ELM327LP Datasheet, PDF (80/94 Pages) ELM Electronics – Automatically searches for protocols

ELM327L
Example Applications (continued)
what you are doing while testing.
The final OBD interface shown is in the bottom left
corner, and is used for the two J1850 standards. The
J1850 VPW standard needs a positive supply of up to
8V while the J1850 PWM needs 5V, so we have
shown a two level supply that can provide both. This
dual voltage supply uses a 317L adjustable regulator
as shown, controlled by the pin 3 output. With the
resistor values given, the selected voltages will be
about 8.0V and 5.5V, which works well for most
vehicles. The 317L is able to maintain regulation with a
minimum of about 2 mA of current, which is provided
by the 470Ω resistor between the output and adjust
pins. A larger 317 regulator will require more current,
and the resistors would need to be scaled down
proportionally. Once suitable voltages have been
generated, they are driven by the Q1-Q2 combination
for the Bus+, and Q3 for the Bus-.
The J1850 VPW input uses a resistor divider,
similar to that which was used for the ISO input.
Typical threshold voltages with the resistors shown will
be about 4.2V (rising) and 2.2V (falling). The J1850
PWM input is a little different in that it must convert a
differential input into a single-ended one for use by the
ELM327. This is done by connecting Q4 across the
input so that it operates as a difference amplifier. The
Q4-D3 series combination sets a threshold voltage of
about 1V (for improved noise immunity), while R11
limits the current flow, and R12 makes sure that Q4 is
off when it should be. The circuit works well as shown,
but the R14 passive pullup time constant can be easily
affected by stray capacitance - be aware of this if
connecting test equipment to pin 13.
Resistor R10 is the final J1850 component. We
added this to help discharge the data bus more rapidly
when it was found that some vehicles showed higher
capacitance than others. The resistor may not be
required for many vehicles - the choice is yours. If you
should see reports of BUS ERRORs with a J1850
vehicle, it may be this capacitance causing problems
(you will need to ‘scope the signal to be sure).
Moving on, the R25-R26 voltage divider shown
connected to pin 2 is used for the vehicle voltage
monitoring circuitry. The two resistors simply divide the
battery voltage to a safer level for the ELM327, and
the capacitor (C2) helps to filter out noise. With VDD >
4V, the ELM327L expects a resistor divider ratio as
shown, and sets nominal calibration constants
accordingly. If your application needs a different range
of values, simply choose your resistor values to
maintain the input within the ELM327’s VSS to VDD
voltage range, and then perform an AT CV to calibrate
the ELM327 to your new ratio. The maximum voltage
that the ELM327 can display is 99.9V (it’s a software
limit, not hardware).
Four LEDs are shown on pins 25 to 28. These
have been provided as a visual means of confirming
circuit activity. They are not essential, but it is nice to
see the visual feedback.
On the right side of the circuit, the ELM327’s
RS232 pins (17 and 18) are shown connected to an
FTDI USB module. This module makes it very easy to
connect the ELM327 circuit to your computer - all you
need is the VCP Driver software, which is available for
free from the FTDI web site (www.ftdichip.com). The
module pinout matches a 9 pin D-sub connector, so
you can simply solder it in where the RS232 circuitry
used to go. Diode D5 and resistor R32 have been
added to the interface to prevent the USB supply from
backfeeding into the ELM327L.
A 10µF capacitor is shown connected to pin 6 of
the ELM327L. This is required for filtering of the IC’s
internal 3.3V supply. Choose a low ESR (<5Ω)
ceramic or tantalum type.
Finally, the crystal shown connected between pins
9 and 10 is a standard 4.000MHz microprocessor type
crystal. The 27pF crystal loading capacitors shown are
typical only, and you may have to select other values
depending on what is specified for the crystal that you
use. The crystal frequency is critical to circuit operation
and must not be altered.
We often receive requests for parts lists to
accompany our Example Applications circuits. Since
this circuit is more complex than most, we have named
and numbered all of the components and provided a
summary parts list (Figure 10). Note that these are
only suggestions for parts. If you prefer another LED
colour, or have a different general purpose transistor
on hand, etc., then by all means make the change. A
quick tip for those having trouble finding a 0.3” wide
socket for the ELM327: many of the standard 14 pin
sockets can be placed end-to-end to form one 0.3”
wide 28 pin socket. For more help with building and
testing the circuit, see our ‘AN02 - ELM327 Circuit
Construction’ application note.
What if you only want to support one of the
ELM327’s protocols? Well, you may do that if you
wish. Simply remove the OBD interfaces that you do
not require, and connect the rest. Since you must
never leave a CMOS input floating (open-circuited),
ELM327L DSA
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