ELM327P datasheet(54/76 Pages) ELM

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Programmable Parameters

The ELM327 contains several programmable

memory locations that retain their data even after

power is turned off. Every time the IC is powered up,

these locations are read and used to change the

default settings for such things as whether to display

the headers, or how often to send ‘wakeup’ messages.

The settings, or parameters, can be altered by the

user at any time using a few simple commands. These

Programmable Parameter commands are standard AT

Commands, with one exception: each one requires a

two-step process to complete. This extra step provides

some security against random inputs that might try to

make changes.

The following pages list the currently supported

Programmable Parameters for this version of the

ELM327. As an example of how to use them, consider

PP 01 (shown on page 55) which sets the default state

for the AT H command. If you are constantly powering

your ELM327 and then using AT H1 to turn the

headers on, you may want to change the default

setting, so that they are always on by default. To do

this, simply set the value of PP 01 to 00:

>AT PP 01 SV 00

This changes the value associated with PP 01, but

does not enable it. To make the change effective, you

must also type:

>AT PP 01 ON

At this point, you have changed the default setting

for AT H1/H0, but you have not changed the actual

value of the current AT H1/H0 setting. From the ‘Type’

column in the table on page 55, you can see that the

change only becomes effective the next time that

defaults are restored. This could be from a reset, a

power off/on, or possibly an AT D command.

With time, it may be difficult to know what changes

you have made to the Programmable Parameters. To

help with that, the ELM327 provides a Programmable

Parameter Summary (PPS) command. This simply

prints a list of all of the supported PPs, their current

value, and whether they are on/enabled (N), or

off/disabled (F). For an ELM327 v1.4b IC, with only the

headers enabled (as discussed above), the summary

table would look like this:

>AT PPS

00:FF F

01:00 N

02:FF F

03:32 F

04:01 F

05:FF F

06:F1 F

07:09 F

08:FF F

09:00 F

0A:0A F

0B:FF F

0C:68 F

0D:0D F

0E:9A F

0F:FF F

10:0D F

11:00 F

12:FF F

13:32 F

14:FF F

15:0A F

16:FF F

17:92 F

18:00 F

19:28 F

1A:FF F

1B:FF F

1C:FF F

1D:FF F

1E:FF F

1F:FF F

20:FF F

21:FF F

22:FF F

23:FF F

24:00 F

25:00 F

26:00 F

27:FF F

28:FF F

29:FF F

2A:38 F

2B:02 F

2C:E0 F

2D:04 F

2E:80 F

2F:0A F

You can see that PP 01 now shows a value of 00,

and it is enabled (oN), while the others are all off.

Another example shows how you might change

the CAN filler byte. Some systems use ‘AA’ as the

value to put into unused CAN bytes, while the ELM327

uses ‘00’ by default. To change the ELM327’s

behaviour, simply change PP 26:

>AT PP 26 SV AA

>AT PP 26 ON

Again, PP 26 is of type ‘D’, so the above change

will not actually take effect until the AT D command is

issued, or the ELM327 is reset.

The Programmable Parameters are a great way to

customize your ELM327 for your own use, but you

should do so with caution if using commercial

software. Most software expects an ELM327 to

respond in certain ways to commands, and may be

confused if the carriage return character has been

redefined, or if the CAN response shows data length

codes, for example. If you make changes, it might be

best to make small changes and then see the effect of

each, so that it is easier to retrace your steps and

‘undo’ what you have done. If you get in too deeply,

don’t forget the ‘all off’ command:

>AT PP FF OFF

No matter what software you use, you might get

into more serious trouble, should you change the baud

rate, or the Carriage Return character, for example,

and forget what you have set them to. The Carriage

Return value that is set by PP 0D is the only character

that is recognized by the ELM327 as ending a

command, so if you change its value, you may not be

able to undo your change. In this case, your only

54 of 76

ELM327DSH

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