Bedienungsanleitung Elm 327 (Seite 77 von 94) (Englisch)

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ELM327

ELM327DSJ Elm Electronics – Circuits for the Hobbyist

www.elmelectronics.com

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

signals off, so you will need to change that in order to

use them.

We are often asked about connecting the ELM327

to 3.3V logic, which is quite popular. Many hope that

they can just insert a resistor or two and have it work.

Unfortunately, that is not the case, mostly because the

ELM327 has Schmitt waveshaping on the RS232 Rx

input (pin 18), and so may need as much as 4V for a

high input (although it often works with less). To

ensure that there’s enough voltage, we recommend

using a level translator circuit such as the TXB0102

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

from ST Microelectronics (www.st.com), or the Analog

Devices (www.analog.com) ADuM1201 as shown

here. We have been using the ADuM1201 with the

Raspberry Pi lately, because it offers galvanic isolation

(to 2500 Vrms) in addition to the level translation, so

protects the Pi from occasional wiring problems. Note

that for complete isolation you must use separate

commons for the 5V and the 3.3V power supplies. The

one disadvantage with the ‘1201 over the others is that

it draws about 1 mA, which can be an issue if you are

trying to use the low power mode with your ELM327.

A popular question that we receive is “Can I

upgrade my firmware with a file download?”. The

answer to this is no, the ELM327 can not be upgraded

in this way - your integrated circuit must be replaced.

The next question which usually follows is “Can I

simply replace an old ELM327 chip with a new one to

upgrade the firmware?”

The answer to this last question is basically yes.

We say basically because there was a change

between versions 1.3a and 1.4b when we added the

power control feature. This change modified how pins

15 and 16 were used (they took on dual roles), and

that may affect your circuit.

If your circuit board is older and used an earlier

version of our chip (ie pre v1.4b), then you have to

look at what pins 15 and 16 were used for. Almost all

of the early boards followed our example and left pin

16 open, and pin 15 tied to +5V. If your circuit did this,

there is no problem - you may simply replace the old

chip with a new one.

If your circuit board used pins 15 and 16 for

handshaking with a microprocessor or other device,

then you may need to take extra steps. Usually this

only requires turning off the low power control by

setting b7 of PP 0E to 0.

If you are replacing a v1.4b or a v2.0 integrated

circuit with a newer one, then there are no concerns.

Please note that there are several clone products

available that use integrated circuits that mimic the

ELM327 at a software level. They do not necessarily

mimic the hardware. For this reason, you should not

replace a clone chip with a genuine ELM chip, unless

you are absolutely certain that all 28 pins of the

integrated circuits are identical.

+3.3V

+5V

n.c.

+5V

your microprocessor

ADuM1201

Busy

RTS

327

+5V

+3.3V

Microprocessor Interfaces (continued)

Upgrading Versions

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