Design Considerations

8 of 9

ELM404DSA

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ELM404

Output Waveforms (continued)

There are a few details to consider when using the

ELM404 to interface to a rotary encoder. The first is

the fact that the signals available from the encoder are

usually only dry contacts closing and opening. You will

need to provide pullup resistors for these signals in

order to use them in a circuit.

The size of the pullup resistor is chosen based on

the encoder specifications. The main concern is the

maximum current carrying capacity, which sets a lower

limit for the pullup resistance. An upper limit for the

resistance is set by the minimum current required for

contact wetting. If you do not provide enough current

through mechanical contacts when they are closed,

they will tend to go open with time. A maximum current

specification is usually in the range of 1 to 10 mA,

while the minimum wetting current would be in the

range of 1 mA. This means that with a 5V supply, a

pullup resistor of 5 to 10 K

Ω is typically required.

The second concern is the use of capacitors on

the ‘A’ and ‘B’ signal lines. Many encoder circuits show

these as a way to provide some pre-filtering of the

signal. That is fine, as long as you realize that the

ELM404 inputs are CMOS and do not have Schmitt

trigger waveshaping. This means that you should keep

the rate of change of the input signal as high as

possible to avoid problems (we usually try to maintain

at least 1V/µsec). Typically, with a 5V supply, a 10 K

Ω

pullup, and TTL thresholds, this means capacitor

values of no more than about 330 pF, while with a

2.0V supply, the limit would be about 100 pF.

One other issue to consider is that during the initial

circuit startup, there is a period (of about 20 msec)

when the ELM404 is being held in a reset state, and

the outputs are in a tristate condition. During this time,

the outputs will sit at the level they were at before

power up (0V) due to stray capacitance having

discharged through the protection diodes. Since the

CS (Chip Select) is an active low output, this may

cause a problem, if your controlled circuit is ready too

fast. If you require that pin 7 (and possibly pin 6) go

high as quickly as possible, you may want to install a

pullup resistor (of about 10 K

Ω) on the output(s) to

After the initial 20 msec period, the ELM404 sets

all pins to their quiescent levels, but does not change

any outputs for an additional 50 msec. This ensures

that the external circuits have had adequate time to

initialize, before being presented with signals to

process.

CS

Clk

U/D

200 µsec

Figure 5. Up or Clockwise Output

CS

Clk

U/D

200 µsec

Figure 6. Down or Counter-clockwise Output