Manual Elm 327 (page 67 of 94) (English)

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67 of 94ELM327DSJ Elm Electronics – Circuits for the Hobbyist

www.elmelectronics.com

ELM327

easily accomplished by sending a space or @

character, if the baud rate is 57.6 kbps or less. If using

higher baud rates, you may wish to consider

temporarily shifting to a lower baud rate and sending a

character, or possibly see if your software can

generate a ‘break’ signal for you. The break signal is

not always the same length, but is typically several

msec long (ie much longer than 128 µsec), and can

often be sent by USB through USB to RS232

conversion devices. You will have to experiment if

using other methods (Bluetooth or WiFi, for example).

The second method to have the ELM327 circuit go

back to full power is by having activity appear on the

OBD input pins. At the first sign of an active level, the

circuit will start its wakeup, and within seconds, will be

at full capability. Note that the logic is configured so

that it must see all quiet on the OBD inputs before it

will allow an automatic wakeup. This does not normally

present a problem, and is implemented so that you

can manually put the circuit to low power (AT LP),

even if there is activity on one of the OBD inputs.

Otherwise, the circuit would wake up immediately after

the AT LP.

When connected to protocols 3, 4 or 5 (ISO 9141-

2 or ISO 14230-4), there may be no OBD activity when

the vehicle is first turned on. In this case, you could not

rely on the activity monitor to wake the circuit and you

would have to look for other means.

The final method that is provided to wake up the

circuit is by way of the ignition monitor input and logic.

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

ELM327 to return to full power operation. 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.

We are often asked if a switched ‘ignition’ voltage

is available at the vehicle’s diagnostic connector for

this purpose. It is not, so you must connect a separate

wire yourself. Often there are convenient places that

you can use for this - possibly the radio power, or the

connection to a convenience outlet.

Note that we present the pin 15 input as if it is an

ignition monitoring circuit, but in fact, it will monitor for

any change in voltage level (as long as the change is

stable for the debounce period). You might consider

connecting to other inputs (such as the 5V from a USB

cable, for example) - just be sure to protect the

ELM327 input circuitry from transients. This usually

just requires a series resistor of 1K to 10K in value.

One alternative is to provide a pullup resistor to

allow waking of the ELM327 with a momentary

pushbutton switch:

2.2KΩ

47KΩ

+5V

If the circuit is in low power mode, a momentary

push of the button will wake it up. This is just one of

the many ways in which you can use this input.

One final note has to do with the changes to the

startup process, beginning with firmware version 2.0.

Initially (with v1.4 and v1.4b), the ELM327 simply used

a warm start command (AT WS) to exit from the low

power mode. The positive feedback that we received

from our ELM329 changes has led us to also change

the wakeup process for the ELM327.

Now, on switching back to full power operation,

the ELM327 performs a warm start, but retains the

following settings:

E0/1 H0/1 L0/1 M0/1

R0/1 D0/1 S0/1 AT0/1/2

CAF0/1 CFC0/1 CSM0/1 CEA

CTM1/5 JTM1/5 AL/NL IIA

In addition, the ISO/KWP baud rate is retained and

the current protocol is not changed (but it is closed, so

will require initialization).

This has discussed how to have the ELM327 go to

low power operation, and wake from it. While in the

low power mode, the question of ‘just how low is the

power consumption’ arises. We discuss that in the

‘Modifications for Low Power Standby Operation’

section, which is on page 84.

Power Control (continued)

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Elm 327 Manual

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