ELM327
Pin Descriptions
MCLR (pin 1)
A momentary logic low applied to this input will reset
the IC. If unused, this pin should be connected to a
logic high (VDD) level.
interface. If it is at a high level during power-up or
reset, the baud rate will be set to 38400 (or the
rate that has been set by PP 0C). If at a low level,
the baud rate will be 9600.
Vmeasure (pin 2)
LFmode (pin 7)
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. If it is
not used, this pin should be tied to either VDD or VSS.
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).
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.
VSS (pins 8 and 19)
Circuit common must be connected to these pins.
XT1 (pin 9) and XT2 (pin 10)
J1850 Bus+ (pin 4)
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 between each of these
pins and circuit common (Vss).
This active high output is used to 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.
Note that this device has not been configured for
operation with an external oscillator – it expects a
crystal to be connected to these pins. Use of an
external clock source is not recommended.
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 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.
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.
Baud Rate (pin 6)
This input controls the baud rate of the RS232
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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.
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