ELM329LSM datasheet(63/87 Pages) ELM

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ELM329L DSA

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ELM329L

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 while in low

power mode, that is not the case beginning with v2.0 -

those 3 pins now remain unchanged.

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, so is ready to go. Note that the CAN

Monitor signal is blocked from causing a switch to the

low power mode while the IC is in a monitoring mode

(AT MA, DM1 or MP). This feature is provided so that

the ELM329 does not power down while you are

troubleshooting with the monitor.

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 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 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 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.

Low Power Mode (continued)

+12V switched

by the ignition

47K

Ω

22K

Ω

0.1uF

329

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