Operation Manual

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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
OK
> AT PP 0C ON
OK
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
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+5V
n.c.
+5V
Tx
Rx
ADuM1201
Busy
RTS
327
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+5V
Tx
+3.3V
3.3V common
Rx
3.3V
Interface