ELM327
Elm Electronics – Circuits for the Hobbyist
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Pin Descriptions
2 of 5
All rights reserved. Copyright 2005 Elm Electronics Inc.
Every effort is made to verify the accuracy of information provided in this document, but no representation or warranty can be
given and no liability assumed by Elm Electronics with respect to the accuracy and/or use of any products or information
described in this document. Elm Electronics will not be responsible for any patent infringements arising from the use of these
products or information, and does not authorize or warrant the use of any Elm Electronics product in life support devices and/or
systems. Elm Electronics reserves the right to make changes to the device(s) described in this document in order to improve
reliability, function, or design.
MCLR (pin 1)
A logic low applied to this input will reset the IC. If
unused, this pin should be connected to a logic high
(VDD) level.
Vmeasure (pin 2)
This analog input is used to measure a 0 to 5V
signal that is applied to it. Care must be taken to
prevent the voltage from going outside of the supply
levels of the ELM327, or damage may occur.
J1850 Volts (pin 3)
This output can be used to control a voltage supply
for the J1850 Bus + output. The pin will output a
logic high level when a nominal 8V is required (for
J1850 VPW), and will output a low level when 5V is
needed (as for J1850 PWM applications). If this
switching
capability
is
not
required
for
your
application, this output can be left open-circuited.
J1850 Bus+ (pin 4)
This active high output is used drive the J1850 Bus +
Line to an active level. Note that this signal does not
have to be used for the Bus - Line (as was the case
for the ELM320), since a separate J1850 Bus - drive
output is provided on pin 14.
Memory (pin 5)
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 is enabled by default.
If it is at a low level, then the default will be to have it
disabled. Memory can always be controlled with the
AT M1 and AT M0 commands at other times.
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. If at a low
level, the baud rate will be 9600.
LFmode (pin 7)
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
ELM327 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 (see the section on AT
Commands).
VSS (pins 8 and 19)
Circuit common must be connected to these pins.
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 normally be
connected between each of these pins and circuit
common (Vss).
VPW In (pin 11)
This is the active high input for the J1850 VPW data
signal. When at rest (bus recessive) this pin should
be at a low logic level. This input has Schmitt trigger
waveshaping, so no special amplification is required.
ISO In (pin 12)
This is the active low input for the ISO 9141 and
ISO 14230 data signal. It is derived from the K Line,
and should be at a high logic level when at rest (bus
recessive). No special amplification is required, as
this input has Schmitt trigger waveshaping.
PWM In (pin 13)
This is the active low input for the J1850 PWM data
signal. It should normally be at a high level when at
rest (ie. bus recessive). This input has Schmitt
trigger waveshaping, so no special amplification is
required.
ELM327QSA
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