By John Griffith, Texas Instruments
Automotive body control modules (BCMs) are electronic control units that manage numerous vehicle comfort, convenience and lighting functions, including door locks, windows, chimes, closure sensors, interior and exterior lighting, wipers, and turn signals. Specifically, BCMs monitor different driver switches and control power to corresponding loads in a car, as shown in Figure 1.
A typical BCM consists of a microcontroller (MCU) that processes automotive 12V battery level driver switch states. The signals connect to the MCU through interface circuits traditionally implemented using discrete passive components such as resistors, capacitors and diodes. You must take care to protect the MCU from battery level voltages, electrostatic discharge (ESD), transients and reverse-battery conditions. Additionally, you need to provide wetting currents for biasing the switch inputs and to keep the switch contact in good condition.
Figure 2 shows an example implementation of how to handle an external ground-connected switch input. Capacitor C2 shunts ESD and transient energy; diode D1 blocks high voltages; resistor R4 sets the wetting current at the switch; resistor R4 (in conjunction with R8) divides the battery level voltages down; and resistors R1 through R3, transistors Q1 and Q2, capacitor C1, and a general-purpose input/output (GPIO) pin enable and disable the wetting currents.
There are three caveats to this discrete approach:
- The MCU, and therefore the MCU’s supply voltage, have to remain active for the wetting currents to be active. This severely affects the minimum amount of current that the module consumes when in low power (key-off) mode.
- The solution requires a large number of passive components, such as transistors and resistors for creating the wetting currents, as well as diodes, resistors and capacitors for each switch input. This makes the overall solution size large.
- The wetting current will vary with battery voltage; for example, if the battery voltage droops by 30%, the wetting current will also droop by 30%.
A multi-switch detection interface (MSDI) is a device that handles all of these issues, aggregating battery- and ground-connected switch status information and communicating back to a microprocessor via the serial peripheral…