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ELM407

Rotary Decoder - 3 wire 4x Interface

Elm Electronics 

ELM407

Output Waveforms (continued)

200 µsec. This will result in a write of the wiper

position to non-volatile memory for many ICs (for

example, the ON Semiconductor CAT5112, or the

Intersil X9C102).

Outputs are usually generated each time that one

of the inputs change, so for each full cycle of both of

the inputs, there will be 4 output pulses. This is what is

known as 4x decoding. The other type of decoder that

is very common is the 2x decoder, as used by the

ELM406. We do not currently offer a product with 1x

decoding - if you require one, you will need to use the

ELM401 and decode the signal with your own logic.

Output pulses do not always occur when one of

the inputs change. After startup, the first output can

not be generated until the ELM407 has seen both of

the inputs change, and no output is generated until

then. Also, if an input is seen to be ‘chattering’ the

ELM407 will block the output. Chattering can occur

when the shaft is wiggled, or when it is subjected to

severe vibration. Figure 7 below shows how the

ELM407 handles the multiple inputs due to chatter. If

this logic were not in place, multiple outputs could

occur due to vibration (as found in an automobile, or

an industrial setting), and the setting could ‘creep’ with

time – even though the shaft was not turned.

Generally, output pulses will occur in groups of

four (between each detent), so resistance values or

settings will repeat consistently as the shaft is rotated

in one direction and then in the other. Depending on

when the shaft changes direction, however, the pulses

may not be generated in groups of four. If you are

designing a circuit and require that the controlled

variable must repeat exactly as the shaft is turned in

either direction, you may prefer to use a 2x decoder

such as the ELM406.

Change in B (expecting a change in A)

causes the ELM407 to anticipate a change

in direction, and generate a Down output

A changes, meaning that

the shaft is now rotating,

and pulses can be output

Multiple pulses are ignored while

the ELM407 waits for a change at

the A input

The shaft direction

changes again

A Input

B Input

CS

Clk

U/D

Figure 7. Input Chattering

ELM407DSA

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www.elmelectronics.com

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