of the encoder inputs
No external filtering needed
2x decoding
Startup delay timer
High current drive outputs
Connection Diagram
PDIP and SOIC
(top view)
VDD
1
8
VSS
A
2
7
Up
B
3
6
Down
Pulse
Width
4
5
Output
Invert
Applications
•
•
•
•
Digital audio potentiometer controls
Variable voltage or temperature circuits
Positioning controls
Tuning circuits
Block Diagram
VDD
A
2
VDD
ELM402DSA
Direction
Decoding
B
3
Rotary
Encoder
Debounce
Circuit
Up
6
Down
Output
Logic
Debounce
Circuit
Pulse
Width
7
4
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Output
Invert
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ELM402
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 should be connected to what is normally
known as the ‘A’ signal from a rotary (quadrature)
encoder. The signal at this pin should lead (ie
change state before) the signal at pin 3 when the
device is being moved in a clockwise or up direction.
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).
B (pin 3)
This input should be connected to what is normally
known as the ‘B’ signal from a rotary (quadrature)
encoder. The signal at this pin should lag behind (ie
change state after) the signal at pin 2 when the
device is being moved in a clockwise or up direction.
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).
Output Invert (pin 5)
This input can be used to invert the output signals at
pins 6 and 7. If pin 5 is at a low level, the outputs will
maintain a low quiescent level, with the pulses going
to the VDD level. If pin 5 is set to a high level, the
outputs will normally be at a high level, with pulses
going low (to Vss). This input can be changed at any
time.
Down (pin 6) and Up (pin 7)
These two outputs pulse as the rotary encoder
moves. For a clockwise movement (the A input
appears first, followed by the B), pulses will appear
at the Up output, and the B output will remain at a
constant level. For counter-clockwise movement, the
Down output will pulse, and the Up output will remain
at a constant level. See the Output Waveforms
section (page 7) for a pictorial view of these outputs.
VSS (pin 8)
Circuit common is connected to this pin. This is the
most negative point in the circuit.
Pulse Width (pin 4)
This input controls the width of the pulses which
appear at pins 6 and 7. If the input is high, the pulse
width will be 2.0 msec, while a low input sets the
pulse width to 0.2 msec. This input can be changed
at any time.
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.
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ELM402
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............................... ELM402P
150 mil SOIC..................................... ELM402SM
Outline Diagrams
The diagrams at the right show the two package
styles that the ELM402 is available in. The first shows
our ELM402P product, which is an ELM402 in a
300 mil DIP package. This is a standard through hole
type dual inline package. The ELM402SM is our
surface mount version of the ELM402. 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
ELM402P
2.54
6.35
max
10.92
ELM402SM
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.
ELM402DSA
Note: all dimensions shown are in mm.
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ELM402
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
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 ELM402 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
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ELM402
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 ELM402 uses electronic means to
remove the bounce.
Figure 2. Actual Rotary Encoder waveform
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ELM402
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 ELM402 employs a two stage system to
remove the bounce and the sliding noise from the
input 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 ELM402 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.
Once the signals from the rotary encoder have
been debounced, they may be used by the direction
logic circuitry. The following section shows what the
ELM402 is able to produce from these signals.
rotary
encoder
input
3 msec
timer
to
output
logic
tc = 1.7 msec
filter
timer
Figure 3. Internal Debouncing Logic
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ELM402
Output Waveforms
Once the ELM402 has some bounce-free signals
to work with, it can generate outputs based on them.
The logic to decode the motion of an encoder
shaft 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 ELM402 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 selfchecking, 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 ELM402 is simply a series of
pulses, as shown in Figures 4 and 5. When the shaft is
moving in a clockwise direction, there will be pulses at
the Up output, while a counter-clockwise rotation
results in pulses at the Down output. This assumes
that the encoder is a standard one, where the ‘A’
signal leads the ‘B’ for a clockwise rotation. Note that
the Up output pulses are always a result of a change
in A followed by a change in B, while the Down output
pulses result from a change in B followed by a change
in A. Figure 4 also shows what happens when the
rotary encoder shaft is ‘wiggled’, generating pulses on
only the A or only the B input - the logic detects this
and ignores them. In order for an output to be
generated, the ELM402 must see a change on one of
the rotary encoder inputs followed by a change on the
other input.
