ELM402
Example Applications
Figure 6 below shows how the ELM402 might be
used with a microprocessor. Pullup resistors of 10 KW
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 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!
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
VDD
VDD
your microprocessor
VDD
port pins
0.01µF
10 KW
10 KW
1
2
3
4
8
7
6
5
VDD
A
B
rotary
encoder
C
Figure 6. Connecting the ELM402 to a microprocessor
+5V
+5V
10 KW
1
2
3
4
8
7
6
5
8
1
7
6
5
A
B
MAX5128
rotary
encoder
H
W
L
4
+5V
C
2
3
0.1µF
Figure 7. Connecting the ELM402 to a MAX5128
ELM402DSA
Elm Electronics – Circuits for the Hobbyist
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