ELM404

Elm Electronics – Circuits for the Hobbyist

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Pin Descriptions

2 of 9

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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).

Clk Invert (pin 4)

This input controls the polarity of the pulse at pin 6. If

pin 4 is at a low level, the Clk output will maintain a

level. If pin 4 is at a high level, then pin 6 will

normally be at a high level, with pulses going low (to

Vss). This input can be changed at any time.

U/D (pin 5)

This ‘Up/Down’ output changes with the direction of

encoder rotation. For a clockwise movement (the A

input appears first, followed by the B), this output will

be set high before the CS output (pin 7) is switched

low. For counter-clockwise movement, the output will

be set low prior to the Chip Select going low. The

U/D output does not change state when the CS and

Clk pulsing is complete - it remains at the level it was

just prior to the CS going low.

Clk (pin 6)

A ‘Clock’ pulse will be output at this pin 200 µsec

after the Chip Select (pin 7) has gone low. The pulse

is 200 µsec wide, and is typically used to cause the

controlled device to change state based on the level

at the U/D pin. The polarity of this output can be

changed at any time by changing the logic level at

pin 4.

CS (pin 7)

This output provides a ‘Chip Select’, or enable

function. As the shaft rotates, a series of clock

pulses are generated at pin 6. Just prior to these

pulses appearing, the CS output will go low, and stay

at that level until 200 µsec after the Clk pulse is

complete. See the Output Waveforms section (on

page 7) for more information.

Circuit common is connected to this pin. This is the

most negative point in the circuit.

ELM404DSA