Understanding Smart Sensors - Nomads.usp

138 Understanding Smart SensorsCPUinterface{Interfacemanagementlogic(IML)Bit timinglogic(BTL)Transceivelogic(TCL)CANbusTransmit buffer(TBF)Receivebuffer 0(RBF0)Errormanagementlogic(EML)Receivebuffer 1(RBF1)Bit streamprocessor(BSP)Microprocessor related logicBus line related logicFigure 6.9 CAN module.provide an interface between application software residing in the MCU ROM(or EPROM) and the CAN module. The routines initialize the CAN module,transmit messages previously stored in RAM, and automatically handlereceived messages [7].Memory, in RAM, ROM, EPROM, or EEPROM, is required for properCAN operation. The smaller X4 version includes 4,096 bytes of EPROM(one-time programmable version) or ROM and 176 bytes of RAM. The X16and X32 versions have 15,120 bytes of (EP)ROM, 256 bytes of EEPROM, and352 bytes of data RAM for more complex programs. The EEPROM on theX16 also provides flexibility for addressing and programming over the network.A user can manufacture a number of identical modules and put the ID inEEPROM when each is installed, to minimize inventory costs. The A/D allowseasy interface for sensors, and the PWM module can be used for digital speedcontrol of power devices. The integrated timers can be used as part of theprogram control. Also, the buffering of the CAN module requires CPU interventionfor each message received or transmitted. Due to the additional functionalitydesigned into the MC68HC05X16, the CAN module occupies lessthan 20% of the active area of the chip, a feature that makes the integratedfunction very cost effective [24].