ELM327P datasheet(63/76 Pages) ELM

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ELM327

ELM327DSH

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switch all of the outputs as was described for the other

methods.

When connecting to pin 15, care should be taken

to not pass excessive current (>0.5 mA) through the

protection diodes, and the circuit should also provide

some filtering for ignition noise. Typically a circuit like

this works well (note that the Schmitt input on pin 15

allows the use of large value capacitors):

+12V switched

by the ignition

47K

Ω

22K

Ω

0.1uF

The new 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

what is done while monitoring. Refer to Figure 7 below

for a pictorial view of these methods.

Either a low level pulse at the RS232 Rx input, or

a low then high level at the IgnMon input will cause the

ELM327 to return to full power operation, and perform

a Warm Start reset. Note that PP 0E bit 2 does not

have to be set for the IgnMon to wake the circuit - the

ELM327 always monitors this pin, and will wake the

circuit after the delay that is set by PP 0E bit 1.

The RS232 input is not as sensitive as normal

when in the low power mode. For this reason, the

RS232 input pulse to wake the system must be at

least 128 usec wide. This is easily accomplished by

sending a space or @ character if the baud rate is less

than about 57.6 kbps, but when you use higher baud

rates, you may have trouble. For higher baud rates,

consider temporarily shifting to a lower baud rate, or

see if your software can generate a ‘break’ signal. If

you are directly connected to a microprocessor, then

you might want to generate your own break signal in

software.

This has discussed some of the software aspects

of using the new Power Control feature. Refer to the

‘Modifications for Low Power Standby Operation’

section (page 70) for a discussion of some of the

electrical design considerations.

Power Control (continued)

128 µsec min

pulse width

1 sec or 5 sec

time delay on pick up

(fast reset)

rising edge (

)

was detected

1 sec

delay

perform a

warm start

(AT WS)

Go to Full Power

• µP to normal

• pin 16 = b6

RS232 Rx

(pin 18)

IgnMon

(pin 15)

Figure 7. Returning to Normal Operation

ELM327P Datasheet(PDF) 63 Page - ELM Electronics

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