ELM407

Elm Electronics – Circuits for the Hobbyist

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Pin Descriptions

2 of 10

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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).

Write Enable (pin 4)

This input controls the state of the Clk output when

the rising edge of the CS output occurs. If pin 4 is

high, the Clk output will be held high during the CS

transition, which generates an EEPROM write for

many digital potentiometer chips.

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 low ‘Clock’ pulse is output at this pin 200 µsec after

the CS output has gone low. The pulse is normally

400 µsec wide, and is typically used to cause the

controlled device to change state based on the level

at the U/D pin. If pin 4 is at a high level, the Clk pulse

will be reduced in length to 200 µsec, and the CS

pulse width extended to 600 µsec, so that Clk is at a

high level while the CS output transitions from the

active low level to the quiescent high level. This will

cause the setting to be written to memory for many

devices.

CS (pin 7)

This output provides a ‘Chip Select’, or enable

function. As the rotary encoder 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 for 400 µsec. This changes a

little if the Write Enable input is at a high level. 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.

ELM407DSA