ELM327LSM datasheet(83/94 Pages) ELM

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Example Applications (continued)

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ELM327L DSA

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ELM327L

internal charge pump circuitry that generates the

voltages required for RS232 communications, in

addition to the analog interface circuitry needed. All

you have to do is provide a few capacitors, and it does

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,

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

327

0.47µF

+5V

327

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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