ELM329
Pin Descriptions
MCLR (pin 1)
A momentary (>2µsec) logic low applied to this input
will reset the ELM329. If unused, this pin should be
connected to a logic high (VDD) level.
that has been set by PP 0C). If at a low level, the
baud rate will always be 9600 bps.
LFmode (pin 7)
Vmeasure (pin 2)
This input is used to select the default linefeed mode
to be used after a power-up or system reset. If it is at
a high level, then by default messages sent by the
ELM329 will be terminated with both a carriage
return and a linefeed character. If it is at a low level,
lines will be terminated by a carriage return only.
This behaviour can always be modified by issuing an
AT L1 or AT L0 command.
This analog input is used to measure a 0 to 5V
signal that is applied to it. The value measured is
scaled by a factor of about 5.7 and may be displayed
using the AT RV command.
Care must be taken to prevent the voltage from
going outside of the supply levels of the ELM329, or
damage may occur. If it is not used, this pin should
be tied to either VDD or VSS.
VSS (pin 8)
Active LED (pin 3)
Circuit common must be connected to this pin.
This output pin is normally at a high level, and is
driven to a low level when the ELM329 has
determined that it has found a valid (active) protocol.
The output is suitable for directly driving most LEDs
through a current limiting resistor, or interfacing to
other logic circuits. If unused, this pin may be left
open-circuited.
XT1 (pin 9) and XT2 (pin 10)
A 4.000 MHz oscillator crystal is connected between
these two pins. Loading capacitors as required by
the crystal (typically 27pF each) will also need to be
connected from each of these pins to circuit common
(Vss).
Note that the behaviour of this pin when the ELM329
is in the low power mode may be modified by the
logic level at pin 11, or with PP 0F, bit 4.
When laying out a printed circuit board, you may
wish to consider placing a guard ring around the
oscillator crystal, pins, and capacitors, to provide a
little isolation between them and the other signals
(particularly the pin 11 CAN input).
Control (pin 4)
The level at this output may be directly controlled
through AT commands. After any reset (powerup,
AT Z, etc.), the output reverts to low level.
Note that this device has not been configured for
operation with an external oscillator, and it expects a
crystal to be connected to these pins. Use of an
external clock source is not recommended. Also,
note that this oscillator is turned off when in the low
power or ‘standby’ mode of operation.
Pin 4 may also be used to provide an output signal
that follows the internal CAN monitor output, by
setting bit 0 of PP 0F to 1. This is a new feature with
v2.0, and is not available with v1.0 ICs.
CAN Monitor and A LED (pin 11)
Memory (pin 5)
This input serves two functions. If a CAN signal is
detected at this pin, the ELM329 assumes that you
wish it to monitor that signal, and possibly control the
low power operation based on it.
This input controls the default state of the memory
option. If this pin is at a high level during power-up or
reset, the memory function will be enabled by
default. If it is at a low level, then the default will be
to have it disabled. Memory can always be enabled
or disabled with the AT M1 and AT M0 commands.
If no CAN signal is detected, the ELM329 assumes
that you are using this pin to control the behaviour of
the Active LED output when the IC is in the low
power mode (as it did for v1.0 ICs). In this mode, if
pin 11 is at a high level when low power operation
begins, the Active LED output will flash for about
16 msec every 4 seconds. If the input is at a low
level when low power operation begins, the
Baud Rate (pin 6)
This input controls the baud rate of the RS232
interface. If it is at a high level during power-up or
reset, the baud rate will be set to 38400 (or the rate
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ELM329DSB
Elm Electronics – Circuits for the Hobbyist
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