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Elm 327 Seite 64

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Power Control
Often, the ELM327 is connected to a vehicle for
only a short time, so power consumption is not of great
concern. Occasionally, the ELM327 may be connected
for longer times, however, possibly without the engine
running. For those applications, it is often desirable to
be able to put the circuit into a low power ‘standby’
state, and have it return to normal operation when
needed. The power control features of the ELM327
were introduced for this.
There are four ways in which the ELM327 can be
placed into the low power standby mode (these are
shown pictorially in Figure 6). None of them will work
without having the master enable (ie bit 7 of PP 0E)
set to ‘1’, which it is by default.
The first method is with an AT command. You may
simply send:
>AT LP
and the IC will go to the low power mode after a one
second delay (which allows the controlling circuit a
little time to perform some housekeeping tasks).
When in Low Power mode, the ELM327 sets all
outputs to their recessive/off state, sets pin 3 (J1850
voltage control) to a low level, and it reverses the state
of the pin 16 control output. The ELM327 will then
reduce its own power level, and begin monitoring for
inputs that would cause a shift back to full power.
The next method allows automatic switching to the
low power mode when there has been no RS232 input
for a period of time - ie the controlling computer has
stopped for some reason. To enable this method, both
b7 and b5 of PP 0E must be set to ‘1’. The time delay
(either 5 or 20 minutes) is set by b4, and the printing of
a warning is enabled with b3. The warning is handy in
some cases – it is the activity alert message (‘ACT
ALERT’) and is sent 1 minute before the timer is about
to time out. When the timer does time out, you will see
a low power alert warning (‘LP ALERT’), and then 2
seconds later, all of the outputs will change as
described above for the AT LP command.
In a similar way, the Activity Monitor may be used
64 of 94ELM327DSJ Elm Electronics – Circuits for the Hobbyist
www.elmelectronics.com
ELM327
The Activity Monitor
The ELM327 contains some firmware that is used
to monitor the OBD input pins (ie pins 11, 12, 13, and
24). These routines continually ‘poll’ the inputs, looking
for any active levels on them. Of course, we don’t want
to look at a pin while we are sending, so there is also
some logic to block those inputs during a send, to filter
the levels, and also to provide timing. Collectively,
these routines are known as the Activity Monitor.
The Activity Monitor software ensures that each
OBD input pin is checked at least once every 4 msec
(and sometimes as often as every few µsec). This may
not be perfect (since a single very short message on
an otherwise quiet bus could be missed), but it does
make sure that the ELM327 detects normal activity on
active busses. The presence or absence of activity can
then be used to cause the ELM327 to go to Low
Power operation, or wake from it. See the next section
for details on that.
If you do not want the Activity Monitor to initiate
Low Power operation, you may set PP 0F so that only
‘ACT ALERT’ reports are provided. Of course, you
may also disable that as well, and make your own
decisions based on what the Activity Monitor is seeing.
To help with this, the ELM327 offers an instruction
that is able to report the current Activity Monitor
Counts. The count is an internal value that represents
the time since OBD activity was detected. The actual
time is given by (AMC value + 1) x 0.65536 seconds.
To use it, send the AMC Command and note the value
returned:
>AT AMC
31
In this case, the value returned is 31 hexadecimal
(ie 49 in decimal), which means that the ELM327 has
not detected any OBD activity in the past 49 x 0.655 =
32.8 seconds. The Activity Monitor Count is limited to
a single byte value (max is FF), and internal logic
prevents it from ‘rolling over’. That is, it will count to FF
and stop there.
After a power on or reset, the Activity Monitor is
initially disabled (to give you time to perform setup
tasks). It will be enabled as soon as you attempt to
send the first OBD message. Also, the monitor is
always disabled while the ELM327 is in a monitoring
mode so that you can use the ELM327 as a reliable
monitor of bus activity (you don’t want it to go to sleep
when it should be monitoring, or to miss anything while
waking from sleep).
… 64 …

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