ELM329
Low Power Mode (continued)
to its low power setting. After a delay of 50 msec
(which gives the power supplies a chance to settle),
pin 16 is then set to its low power level, and then after
a further 50 msec, the chip reduces its power needs.
Note that while firmware version 1.0 also set M0, M1
and the Control output to a low level, that is not the
case with v2.0 - those 3 pins will now maintain their
current levels/function while in the low power mode.
The CAN Monitor (new with v2.0) offers another
way in which the ELM329 can be switched to the low
power mode. This module continually monitors pin 11
for a signal, and can initiate the low power mode if the
signal disappears for more than 2 (or 10) minutes. To
use this monitor, simply connect pin 11 to pin 24 (by
default, the module is enabled with a timeout setting of
10 minutes). Note that the CAN Monitor signal is not
able to cause a switch to the low power mode if the IC
is in a monitoring mode (AT MA, DM1 or MP). This
logic is provided to aid in troubleshooting (depending
on the activity, you likely do not want the circuit to
power down while trying to capture rare data, or to be
slow powering up).
The third method is very similar in function to the
CAN Monitor. It allows automatic switching to the low
power mode when there has been no RS232 input for
a period of time (a good method if you have a system
where the controlling computer may be turned off at
any time). This method does not require any wiring
changes for the input as the connection is made
internally. This monitor provides slightly longer time
delays of either 5 or 20 minutes, to allow for the longer
time delays that you might encounter with a human
operator.
The ignition/voltage monitor method uses a short
internal delay (‘debounce’) timer to be sure that the
low level is a legitimate ‘key off’, and not just noise
spikes. As with the previous two methods, when the
low power mode is initiated, the ELM329 will send an
alert message (‘LP ALERT’), wait 2 seconds, and will
then begin low power mode.
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. So, if the ELM329
reports being interrupted (ie ‘STOPPED’), 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 ELM329 into low power mode, you
will need a means to wake it up. There are several
ways in which you may do this (note that it does not
have to be the same method that put it into the low
power mode). Figure 7 is a block diagram which
shows the possible ways.
The first way that a wakeup may occur is by the
CAN Monitor sensing that there has been a change
from no CAN activity to there being activity (this is
shown as a ‘rising edge’ condition in the diagram). In
addition to this change in CAN activity, you may also
require that the CAN input was previously active by
setting (PP 0F) bit 3 to 0 (it is set to 1 by default). This
bit 3 switch was provided to more or less ensure that
the circuit only wakes up on CAN activity if that was
the cause of it going to low power mode. PP 0F bit 7
does not have to be set in order for the circuit to ‘wake
up’ on CAN activity.
The final method that may be used to enter the
low power mode is by a low level appearing at the
ignition monitor input (pin 15 - IgnMon). It is enabled
by setting both b2 and b7 of PP 0E to ‘1’. Note that
when connecting to pin 15, care must be taken to not
pass excessive current (>0.5 mA) through the internal
protection diodes. Typically a circuit like this works well
(note that the Schmitt trigger input on pin 15 allows the
use of large value filtering capacitors):
The other two ways that may be used to ‘wake’ the
circuit are as shown in Figure 7. The first is with an
RS232 input that goes to the active (low) level for at
least 128 µsec. This may be accomplished by sending
a space or @ character if the baud rate is less than
about 57.6 kbps. At higher baud rates, it may be more
difficult to generate this width, so you might 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
be able to generate a break signal or pulse output in
software.
47KW
+12V switched
by the ignition
0.1uF
22KW
16
15
14
329
ELM329DSB
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