Understanding Smart Sensors - Nomads.usp

258 Understanding Smart Sensorsbe added to the control capability. A total of 32 outputs are controlled withonly four connections (the IRQ, interrupt request not, is optional) to theMCU.A high-level of smart-power IC device complexity is demonstrated in aparallel-serial control. The device has six outputs that can be selected independentlyon six dedicated parallel input pins by an input command (voltage aboveVth) from an MCU. Inputs 0, 1, 4, and 5 also can be simultaneously selected.That allows the two output devices to be connected in parallel for reducing theon-resistance. Parallel control provides the fastest activation of the loads, whichallows real-time control using PWM techniques. The drawback is that thehighest number of connections must be made and the highest number of tracesmust be routed on the printed circuit board. Serial control of the six outputs isalso possible using an SPI interface, as described earlier. Mixed control of theoutputs can be achieved with various combinations of parallel and serial controlof the outputs.Fault diagnostics and control are similar to those in the OSS. However,an advanced SPI provides serial diagnostic information to the MCU, whichincludes immediate parity checking (nth word) to confirm that the smartpowerIC received the word sent by the MCU. In the initial SPI implementation(the OSS), fault reporting is performed in the n − 1 word. The presentcommand to the OSS is compared to the outputs from the previous word todetermine the status. The time of the active filter period, which can be severalclock cycles, is required before the next word can determine if a fault exists. Theadvanced SPI uses an exclusive OR to provide fault information when the currentword is written.Automotive applications have required diagnostics beyond those typicallyfound in smart-power ICs. For example, the California Air Resource Board’slatest OBDII legislation requires that the actual movement of the pintle intransmission solenoids be monitored when the output device is activated. Thatis done to indicate that the transmission is in the proper gear and that thetorque converter has full lockup. Simply sensing the current going through theoutput device is insufficient to determine that the pintle has moved as a resultof the output device being turned on. The differential sensing circuit in Figure11.5(a) is used to detect the change that occurs when the solenoid’s armature isattracted into the winding. The negative slope, shown in the current waveformin Figure 11.5(b), is an indication of the physical movement. Either low-side orhigh-side sensing can be performed with low-pass, high-pass filters and a comparatorproviding the signal conditioning. The event counter can be adjustedfrom 15 to 64 to establish the confidence level of detecting movement. Theon-resistance of the power driver can be used as a sense resistor to convert thecurrent into a voltage level. A peak detector also can be used to detect the peak