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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.
7
9
+3.3V
+5V
n.c.
+5V
your microprocessor
Rx
Rx
ADuM1201
Busy
RTS
327
8
1
2
3
4
5
6
7
8
+5V
+3.3V
Microprocessor Interfaces (continued)
Upgrading Versions
77


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