How Car Computers Work: ECU Components

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 (see How Analog and Digital Recording Works for more details).
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.