Bedienungsanleitung Elm 327 (Seite 83 von 94) (Englisch)

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ELM327

ELM327DSJ Elm Electronics – Circuits for the Hobbyist

www.elmelectronics.com

Example Applications (continued)

value that is stored in Programmable Parameter 0C,

then enable it:

>AT PP 0C SV 23

> AT PP 0C ON

That is all that is needed to semi-permanently

change the ELM327. We say semi, because it stays in

effect through power downs, and resets, but you can

change it again if you wish. If you now reset the

ELM327 (send AT Z or power down then up), the

ELM327 will begin operating at the new rate (of

115200 bps, rather than to 38400 bps). Change your

software setting to also be 115200 bps, and you

should be communicating. If you go through the

calculations, you will note that the ELM327 baud rate

is actually off by about 0.8%, but modern UARTs can

typically handle rate errors of a few % without any

problems.

When working with the CP2102, we do caution

that it is very small and difficult to solder by hand, so

be prepared for that. Also, if you provide protection on

the data lines with transient voltage suppressors

(TVS’s), be careful when choosing devices, as some

exhibit a very large capacitance and will affect the

transmission of the USB data.

Our final circuit (Figure 14) shows one way to

interface the ELM327 to circuits that operate at a

different voltage level. We show 3.3V as an example,

but it can actually be anything from 2.7V to 5.5V.

The circuit uses the ADuM1201 iCoupler chip from

Analog Devices (www.analog.com). In addition to

acting as a level translator, this device also provides

isolation (galvanic, to 2500 Vrms) between the two

sides. This is often desired in order to keep the vehicle

circuit completely separate from the computer circuit.

Typically, one might use a standard level shifter IC

to interface to 3.3V – for example the TXB0102 by

Texas Instruments (www.ti.com), or the ST2129 from

ST Microelectronics (www.st.com), but the ADuM1201

offers several other advantages. The main difference

is that it offers isolation, as mentioned, but it also can

be used with 2.7V to 5.5V on either side, it provides a

high output if the input side is unpowered, and it

typically uses less current and is much faster than

many opto-isolator solutions (the ‘1201 minimum data

is 1 MHz). Of course there are a couple disadvantages

too. It does use current (almost 1 mA), so may be an

issue if trying to reduce sleep current to a minimum,

and it does cost more than devices like the TXB0102.

Many wireless modules (WiFi or Bluetooth®) use

serial interfaces just like what we have shown here for

the RS232 connections. Connecting to them should

not be very difficult if you follow the manufacturers

directions (and perhaps consider using devices like the

ADuM1201 or the TXB0102). If you are considering

using a Bluetooth interface, you might read our ‘AN-04

ELM327 and Bluetooth®’ application note first.

This has provided a few examples of how the

ELM327 integrated circuit might typically be used.

Hopefully it has been enough to get you started on

your way to many more. The following section shows

how you might be able to optimize these circuits to

reduce power consumption…

Figure 14. Connecting to a 3.3V System

+5V

n.c.

+5V

ADuM1201

Busy

RTS

327

+5V

+3.3V

3.3V common

3.3V

Interface

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