Issue
155 June 2003
Good
Vibrations
DOWN
TO EARTH
I
was prompted to go on the theremin jag by Motorola’s
introduction of an intriguing-sounding electronic field
imaging device. That’s because the basic premise behind
the new chip and Theremin’s instrument is the same:
exploit the capacitance of a human (or another electromagnetic
object) to detect proximity.
Admittedly
more practical than whimsical, the chip was developed
to serve a compelling application: occupant detection
for smart air bag systems in cars. According to the
press release, the chip is already at work doing just
that with Honda mentioned as one of the first customers.
Needless
to say, the well-reported stories of the early “let
’er rip” (your head off) airbag designs set the stage
for technology to come to the rescue. I see a future
where your car seat is a veritable cocoon of front and
side airbags and curtains, all working in concert to
soak up as many Gs as possible. But doing the job right
requires not only knowing if a seat is occupied, but
also identifying the occupant’s size, height, seating
position, and so on.
Hey,
how about an Off-Baby-Kid-Adult switch for the airbag?
Oops, that wouldn’t be passive enough, not to mention
the problem of multiple languages, occupants who can’t
read or forget their glasses, and so on.
The
engineer in me loves to brainstorm the options. How
about a scanning ultrasonic dish that maps the interior?
Dash cams with image processing? Piezo strain gauge
seat covers? But the reality of automotive applications
is that the solution has to be reliable and durable
without inducing sticker shock. Enter the MC33794 (see
Figure 1).
|
(Click
here to enlarge)
|
Figure
1—Although the design was prompted by the need for
smarter airbags, the MC33794 electric field imaging
device raises other intriguing application possibilities. |
Before
getting into the e-field stuff, let’s talk about some
overall design-in factors. First, just a glance at the
chip’s pinout highlights its automotive roots. For instance,
there’s an ISO 9141 transceiver (ISO IN, ISO OUT, and
ISO 9141). If you’ve gone under the hood (literally)
of your modern car, then you’ll recognize this as the
connection to automotive networks such as On-Board Diagnostics
(OBD II). It’s the wire hooked to the connector under
your dash that the mechanic plugs a scanner into in
order to diagnose problems and turn off the Check Engine
light by clearing trouble codes.
The
chip also includes a lamp circuit and related pins (LAMP
CNTL, LAMP GND, LAMP SENSE, and LAMP MON), perhaps intended
to drive a Check Airbag light. Hook everything up right,
and you can not only turn the lamp on and off, but also
check whether it’s shorted or burned out.
And
I’m not talking about a wimpy LED, but something more
like a headlight—the built-in current limit is a whopping
1.7-A max! Along with the ISO 9141 interface (120-mA
current limit), that explains the need for the fancy
heatsink package this chip utilizes. If you aren’t planning
an automotive application, feel free to hook up a motor
or some other high-current device.
One
welcome by-product of the MC33794’s road-going roots
is that automotive stuff is designed to take a licking
and keep on ticking. For instance, an extended temperature
range (–40° to 85°C) is standard, and this nominally
12-V chip happily runs on anything from 9 to 18 V. It
can even tolerate the infamous load dump (40-V spike)
and double battery (26.5 V for up to 1 min.) hazards.
It’s
also obvious that the chip is designed to work in concert
with an external MCU as evidenced by a built-in regulator
(5 V at 75 mA) with voltage monitoring, power-on reset
(POR) circuit, and even a watchdog timer. The good news
is that some or all of these may come in handy or allow
for the use of a simpler, lower-cost MCU.
However,
it’s a bit disappointing that none of the characteristics
are programmable. For instance, the watchdog timeout
is imprecise—anywhere between 50 and 250 ms—and there’s
no way to adjust it. The same goes for the POR circuit,
which may assert the RST output for anywhere from 9
to 50 ms, take it or leave it. The 5-V regulator voltage
monitor can trigger anywhere between 4 and 4.72 V on
the low side and 5.26 and 5.83 V on the high side—a
rather loose spec, and certainly problematic when working
with a typical 5-V ±10% part.
One
nice touch is that three monitoring pins are provided
for PWR (chip power), VDD (output of the 5-V regulator),
and the LAMP. These include built-in voltage dividers
that scale down the voltage for connection to a 0- to
5-V A/D converter. For instance, the PWR MON pin divide
ratio is 1:4, so a 0- to 5-V A/D converter can monitor
a range of 0 to 20 V on the PWR pin.
Even
if you don’t need the ’33794’s automotive or MCU support
features, keep in mind that at $3.09 in quantities of
10,000, you’re not really paying much for them anyway.
