ELM 329 - Page 78

Example Applications (continued)
ELM329
78 of 83ELM329DSC Elm Electronics – Circuits for the Hobbyist
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Figure 13. An Alternative USB Interface
2 (D-)
USB
Interface
(type ‘B’
connector)
+5V
+5V
1 (+5)
4 (SG)
10
11
12
13
14
15
16
17
18
19
20
21
22
+5V
3 (D+)
SiLabs
CP2102
0.1µF
5.0V
TVS
1µF
7
6
26 25
3
4
5
8
329
Note: The CP2102 baud rate is
set during the COM port
configuration on the PC
during powerup, then the chip will draw enough current
to drop the 5V supply to such a level that the ELM329
is not able to start up. Do not install any pull-down
resistors on pin 18 of the MAX3222! If you build the
circuit of Figure 12, you should not encounter any
problems.
The final circuit (Figure 13) shows how you might
connect the ELM329 to a different USB converter.
While there are a few single IC products on the market
that allow you to connect an RS232 system directly to
USB, we show the CP2102 by Silicon Laboratories
(www.silabs.com) here. It has proven to be fairly easy
to connect and use. These ICs provide a very simple
and relatively inexpensive way to ‘bridge’ between
RS232 and USB, and as you can see, require very few
components to support them.
If using 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 of which ones you choose, as some exhibit a
very large capacitance and will affect the transmission
of the USB data. Note also that the circuit as
presented will operate at a 38400 bits per second rate.
If you want to take full advantage of the speed of the
USB interface, you will need to change PP 0C to
obtain a higher baud rate, and then select the same
rate for your COM port operation.
This has provided some examples of how the
ELM329 might be used in a circuit. Hopefully it has
been enough to get you started on your way to many
more.