ELM402

Elm Electronics – Circuits for the Hobbyist

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Pin Descriptions

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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 K

Ω), 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 K

Ω), but no external capacitors

are needed (as the internal debounce circuitry

provides the filtering).

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.

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

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.

Circuit common is connected to this pin. This is the

most negative point in the circuit.

ELM402DSA