ELM329 Datasheet, PDF (14/76 Pages) ELM Electronics – CAN Interpreter

ELM329
AT Command Descriptions (continued)
This three digit variation of the CM command is used
to provide mask values for 11 bit ID systems (the most
significant bit is always ignored).
Note that a common storage location is used
internally for the 29 bit and 11 bit masks, so an 11 bit
mask could conceivably be assigned with the next
command (CM hh hh hh hh), should you wish to do the
extra typing.
CM hh hh hh hh [ set the CAN ID Mask to hhhhhhhh ]
This command is used to assign mask values for
29 bit ID systems. See the discussion under the
CM hhh command as it is essentially identical, except
for the length. Note that the three most significant bits
that you provide in the first digit will be ignored.
CP hh
[ set CAN Priority bits to hh ]
This command is used to modify the five most
significant bits of a 29 bit CAN ID for sending
messages (the other 24 bits are set with one of the AT
SH commands). Many systems use these bits to
assign a priority value to messages, and to determine
the protocol. Any bits provided in excess of the five
required are ignored, and not stored by the ELM329 (it
only uses the five least significant bits of this byte).
The default value for these priority bits is hex 18,
which can be restored at any time with the AT D
command.
CRA
[reset the CAN Rx Addr]
The AT CRA command is used to restore the CAN
receive filters to their default values. Note that it does
not have any arguments (ie no data).
CRA xyz
[set the CAN Rx Addr to xyz]
Setting the CAN masks and filters can be difficult
at times, so if you only want to receive information
from one address (ie. one CAN ID), then this
command may be very welcome. For example, if you
only want to see information from 7E8, simply send AT
CRA 7E8, and the ELM329 will make the necessary
adjustments to both the mask and the filter for you.
If you wish to allow the reception of a range of
values, you can use the letter X to signify a ‘don’t care’
condition. That is, AT CRA 7EX would allow all IDs
that start with 7E to pass (7E0, 7E1, etc.). For a more
specific range of IDs, you may need to assign a mask
and filter.
CRA wwxxyyzz [set the CAN Rx Addr to wwxxyyzz]
This command is identical to the previous one,
except that it is used to set 29 bit CAN IDs, instead of
11. Sending AT CRA will also reverse the changes
made by this command.
CS
[ show the CAN Status counts ]
The CAN protocol requires that statistics be kept
regarding the number of transmit and receive errors
detected. If there should be a significant number of
errors (due to a hardware or software problem), the
device will go off-line in order to not affect other data
on the bus. The AT CS command lets you see both
the transmitter (Tx) and the receiver (Rx) error counts,
in hexadecimal. If the transmitter should be off (count
>FF), you will see ‘OFF’ rather than a specific count.
CSM0 and CSM1 [ CAN Silent Monitoring off or on ]
The ELM329 was designed to be completely silent
while monitoring a CAN bus. Because of this, it is able
to report exactly what it sees, without colouring the
information in any way. Occasionally (when bench
testing, or when connecting to a dedicated CAN port),
it may be preferred that the ELM329 does not operate
silently (ie you may want it to generate ACK bits, etc.),
and this is what the CSM command is for. CSM1 turns
silent monitoring on (no ACKs), CSM0 turns it off. The
default value is CSM1, but it may be changed with
PP 21.
CV dddd
[ Calibrate the Voltage to dd.dd volts ]
The voltage reading that the ELM329 shows for an
AT RV request can be calibrated with this command.
The argument (‘dddd’) must always be provided as 4
digits, with no decimal point (it assumes that the
decimal place is between the second and the third
digits).
To use this feature, simply use an accurate meter
to read the actual input voltage, then use the CV
command to change the internal calibration (scaling)
factor. For example, if the ELM329 shows the voltage
as 12.2V while you measure 11.99 volts, then send
AT CV 1199 and the ELM329 will recalibrate itself for
that voltage (it will actually read 12.0V due to digit
roundoff). See page 25 for some more information on
how to read voltages and perform the calibration.
ELM329DSB
Elm Electronics – Circuits for the Hobbyist
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