Operation Manual

Example Applications (continued)
82 of 94ELM327DSJ Elm Electronics – Circuits for the Hobbyist
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ELM327
the rest.
We do caution that the MAX3222E does seem to
place some extra demands on the 5V power supply. It
should work fairly well with the 7805 regulator that we
show in Figure 9, but if you have chosen a lower
current device like the 78L05 or LP2950, you may
experience occasional ‘LV RESET’s. If you do, we
recommend adding two capacitors to the circuit of
Figure 9. We suggest that a 10 µF 50V capacitor be
connected in parallel with C1, and also that a 33 µF
10V unit be installed across C5 (the 5V ‘rail’). This
should eliminate problems, but if it does not, you may
also wish to consider a more powerful regulator (such
as the 7805 that we show in Figure 9) if you do not
already have one installed.
The USB interface of Figure 13 provides another
way to connect the ELM327 to USB systems. It uses a
Silicon Laboratories (www.silabs.com) CP2102 chip to
convert between the ELM327’s serial data and USB.
One of the advantages of going to a USB interface
is the high serial baud rates that you may experience.
In order to use these higher rates, you will have to
program both the USB interface and the ELM327
interface for them.
The CP2102 interface baud rate is actually
configured by driver software. When you set the baud
rate in your terminal program, the software does what
is necessary to configure the CP2102 for operation at
that rate and you do not need to do anything more.
The ELM327 initially only uses a 38,400 bps rate,
however, and must be told to use anything different.
In order to change from the standard 38.4 k baud
rate, you must first set your software to 38.4 kbps, and
power up your ELM327 circuit. Make sure that it is
working, as described in the ‘Communicating with the
ELM327’ section, before you do anything else. When
you are confident that all is well, you can then change
the baud rate. Before doing so, we caution that you
should check to be sure that your software actually
supports the desired rate (as several can not handle
more than about 250 kbps).
The CP2102 chip is able to support a 115.2 kbps
rate natively, and almost all software should be able to
support it as well, so we will use that rate to provide an
example.
First, while connected to the ELM327 at 38.4 kbps,
we need to change the default rate to 115.2 kbps. No
need to worry that this will affect your communications,
as it will not take effect until the ELM327 has been
reset. To change the data rate, simply change the
Figure 12. An High Speed
RS232 Interface (≤ 250 kbps)
2 (RxD)
3 (TxD)
5 (SG)
1 (DCD)
4 (DTR)
6 (DSR)
7 (RTS)
8 (CTS)
0.1µF
1
2
3
4
5
6
7
8
9
RS232
Interface
(DB9F)
+5V
0.1µF
10
11
12
13
14
15
16
17
18
+5V
15
16
17
18
327
0.47µF
0.47µF
0.47µF
+5V
15
16
17
18
19
327
2 (D-)
USB
Interface
(type ‘B’
connector)
+5V
1 (+5)
4 (SG)
3 (D+)
SiLabs
CP2102
0.1µF
5.0V
TVS
1µF
7
6
26 25
3
4
5
8
Figure 13. An Alternative USB Interface