Multiple Switch Detection Interface (MSDI) breakout board

• 8 programmable inputs (switches to battery or ground)
• 14 switch-to-ground inputs
• Switch input voltage range from -1.0 V to 36 V
• Selectable wetting current (2, 6, 8, 10, 12, 14, 16, or 20 mA)
• Selectable wake-up on change of state
• Interfaces directly to an MCU using 3.3 V / 5.0 V SPI protocol
• Active interrupt (INT_B) on change-of-switch state
• Integrated battery and temperature sensing
• Fully functional operation 5.5 V ≤ VBATP ≤ 36 V
• Full parametric operation 7.0 V ≤ VBATP ≤ 28 V
• Typical standby current IBATP = 30 μA and IDDQ = 10 μA
• -40 to 85 Celius
• Reserve battery protection on VBATP & switch inputs
• All switch inputs are protected against Short to Grounds and Short to Battery and Electrostatic Discharge (ESD)

This board features the NXP® MC33975. This IC chip is a Multiple Switch Detection Interface (MSDI) designed to detect the closing and opening of 22 switch contacts. The switch status, either open or closed, is transferred to the microprocessor unit (MCU) through a serial peripheral interface (SPI). The device also features a 22-to-1 analog multiplexer for reading the voltages on the switch inputs, in addition too, monitoring the battery voltage and IC temperature.

The inputs are capable of surviving reverse battery without a blocking diode and also contain an internal blocking diode from the input to the power supply (VBATP), to ensure there is no back feeding of voltage/current into the IC, when the voltage on the input is higher than the VBATP pin.

The MSDI has two modes of operation, Normal and Sleep.

Normal Mode allows programming of the device and supplies switch contacts with pull-up or pull-down current as it monitors the switches.

Sleep Mode provides low quiescent current and monitors the switches periodically(polling). The polling times are programable from 3ms to 64ms, with increments of 3ms. There is also a setting to slow the polling rate by 4x; resulting in a lower quiescent current.