ELM408 查看數據表（PDF） - Elm Electronics

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ELM408

Rotary Decoder - 2 wire Interface

Elm Electronics 

ELM408

Output Waveforms (continued)

decoding (there are two sets of output pulses for each

set of waveforms from the A and B inputs), while the

second (Figure 9) shows 4x decoding. In both cases,

the Write Enable input is high, but this has no bearing

on whether the sets of pulses are generated or not.

Figure 8 also shows what happens when the

rotary encoder shaft is ‘wiggled’, generating pulses on

only the A or only the B input - the logic detects this

and ignores them. In order for an output to be

generated, the ELM408 must see a change on one of

the rotary encoder inputs followed by a change on the

other input. The 4x decoding is a little different in this

respect. It will generate one pulse in anticipation of a

change of direction when the same input (A or B)

changes two times in a row. Figure 10 on the next

page shows how this typically works. If the logic to

detect and ignore multiple inputs such as this were not

in place, then 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 actually 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 your

application demands that the controlled variable must

repeat exactly as the shaft is turned in either direction,

you may prefer to use a 2x decoder setting.

A Input

B Input

CS

U/D

A ‘wiggle’ or vibration causes B to change,

but A does not, so the pulse is ignored

debounce

delay

direction changes

Figure 8. 2x Output signals (pin 5 = Low)

A Input

B Input

CS

U/D

ELM408DSA

direction changes

Figure 9. 4x Output signals (pin 5 = High)

Elm Electronics – Circuits for the Hobbyist

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