ELM329LP datasheet(46/87 Pages) ELM

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ELM329L DSA

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ELM329L

Using Higher RS232 Baud Rates

The RS232 serial interface has been maintained

throughout the ELM OBD products, largely due to its

versatility. Older computers, microprocessors and

PDAs can use it directly, as can USB, Bluetooth,

ethernet and wifi devices. It is simply one of the most

versatile interfaces available. Depending on the type of

interface that you use, you may wish to adjust the

ELM329 baud rate in order to take full advantage of

the speed available.

RS232 interfaces are demonstrated later in this

data sheet (Figures 11 and 12). Note that while a

MAX3222E is shown in Figure 12, there are many

other single IC solutions that are available. For

example, the popular MAX232 series of ICs is also

available

from

Maxim

Integrated

Products

(http://www.maxim-ic.com/) and there are devices

such

the

ADM232A

from

Analog

Devices

(http://www.analog.com/) which are popular. These are

all excellent circuits that can be used for higher speed

connections. We do caution that many of these

devices are only rated for operation up to 120 kbps,

however, so may not be suitable for very high data

rates - be sure to check the manufacturers data sheet

before committing to a design.

An RS232 interface needs relatively large voltage

swings, which are difficult to maintain at high data

rates when there are large cable capacitances to

contend with. (A typical interface is often limited to

about 230.4 kbps under ideal conditions.) If you need

to operate the ELM329 at these speeds or higher, it is

recommended that you consider alternatives.

One popular alternative is a USB data connection.

The USB interface is capable of very high data transfer

rates, certainly much higher than the ELM329 is

capable

of.

Several

manufacturers

offer

special

‘bridge’ circuits that simplify connecting an RS232

device (such as the ELM329) directly to the USB bus.

Examples

are

the

CP2102

from

Silicon

Labs

(http://www.silabs.com/) and the FT232R or DB9-USB

module from Future Technology Devices (see their

web site at http://www.ftdichip.com/). If planning to use

the higher baud rates, USB interfaces are essential.

We are often asked if it is possible to use a direct

connection to a microprocessor. That is certainly an

option, and one that allows a full speed connection at

essentially zero cost. If you are developing such an

interface, refer to page 73 for more information.

The default configuration for the ELM329 provides

an RS232 data rate of either 9600 baud, or 38400

baud, depending on the voltage level at pin 6 during

power up or reset. While the 9600 baud rate is not

adjustable, the 38400 one is. There are two ways that

the rate can be changed – either permanently with a

Programmable Parameter, or temporarily with an AT

command.

Programmable Parameter ‘0C’ is the memory

location that allows you to permanently store a new

baud rate which replaces the 38.4 kbps high speed

rate. The value is stored in EEPROM and is not

affected by power cycles or resets (but changing this

value may affect the operation of some software

packages, so be careful how you use it).

If you store a new value in PP 0C, then enable it,

and if pin 6 is at a high level during the next powerup,

then your stored rate will become the new data rate.

As an example, perhaps you would like to have the

ELM329 use a baud rate of 57.6 kbps, rather than the

factory setting of 38.4 kbps. To do this, determine the

required value for PP 0C, store this value in PP 0C,

and then enable the PP.

The value stored in PP 0C is actually an internal

divisor that is used to determine the baud rate (it will

be 4000 kbps divided by the value of PP 0C). To

obtain a setting of 57.6, a baud rate divisor of 69 is

required (4000/69 is approximately 57.6). Since 69 in

decimal is 45 in hexadecimal, you need to tell the

ELM329 to set the value of PP 0C to 45, with this

command:

>AT PP 0C SV 45

then enable the new value for use:

>AT PP 0C ON

from that point on, the default data rate will be 57.6K,

and not 38.4K. Note that the value that you write does

not become effective until the next full reset (a power

off/on, AT Z, or MCLR pulse).

If you are designing your own circuitry, you will

know what your circuit is capable of, and can assign a

value to PP 0C. Software developers will not usually

know what hardware is to be connected, however, so

will not know what the limitations are. For these users,

we have provided the BRD command.

This command allows a new baud rate divisor to

be tested, and then accepted or rejected depending on

the results of the test. See the chart at the right, which

shows how the command works.

As can be seen, the software (PC) first makes a

request for a new baud rate divisor, using this AT

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