ELM327LSM datasheet(67/94 Pages) ELM

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ELM327L DSA

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ELM327L

described for the other methods.

When connecting to pin 15, care should be taken

to not allow excessive current (ie >0.5 mA) to pass

through the internal protection diodes. Typically all that

is required is a series resistor, but adding a capacitor

helps to filter out ignition noise (note that the Schmitt

input on pin 15 allows the use of large value

capacitors). A second resistor ensures a discharge

path for the capacitor, while raising the threshold

voltage. A circuit like this works well:

+12V switched

by the ignition

47K

Ω

47K

Ω

0.1uF

Note that the AT IGN command can always be

used to read the level at pin 15, regardless of the

setting of the PP 0E enable bits. This may be used to

advantage if you wish to manually shut down the IC,

using your own timing and criteria. Recall that the

alternate function for pin 15 is the RTS input which will

interrupt any OBD processing that is in progress. If the

ELM327 reports an interrupt with the ‘STOPPED’

message, you can then check the level at pin 15 with

the AT IGN command, and make your own decisions

as to what should be done. For that matter, you don’t

even need to reduce the power based on the input -

you might possibly do something entirely different.

Having put the ELM327 into Low Power mode,

you will need a method to wake it up. This is done by

‘interrupting’ the IC in ways that are very similar to that

used to put it into Low Power mode.

Figure 7 shows the three ways to ‘wake’ the

ELM327 from low power mode (other than toggling the

power, or pulsing pin 1). Any of these can wake the IC

- they do not have to be the cause of its going to low

power.

The first way is with a low level pulse at the RS232

Rx input (pin 18). The RS232 circuitry is not as

sensitive as normal when in the low power mode, so to

be sure that your input is seen, the ELM327 requires

that the pulse width be at least 128 usec wide. This is

Power Control (continued)

Figure 7. Returning to Normal Operation

OBD

inputs

1 or 5 sec

timer

IgnMon

(pin 15)

the voltage

is now high

RS232 Rx

(pin 18)

the input

is active

1 sec

delay

RS232

detector

voltage

monitor

activity

monitor

the voltage was low

an input is

now active

no input was active

128 µsec min

pulse width

perform a

warm start

(see text)

Go to Full Power

• µP to normal

• pin 16 = b6

ELM327LSM Datasheet(PDF) 67 Page - ELM Electronics

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