The Up or Down output pulse width is set by the
level at pin 4. If pin 4 is at a high level, the pulse width
will be approximately 2.0 msec in duration, while a low
level on pin 4 will result in 200 µsec wide pulses. Pin 5
can be used to invert the output, as shown in Figure 5.
Note that the ELM402 uses what is known as 2x
decoding of the signals. This means that for each
motion of the shaft between the detents, when the ‘A’
and ‘B’ outputs go through one complete cycle, there
will be two output signals. The other type of decoder
that is very common is the 4x decoder, as used by the
ELM403. We do not currently offer a product for 1x
decoding - if you require that, you will need to use the
ELM401 and decode the signal with your own logic.
A ‘wiggle’ or vibration causes B to change,
but A does not, so the pulse is ignored
A Input
B Input
debounce
delay
Up
Down
Figure 4. Output signals (pin 5 = 0V)
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ELM402
Output Waveforms (continued)
A Input
B Input
debounce
delay
Up
Down
Figure 5. Output signals (pin 5 = VDD)
Design Considerations
There are a few details to consider when using the
ELM402 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
ELM402 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 ELM402 is being held in a reset state, and
ELM402DSA
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 are
using the output invert feature, the outputs will not rise
to their quiescent high state until after this reset period
is over. If it is essential that they go high as quickly as
possible, you may want to install a pullup resistor (of
about 10 KΩ) on each output to ensure that the
voltage goes quickly to VDD.
After the initial 20 msec period, the ELM402 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.
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ELM402
Example Applications
Figure 6 below shows how the ELM402 might be
used with a microprocessor. Pullup resistors of 10 KΩ
are used with the rotary encoder to provide voltages
for the A and B inputs. Power for the ELM402 has
been obtained from the microprocessor circuit, but a
small capacitor has been added to ensure that the
supply is filtered. The output is not inverted (pin 5 is at
0V), so pins 6 and 7 are normally at 0V, providing
positive-going pulses only when the rotary encoder is
turned. To allow the microprocessor plenty of time to
poll the signals, an output period of 2.0 msec has been
chosen, by setting pin 4 to VDD. The positive supply
has been shown as VDD, as the ELM402 will operate
with any voltage from 2.0 to 5.5 volts.
The other schematic (Figure 7) shows how the
ELM402 might be used with a digital potentiometer
chip, in this case the Maxim MAX5128. Note that the
ELM402 does not support the write function for chips
such as this, so every time that the circuit is powered
up, the wiper returns to the midpoint position. Since
the Maxim data sheet specifies a rather large bypass
capacitor (0.1 µF), we have shown it here, but you
may not need a capacitor that big if there are no writes
being performed. The ELM402 has been set for
0.2 msec positive going output pulses which are
considerably more than the 80 nsec minimum time. As
the MAX5128 requires positive-going pulses, we have
not put any pullups on the ELM402 outputs, but this
may be a consideration with other potentiometers.
That’s about all there is to using the ELM402.
Connect pullup resistors to your rotary encoder,
connect the encoder signals to the ELM402, then
connect the ELM402 to the controlled circuit. Use a
common supply for all the devices, and add a 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
your microprocessor
VDD
0.01µF
10 KΩ
10 KΩ
A
port pins
1
8
2
7
3
6
4
5
VDD
B
rotary
encoder
C
Figure 6. Connecting the ELM402 to a microprocessor
+5V
10 KΩ
A
B
rotary
encoder
+5V
1
8
2
7
3
6
4
5
8
7
6
5
MAX5128
H
+5V
C
0.1µF
1
2
W
3
L
4
Figure 7. Connecting the ELM402 to a MAX5128
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