ELM327P datasheet(59/76 Pages) ELM

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ELM327DSH

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ELM327

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.

Most people will construct their ELM327 circuits

with an RS232 interface, mainly because it is relatively

easy and inexpensive to do. A circuit such as the one

shown in Figure 9 requires very few components and

works extremely well at speeds of up to 57600 bps.

Depending on your interface’s RS232 voltages, your

wiring practices, and your choice of components, it

may also work well at speeds as high as 115200 bps,

but that is the approximate limit of such a circuit, and

any design using this speed should be thoroughly

tested.

Users that would like to operate at speeds in the

range of 115200 bps or higher may wish to look at

some of the single IC solutions that are available.

These include

devices such as the ADM232A from

Analog

Devices

(http://www.analog.com/),

the

popular MAX232 series of ICs from Maxim Integrated

Products (http://www.maxim-ic.com/). These are all

excellent devices that can be used for speeds of up to

115.2 kbps. We do caution that many of these types of

devices are only rated for operation up to 120 kbps,

however, so may not be suitable for higher 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 higher rates

with 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

ELM327 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 500 kbps limit of

the ELM327. Several manufacturers offer special

‘bridge’ circuits that simplify connecting an RS232

device (such as the ELM327) directly to the USB bus.

Examples

are

the

CP2102

from

Silicon

Labs

(http://www.silabs.com/) or the FT232R from Future

Technology

Devices

(http://www.ftdichip.com/).

planning to use the higher baud rates (ie up to 500

kbaud), these 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 64 for more details.

The default configuration for the ELM327 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 (beginning with v1.2 of

the IC). 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 store a new baud rate which

replaces the 38.4 kbps high speed rate. The value is

stored in ‘non-volatile’ memory (EEPROM) that 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. (If

it has not been enabled, the rate will revert to the

factory default of 38.4 kbps.) As an example, perhaps

you would like to have the ELM327 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

ELM327 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 (until PP 0C is turned off), 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,

ELM327P Datasheet(PDF) 59 Page - ELM Electronics

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