Figure 8-8 Interconnect Driver and Receiver

In mains-wired smoke alarm systems, the alarms can alert each other of smoke conditions with a wired interconnect bus. The TPS8802 has a driver and comparator to interface with the interconnect bus. The driver pulls the bus high when smoke is detected and low when smoke is cleared. The driver is current limited to handle short circuit conditions, and has a diode on the high side driver to prevent the bus from driving VBST. The hysteretic comparator senses when the bus is pulled high, filters the signal with a digital deglitch, and outputs the result to the INT_MCU pin and STATUS1 register. The comparator output is synchronized with the 32 kHz clock. The hysteresis has two settings and the deglitch is programmable from 0 ms to 20 ms. A 35-MΩ resistor prevents the INT_UNIT pin from floating, and a switchable 100-kΩ resistor pulls down the bus to prevent leakage from causing a false alarm. When the comparator outputs a high signal through the deglitch filter, the INT_UNIT register bit is latched high in the STATUS1 register.

The INT_MCU pin has the additional function to output status interrupt signals. The STATUS_INT bit in the MASK register enables interrupt signals to output through the INT_MCU pin. When the interconnect driver is enabled, the interrupt signal output is disconnected to allow the microcontroller to drive the INT_MCU pin.