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Elm 327 Manual

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66 of 94ELM327DSJ Elm Electronics – Circuits for the Hobbyist
www.elmelectronics.com
ELM327
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
47KΩ
47KΩ0.1uF
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
b1
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
… 66 …


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