ELM329LP datasheet(73/87 Pages) ELM

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ELM329L

ELM329L DSA

Elm Electronics – Circuits for the Hobbyist

www.elmelectronics.com

A very common question that we receive is ‘Can I

connect the ELM329 directly to my own circuit, or must

I use the interface shown?’ Certainly you may connect

directly to our ICs, and you do not need to use an

RS232 or USB interface. There are a few items to

consider, however.

The ELM329 is actually a microprocessor that

contains a standard UART type interface, connected to

the RS232 Tx and Rx pins. The logic type is CMOS,

and the Rx pin has Schmitt trigger waveshaping, so

you should be able to connect directly to these pins

provided that the two devices share the same power

supply (VDD), and that they are not physically more

than about 10 to 20 inches apart (CMOS circuits are

subject to latch-up from induced currents, which may

be a problem if you have long leads).

The normal (idle) levels of the ELM329 transmit

and receive pins are at the VDD (logic high) level. Most

microprocessors and RS232 interface ICs expect that

to be the idle level, but you should verify it for your

microprocessor before connecting to the ELM329. The

connections are straightforward - transmit connects to

receive, and receive connects to transmit, as shown

below. Don’t forget to set both devices to the same

baud rate.

The ELM329 also provides a hand-shaking feature

that may simplify the flow of data for you. The interface

consists of two pins - an input an an output. The input

is called ‘request to send’ (RTS), and it is used to

interrupt the ELM329, just the same as tapping a key

on the keyboard when using a terminal program. The

output pin (‘Busy’) is used by the ELM329 to tell your

system that it is processing data.

To use the handshaking feature, set one of your

microprocessor port pins to normally provide a high

output, and connect it to the RTS input (pin 15). Use

another port pin as an input to monitor the ELM329

Busy output (pin 16). When you want to send a

command, simply check the Busy output first. If it is at

a high logic level, then either wait for it to go low, or if

you need to interrupt the IC, then bring the RTS line

low and wait for the Busy line to go low. (You might

want to consider using an edge triggered interrupt on

the Busy output, if one is available). When Busy does

go low, restore your RTS line to a high level, and then

send your command to the ELM329. No need to worry

about the ELM329 becoming busy again after you

raise the RTS line at this point – once Busy goes low,

the ELM329 will wait (indefinitely) for your command. If

you do not use the RTS input on the ELM329, it must

be connected to a high logic level, as shown. Note that

the default setting for PP 0E turns these hand-shaking

Microprocessor Interfaces

VDD

+5V

329L

your microprocessor

The ELM329L and your

microprocessor must

use the same VDD supply

Busy RTS

VDD

ELM329LP Datasheet(PDF) 73 Page - ELM Electronics

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