ELM325
Pin Descriptions
VDD (pin 1)
RS232 Tx (pin 6)
This pin is the positive supply pin, and should always
be the most positive point in the circuit. Internal
circuitry connected to this pin is used to provide
power on reset of the microprocessor, so an external
reset signal is not required. Refer to the Electrical
Characteristics section for further information.
This is the RS232 (serial) data transmit output. The
signal level is compatible with most interface ICs (the
output is high when idle), and there is sufficient
current drive to allow interfacing using only a PNP
transistor, if desired.
RS Rx LED (pin 7), RS Tx LED (pin 8),
J Rx LED (pin 9) and J Tx LED (pin 10)
XT1 (pin 2) and XT2 (pin 3)
A 3.579545 MHz oscillator crystal (often known
simply as a ‘3.58 MHz’ crystal) is to be connected
between these two pins. Loading capacitors as
required by the crystal (typically 27pF) also need to
be connected from each of these pins to circuit
common (Vss). A crystal is preferred, but you may
also use a ceramic resonator.
These four pins normally output a high level, and are
driven low when the ELM325 is transmitting or
receiving data. The internal circuitry is suitable for
directly driving most LEDs through current limiting
resistors, or interfacing to other logic circuits. When
using lower VDD levels, the LED should be chosen so
that the forward voltage drop is not more than the
supply level. If unused, these pins should be left
open-circuited.
Note that this device has not been configured for
operation with an external oscillator, and it expects a
crystal or resonator to be connected between these
pins. Use of an external clock source is not
recommended.
DE (pin 11)
This is the J1708 transmit data output, which should
be connected to the DE pin on an RS485 transceiver
integrated circuit. The polarity of this signal may be
changed by changing the level on the InvDE pin.
InvDE (pin 4)
This input is used to invert the output at the DE pin.
This allows the ELM325 to be connected to a variety
of RS485 interface circuits without the need for an
external inverter.
RE (pin 12)
This is a receiver enable output, which may be used
to enable or disable the RS485 receiver. For some
circuits this might result in a substantial reduction in
current, if the receiver is not being used.
Normally, the InvDE pin is connected to a high (VDD)
level. This causes the DE output (pin 11) to be at a
low level during idle, and go to a high level for the
start bit (ie. when active). This is required for most
RS485 interface ICs such as the 75176, DS485,
LTC485, ADM485, MAX485, and SN65HVD3080E.
If you are using a device that requires an active low
DE output (such as the DS36277), then connect the
InvDE pin to a low level.
This pin is set to a low level on powerup, or ELM325
reset. The level may be controlled with the AT RE0
and RE1 commands. If unused, this pin should be
left open-circuited.
RO (pin 13)
This is the J1708 receive data input, and should be
connected to the receive output (RO) pin of the
RS485 transceiver IC. This is a standard CMOS
input that accepts TTL level signals.
RS232 Rx (pin 5)
This is the RS232 (serial) data receive input. The
signal level is compatible with most interface ICs
(when at idle, the level should be high), but can be
used with other interfaces as well, since the input
has Schmitt trigger input wave shaping.
VSS (pin 14)
Circuit common must be connected to this pin.
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ELM325DSA
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