ECU Components

The processor is packaged in a module with hundreds of other
components on a multi-layer circuit board. Some of the other
components in the ECU that support the processor are:

* Analog-to-digital converters - These devices read the
outputs of some of the sensors in the car, such as the oxygen
sensor. The output of an oxygen sensor is an analog voltage,
usually between 0 and 1.1 volts (V). The processor only
understands digital numbers, so the analog-to-digital
converter changes this voltage into a 10-bit digital number.

* High-level digital outputs - On many modern cars, the
ECU fires the spark plugs, opens and closes the fuel
injectors and turns the cooling fan on and off. All of these
tasks require digital outputs. A digital output is either on
or off -- there is no in-between. For instance, an output for
controlling the cooling fan might provide 12 V and 0.5 amps
to the fan relay when it is on, and 0 V when it is off. The
digital output itself is like a relay. The tiny amount of
power that the processor can output energizes the transistor
in the digital output, allowing it to supply a much larger
amount of power to the cooling fan relay, which in turn
provides a still larger amount of power to the cooling fan.

* Digital-to-analog converters - Sometimes the ECU has to
provide an analog voltage output to drive some engine
components. Since the processor on the ECU is a digital
device, it needs a component that can convert the digital
number into an analog voltage.

* Signal conditioners - Sometimes the inputs or outputs
need to be adjusted before they are read. For instance, the
analog-to-digital converter that reads the voltage from the
oxygen sensor might be set up to read a 0- to 5-V signal, but
the oxygen sensor outputs a 0- to 1.1-V signal. A signal
conditioner is a circuit that adjusts the level of the
signals coming in or out. For instance, if we applied
a signal conditioner that multiplied the voltage coming from
the oxygen sensor by 4, we'd get a 0- to 4.4-V signal, which
would allow the analog-to-digital converter to read the
voltage more accurately.

* Communication chips - These chips implement the various
communications standards that are used on cars. There are
several standards used, but the one that is starting to
dominate in-car communications is called CAN (controller-area
networking). This communication standard allows for
communication speeds of up to 500 kilobits per second (Kbps).
That's a lot faster than older standards. This speed is
becoming necessary because some modules communicate data onto
the bus hundreds of times per second. The CAN bus
communicates using two wires.

Advanced Diagnostics

Another benefit of having a communications bus is that each
module can communicate faults to a central module, which
stores the faults and can communicate them to an off-board
diagnostic tool.

This can make it easier for technicians to diagnose problems
with the car, especially intermittent problems, which are
notorious for disappearing as soon as you bring the car in
for repairs.