2–101
Motorola Sensor Device Data
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Prepared by Brandon Loggins
Automobile manufacturers require all system electronics to
pass stringent electromagnetic compatibility (EMC) tests.
Airbag systems are one of the systems that must perform
adequately under EMC tests. There are different types of tests
for EMC, one of which is testing the tolerance of the system
to high frequency conducted emissions. One of the most
stringent methods for EMC evaluation is the Bulk Current
Injection (BCI) test. The entire airbag system must continue to
function normally throughout the BCI test. This application
note will discuss how to reduce susceptibility to BCI for the
Motorola accelerometer but the information presented here
can be applied to other electronic components in the system.
BCI TEST SETUP
The BCI test procedure follows a published SAE engineering
standard, “Immunity to Radiated Electric Fields ~ Bulk Current
Injection (BCI)”, or SAE J 1113/401. For an airbag module, this
involves injecting the desired current into the wiring harness by
controlling current in the injection probe. The test frequency can
vary from one to several hundred MHz. There are at least 20
frequency steps per octave required for the test, but as many
as 50 steps per octave can be used. The injection probe is
placed on the harness in one of three distances from the airbag
module connector: 120, 450 and 750 mm. There is a monitor
pickup probe present to measure the amount of current being
injected. It is placed 50 mm from the airbag module. This feeds
back to the system to ensure that the desired test current is
being injected on to the wiring harness. Figure 1 shows the
setup for the BCI test. (For more details, see the SAE J
1113/401 Test Procedure).
Figure 1. BCI Test Setup
ANECHOIC CHAMBER WALL
PC
LOAD BOX
INJECTION
PROBE
PICKUP
PROBE
WIRING HARNESS
AIRBAG
MODULE
70, 450, or 750 mm
50 mm
BASEPLATE CONNECTED
TO GROUND
The harness connects the airbag module to a load box. This
load box provides simulated loads for terminating the
remainder of the airbag system (firing ignitors, etc.). The data
coming back is translated from J1850 to RS232 to be
communicated to a dummy terminal on a PC. For safety
reasons, this test is typically performed inside an anechoic
chamber to shield high frequency emissions from equipment
and humans.
BCI TEST PROCEDURE FOR THE
MMA2202D ACCELEROMETER
The accelerometer is evaluated in the following manner. In
an airbag system, the microcontroller’s A/D converter digitizes
the accelerometer output. The microcontroller sends this
value to the communication ASIC which translates the logic
from board level logic to RS232, then sends the value back
SEMICONDUCTOR APPLICATION NOTE
REV 2
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Freescale Semiconductor, Inc.
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