ELM329LP datasheet(75/87 Pages) ELM

75 / 87 page

Figure 8. The J1962 Vehicle Connector

75 of 87

ELM329L

ELM329L DSA

Elm Electronics – Circuits for the Hobbyist

www.elmelectronics.com

Example Applications

The SAE J1962 (ISO 15031-3) standard dictates

that all OBD compliant vehicles must provide a

standard connector near the driver’s seat, the shape

and pinout of which is shown in Figure 8 below. The

circuitry described here can be used to connect to this

J1962 plug without modification to your vehicle.

The male J1962 connector required to mate with a

vehicle’s connector may be difficult to obtain in some

locations, and you might be tempted to improvise by

making your own connections to the back of your

vehicle’s connector. If doing so, we recommend that

you do nothing that would compromise the integrity of

your vehicle’s OBD network. The use of any connector

which could easily short pins (such as an RJ11 type

telephone connector) is definitely not recommended.

The circuit on page 76 (Figure 9) shows how the

ELM329 might typically be used. Circuit power is

obtained from the vehicle via OBD pins 16 and 5, and

after a protecting diode and some capacitive filtering,

is presented to a five volt regulator. (Note that a few

vehicles have been reported to not have a pin 5 – on

these you will need to use pin 4 instead of pin 5.) The

regulator powers several points in the circuit as well as

an LED (for visual confirmation that power is present).

We have shown an LP2950 for the regulator as that

type has very low quiescent current which is important

if you are going to use the low power feature of the

ELM329.

Note that there are some rather large capacitors

(C2 and C4) shown on the input and the output of the

regulator.

Without

these

capacitors,

you

might

experience ‘LV RESET’s (for example, a MAX3222E

transceiver creates large transients as it is switched on

and off so needs this). Testing has shown that the

values shown for C2 and C4 should be adequate in

most cases, but if you do see the occasional ‘LV

RESET’, you may want to increase them further.

The FTDI serial to USB converter shown handles

all

the

RS232

communications

between

the

ELM329 and the controlling computer. It is capable of

very high speed communications so is a good choice

for most applications. We have added diode D3 and

resistor R12 to this circuit to reduce backfeeds from

the USB system (without them, the FTDI interface is

able to provide enough current to at least partially

power the ELM329 circuit).

The top left corner of Figure 9 shows the CAN

interface circuitry. We do not advise making your own

interface using discrete components – CAN buses

may have a lot of critical information on them, and you

can easily do more harm than good if you fail. It is

strongly recommended that you use a commercial

transceiver chip as shown. The Microchip MCP2561 is

used in our circuit, but most major manufacturers also

produce CAN transceiver ICs – look at the NXP

PCA82C251, the Texas Instruments SN65LBC031,

the infineon TLE7250G, and the Linear Technology

LT1796, to name only a few others. Be sure to pay

attention to the voltage limits – depending on the

application the IC may have to tolerate 24V, and not

just 12V.

The voltage monitoring circuitry for the AT RV

command is shown connected to pin 2 of the ELM329.

The two resistors (R10 & R11) simply divide the

battery voltage to a safe level for the ELM329, and the

capacitor (C10) filters out noise. As shipped, the

ELM329 expects a resistor divider ratio as shown, and

sets nominal calibration constants assuming that. If

your application needs a different range of values,

simply choose the resistor values to maintain the input

within the specified 0-5V limit, and then perform an AT

CV to calibrate the ELM329 to your new divider ratio.

The maximum voltage that the ELM329 can show is

99.9V.

A 10µF capacitor (C9) is shown connected to pin 6

of the ELM329L. This is required for filtering of the IC’s

internal 3.3V supply. Choose a low ESR (<5

Ω)

ceramic or tantalum type.

The only remaining components are the LEDs and

the crystal. The LEDs are standard ones, and may be

any colour that you require - we only offer suggestions

here. The crystal is a 4.000MHz microprocessor type,

while the 27pF loading capacitors shown are typical

only, (you may have to select other values depending

on what is specified for the crystal that you use). This

crystal frequency is critical to the circuit operation and

must not be altered. Do not substitute a resonator for

the crystal, as it will not have the accuracy required.

As always, you are not limited to the circuit of

Figure 9. It is only a starting point that you can build

on. The following pages show some alternative

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