ELM327P datasheet(7/51 Pages) ELM

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ELM327

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AT Commands (continued)

AT0, AT1 and AT2

[ Adaptive Timing control ]

When receiving responses from a vehicle, the

ELM327 has traditionally waited the time set by the

AT ST hh setting for a response. To ensure that the IC

would work with a wide variety of vehicles, the default

value was set to a conservative (slow) value. Although

it was adjustable, many people did not have the

equipment or experience to determine a better value.

The new Adaptive Timing feature will automatically

set the timeout value for you, based on the actual

response times that your vehicle is responding in. As

conditions such as bus loading, etc. change, the

algorithm learns from them, and makes appropriate

adjustments. Note that it always uses your AT ST hh

setting as a maximum setting, however. With this new

Adaptive Timing, sampling rates are often doubled or

tripled from those typically experienced with prior

versions.

There are three adaptive timing settings that are

available for use. By default, Adaptive Timing option 1

(AT1) is selected, and is the recommended setting.

AT0 is used to disable Adaptive timing (usually used

when experimenting), while AT2 is a more agressive

version of AT1 (the effect is more noticeable for very

slow connections – you may not see much difference

with faster OBD systems). The J1939 protocol does

not support Adaptive Timing – responses for J1939

use fixed timeouts as set in the standard.

[ perform an OBD Buffer Dump ]

All messages sent and received by the ELM327

are stored temporarily in a set of twelve memory

storage locations called the OBD Buffer. Occasionally,

it may be of use to view the contents of this buffer,

perhaps to see why an initiation failed, to see the

header bytes in the last message, or just to learn more

of the structure of OBD messages. You can ask at any

time for the contents of this buffer to be ‘dumped’

(printed) – when you do, the ELM327 sends a length

byte (representing the length of the message in the

buffer) followed by the contents of all twelve OBD

buffer locations.

The length byte represents the actual number of

bytes received, whether they fit into the OBD buffer or

not. This may be useful when viewing long data

streams (with

AT AL), as the number accurately

represents the number of bytes received, mod 256.

Note that only the first twelve bytes received are

stored in the buffer.

[ Bypass the Initialization sequence ]

This command should be used with caution. It

allows an OBD protocol to be made active without

requiring any sort of initiation or handshaking to occur.

The initiation process is normally used to validate the

protocol, and without it, results may be difficult to

predict. It should not be used for routine OBD use, and

has only been provided to allow the construction of

ECU simulators and training demonstrators.

BRD hh

[ try Baud Rate Divisor hh ]

This command is used to change the RS232 baud

rate divisor to the hex value provided by hh. The actual

baud rate (in kbps) will be 4000 divided by this divisor.

For example, a setting of 115.2kbps would require a

divisor of 4000/115.2 or 35. In hexadecimal notation,

35 is written as 23, so the actual command that needs

to be sent would be AT BRD 23.

Since the ELM327 may be able to operate at

much higher rates than some interfaces can support,

the BRD command allows requested rates to be tested

before they are committed to (with automatic fall-back

to the previous baud rate if there are problems). In

use, the command is sent to request a change in the

baud rate, and the ELM327 responds with the familiar

“OK”. After that, an internal timer begins waiting, to

ensure that the controlling computer has sufficient time

to change their baud rate to the new rate. The ELM327

then sends the poweron message at the new baud

rate, and begins waiting while the controlling computer

assesses what has been received. If the AT I message

was received without errors, the controlling computer

sends a carriage return character, and if received by

the ELM327, the rate will be retained. If the controlling

computer sees errors (or worse, nothing), it provides

no response, and switches back to the initial baud

rate. If the ELM327 times out after receiving no

response, or has received something that does not

appear to be a carriage return character, it will revert

back to the former baud rate. A more detailed

discussion of this entire process is provided in the

‘Using Higher RS232 Baud Rates’ section.

Any new baud rate that is set in this manner is

retained across calls to set defaults (AT D), and for

warm starts (AT WS), but will not survive a hardware

reset (a power off/on or a call to AT Z). If you are in the

habit of calling AT Z in your code, we advise using AT

WS instead.

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