ELM327LSM datasheet(65/94 Pages) ELM

65 / 94 page

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

65 of 94

ELM327L DSA

Elm Electronics – Circuits for the Hobbyist

www.elmelectronics.com

ELM327L

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

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).

ELM327LSM Datasheet(PDF) 65 Page - ELM Electronics

ELM327LSM Datasheet(HTML) 65 Page - ELM Electronics

Html Pages

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60 61 62 63 64 65 66 67 68 69 70 71 72 73 74 75 76 77 78 79 80 81 82 83 84 85 86 87 88 89 90 91 92 93 94

Datasheet Download

Link URL