ELM327P datasheet(18/51 Pages) ELM

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ELM327

ELM327DSC

Elm Electronics – Circuits for the Hobbyist

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Reading the Battery Voltage

Before proceding to 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. 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

12.6V

Note that we did not use upper case characters in

this example, mostly out of laziness. 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 to the ELM327. Also note that we have shown a

space character (‘ ’) between the ‘at’ and the ‘rv’. This

is only to separate the commands and make them

more readable. You do not have to add spaces, or if

you wish, you can add many spaces – it does not

affect the internal interpretation of the command.

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 too many changes. Perhaps

in this case, you have connected your multimeter, and

find that it reads 12.47V, and you would like the

ELM327 to read the same. Simply calibrate it to that

voltage using the CV command:

>at cv 1247

At this point, the internal calibration values have

been changed, and the ELM327 knows that the

voltage at the input is actually 12.47V. You should not

provide the decimal point in the CV value, as the IC

knows that it should be between the second and the

third digits. 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. Note that the second decimal place is

always maintained internally for accuracy and use in

the calculations, but it is never displayed.

The ELM327 can 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 used a 9V

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

you must add a leading zero to that when calibrating

the IC:

>at cv 0932

The other AT Commands are used in the same

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

with the command you want to send, then any

arguments that are required for that command, and

press return (or enter, depending on your keyboard).

You can place space characters as often as you wish

if it improves the readability for you, as they are

ignored by the ELM327.

ELM327P Datasheet(PDF) 18 Page - ELM Electronics

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