ELM329LP datasheet(78/87 Pages) ELM

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Example Applications (continued)

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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 a 7805 regulator, but if you

have chosen a lower current device like the 78L05 or

LP2950 (as in Figure 9), you may experience

occasional

‘LV

RESET’s.

For

this

reason,

recommend adding the larger capacitors shown as C2

and C4 in Figure 9. This should eliminate problems,

but if it does not, you may also wish to consider a

more powerful regulator (such as the 7805) if you do

not already have one installed.

The USB interface of Figure 13 provides another

way to connect the ELM329 to USB systems. It uses a

Silicon Laboratories (www.silabs.com) CP2102 chip to

convert between the ELM329’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 ELM329

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 ELM329 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 ELM329 circuit. Make sure that it is

working, as described in the ‘Communicating with the

ELM329’ 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 ELM329 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 ELM329 has been

reset. To change the data rate, simply change the

value that is stored in Programmable Parameter 0C,

then enable it:

Figure 12. A High Speed

RS232 Interface (

≤ 250 kbps)

2 (RxD)

3 (TxD)

5 (SG)

1 (DCD)

4 (DTR)

6 (DSR)

7 (RTS)

8 (CTS)

0.1µF

RS232

Interface

(DB9F)

+5V

0.1µF

+5V

329

0.47µF

+5V

329

2 (D-)

USB

Interface

(type ‘B’

connector)

+5V

1 (+5)

4 (SG)

3 (D+)

SiLabs

CP2102

0.1µF

5.0V

TVS

1µF

26 25

Figure 13. An Alternative USB Interface

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