ELM327
Power Control (continued)
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
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, and perform
a 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.
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. If
you also provide a pullup resistor, you don’t need a
voltage input, and can use a mechanical switch. This
circuit shows a momentary pushbutton connected:
+5V
47KW
2.2KW
16
15
14
If the circuit is in low power mode, a momentary
push of the button should wake it up. This is just one
of the many ways in which you can use this logic.
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 process with the ELM327. Now, on switching back
to full power operation, the ELM327 retains the
following settings:
E0/1
H0/1
D0/1
L0/1
S0/1
M0/1
R0/1
AT0/1/2
CAF0/1
JTM1/5
CFC0/1 CSM0/1 CEA
AL/NL IIA
Also, the current protocol is not changed, but it is
reset (with AT PC), and the ISO/KWP baud rate is
retained.
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 77.
We are often asked if a switched ‘ignition’ voltage
is available at the vehicle’s diagnostic connector for
this purpose. It is not - you must connect a separate
wire yourself. Often there are convenient places that
you can use for this - possibly 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
ELM327DSI
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