ELM327P datasheet(27/76 Pages) ELM

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ELM327

ELM327DSH

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Reading the Battery Voltage

Before learning the OBD Commands, we will show

an example of how to use an AT Command. We will

assume that you have built (or purchased) a circuit

which is similar to that of Figure 9 in the Example

Applications section (page 67). This circuit provides a

connection to read the vehicle’s battery voltage, which

many will find very useful.

If you look in the AT Command list, you will see

there is one command that is listed as RV [Read the

input Voltage]. This is the command which you will

need to use. First, be sure that the prompt character is

shown (that is the ‘>’ character), then simply enter ‘AT’

followed by RV, and press return (or enter):

>AT RV

Note that we used upper case characters for this

request, but it was not required, as the ELM327 will

accept upper case (AT RV) as well as lower case

(at rv) or any combination of these (At rV). It does not

matter if you insert space characters (‘ ’) within the

message either, as they are ignored by the ELM327.

A typical response to this command will show a

voltage reading, followed by another prompt character:

12.6V

The accuracy of this reading depends on several

factors. As shipped from the factory, the ELM327

voltage reading circuitry will typically be accurate to

about 2%. For many, this is all that is needed. Some

people may want to calibrate the circuitry for more

accurate readings, however, so we have provided a

special ‘Calibrate Voltage’ command for this.

To change the internal calibration constants, you

will need to know the actual battery voltage to more

accuracy than the ELM327 shows. Many quality digital

multimeters can do this, but you should verify the

accuracy before making a change.

Let us assume that you have connected your

accurate multimeter, and you find that it reads 12.47V.

The ELM327 is a little high at 12.6V, and you would

like it to read the same as your meter. Simply calibrate

the ELM327 to the measured voltage using the CV

command:

>AT CV 1247

Note that you should not provide a decimal point in

the CV value, as the ELM327 knows that it should be

between the second and the third digits.

At this point, the internal calibration values have

been changed (ie. written to EEPROM), and the

ELM327 now knows that the voltage at the input is

actually 12.47V. To verify that the changes have taken

place, simply read the voltage again:

>AT RV

12.5V

The ELM327 always rounds off the measurement

to one decimal place, so the 12.47V actually appears

as 12.5V (but the second decimal place is maintained

internally for accuracy and is used in the calculations).

The ELM327 may be calibrated with any reference

voltage that you have available, but note that the CV

command always expects to receive four characters

representing the voltage at the input. If you had used a

9V battery for your reference, and it is actually 9.32V,

then you must add a leading zero to the actual voltage

when calibrating the IC:

>AT CV 0932

If you should get into trouble with this command

(for example, if you set calibration values to something

arbitrary and do not have a voltmeter on hand to

provide accurate values), you can restore the settings

to the original (factory) values with the CV 0000

command. Simply send:

>AT CV 0000

The other AT Commands are used in the same

manner. Simply type the letters A and T, then follow

with the command you want to send and any

arguments that are required. Then press return (or

enter, depending on your keyboard). Remember - you

can always insert space characters as often as you

wish if it improves the readability for you, as they are

ignored by the ELM327.

ELM327P Datasheet(PDF) 27 Page - ELM Electronics

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