ELM329L
Pin Descriptions (continued)
RS232Rx (pin 18)
CAN Tx (pin 23) and CAN Rx (pin 24)
This is the RS232 receive data 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 wave shaping.
These are the two CAN interface signals that must
be connected to a CAN transceiver IC (see the
Example Applications section for more information).
If unused, pin 24 must be connected to a logic high
(VDD) level.
VSS (pin 19)
RS232 Rx LED (pin 25), RS232 Tx LED (pin 26),
OBD Rx LED (pin 27) and OBD Tx LED (pin 28)
Circuit common must be connected to this pin.
These four output pins are normally high, and are
driven to low levels when the ELM329 is transmitting
or receiving data. These outputs are suitable for
directly driving most LEDs through current limiting
resistors, or interfacing to other logic circuits. If
unused, these pins may be left open-circuited.
VDD (pin 20)
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.
Note that pin 28 can also be used to turn off all of the
Programmable Parameters, if you can not do so by
using the normal interface - see page 66 for more
details.
M1 (pin 21) and M0 (pin 22)
These two output pins are provided for use with CAN
transceiver ICs, as are typically used for Single Wire
CAN applications. The ELM329 will set both outputs
to a high level (‘Normal’) after startup, but the level at
these pins may be changed at any time with the
AT TM commands, and the level after powerup may
be set with PP20.
Unused Pins
When people only want to implement a portion of what the ELM329L is capable of, they often ask what to do with
the unused pins. The rule is that unused outputs may be left open-circuited with nothing connected to them, but
unused inputs must always be terminated. The ELM329L is a CMOS integrated circuit that can not have any inputs
left floating (or you might damage the IC). Connect unused inputs as follows:
Pin
1
2
*
5
*
7
*
11
*
12
*
13
*
15
H
18
H
24
H
Level
H
The inputs that are shown with an asterisk (*) may be connected to either a High (VDD) or a Low (VSS) level.
ELM329L DSA
Elm Electronics – Circuits for the Hobbyist
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