ELM327LP datasheet(52/94 Pages) ELM

52 / 94 page

Using Higher RS232 Baud Rates (continued)

* the 75 msec time is adjustable

with the AT BRT hh command

The sequence begins with the PC making a

request for a new baud rate divisor, with the BRD

command. For example, to try the 57.6K rate that was

previously discussed, the controlling PC would send:

AT BRD 45

If the ELM327 firmware is a very old version, it will

not support this command and will return with the

familiar ‘?’ response. If it does support the command, it

will respond with ‘OK’, so the software knows whether

to proceed or not. No prompt character follows the

‘OK’ reply; it is followed only by a carriage return

character (and optionally, a linefeed character).

Having sent the ‘OK’, the ELM327 then switches

to the new (proposed) baud rate, and then simply

waits the time set by the BRT command (it is 75 msec

by default). This period is to allow the PC sufficient

time to change its baud rate. When the time is up, the

ELM327 then sends the ID string (currently ‘ELM327

v2.1’) to the PC at the new baud rate (followed by a

carriage return character and optionally, a linefeed

character) and waits for a response.

Knowing that it should receive the ELM327 ID

string, the PC software compares what was actually

received to what was expected. If they match, the PC

responds with a carriage return character, but if there

is a problem, the PC sends nothing. The ELM327 is

meanwhile waiting for a valid carriage return character

to arrive. If it does (within 75 msec), the proposed

baud rate is retained, and the ELM327 says ‘OK’ at

this new rate. If it does not see a carriage return in the

75 msec ‘window’, the baud rate will revert back to the

original rate. Note that the PC might correctly output

the carriage return at this new rate, but the interface

circuitry could corrupt the character, and the ELM327

might not see a valid response, so your software must

check for an ‘OK’ response before assuming that the

new rate has been accepted.

Using this method, a program can quickly try

several baud rates, and determine the most suitable

one for the connected hardware. The new baud rate

will stay in effect until reset by an AT Z, a Power

off/on, or a MCLR input. It is not affected by the AT D

(set Defaults), or AT WS (Warm Start) commands.

Request for a new

baud rate divisor:

AT BRD hh

ELM327 responds

with ‘OK’

(if it is supported)

ELM327 sends

the AT I string

Program switches to

the new baud rate,

and waits for input

ELM327 waits

up to 75 msec*

for a carriage return

If the Rx is good,

Program sends a

carriage return

ELM327 says ‘OK’

(and remains at the

new baud setting)

Baud rate reverts

to the previous

baud rate

ELM327

received

yes

Print a prompt,

and wait for the

next command

ELM327 switches to

new baud rate and

waits for 75 msec*

ELM327L

52 of 94

ELM327L DSA

Elm Electronics – Circuits for the Hobbyist

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