ELM329LSM datasheet(71/87 Pages) ELM

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ELM329L

ELM329L DSA

Elm Electronics – Circuits for the Hobbyist

www.elmelectronics.com

We are occasionally asked what the maximum

data rate is that the ELM329 can handle. This is often

after someone has tried to monitor all data using the

default settings and has received a ‘BUFFER FULL’

error. It is difficult to say exactly what the maximum

rate is, however, as several factors are involved.

The CAN ‘engine’ inside the ELM329 is actually

configured with one receive register that accepts

messages from the CAN data bus, and another that

accepts messages from the first, in a ‘bucket brigade’

fashion. As long as the firmware empties the second

not be any overflow problems with this component.

The ELM329 actually moves the data quickly to

temporary storage, so this is never a problem.

It would be nice if all the firmware had to do was to

empty the second register, and wait for it to fill again,

Maximum CAN Data Rates

but that is not so. It must also check for errors,

possibly queue a CAN response, format the received

message, convert it to ascii, load it into the RS232

transmit

buffer,

any

other

processing

that’s

required, and then prepare for the next message.

These tasks can take a considerable time, depending

on what formatting options you have chosen, and the

baud rate that you select.

The

diagram

below

shows

these

processes

grouped into blocks. The times shown are typical, and

as you can see vary with both the length of the CAN

message and the CAN baud rate.

When a CAN message arrives, the ELM329

moves quickly to move the received bytes from the

receive registers. The data is then formatted (as ASCII

bytes) and placed into the RS232 transmit buffer, for

sending to the controlling processor. As long as

CAN Data

message

next message

message from ECU

11 bit/500 = 220 µsec

29 bit/250 = 520 µsec

min time

11 bit/500 = 16 µsec

29 bit/250 = 32 usec

Gap at 40% Bus Loading:

11 bit/500 = 354 µsec

29 bit/250 = 828 µsec

Moving Data

Processing Data

Sending Data

Rx register is empty -

can accept next message

moves and error checks

500k = 50 µsec

250k = 50 µsec

data has been stored

in the send buffer

formatting & conversion

11 bit ID = 138 µsec

29 bit ID = 156 µsec

total send time is typically (in µsec):

38.4k

115.2k

500k

11 bit ID

5180

1740

400

29 bit ID

6475

2175

500

this is a background task - the

ELM329 can do other things at

the same time

ELM329:

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