Understanding Smart Sensors - Nomads.usp

Communications for Smart Sensors 1376.8.2 MCU With Integrated CANSeveral semiconductor manufacturers have implemented the CAN protocol ona variety of microcontrollers. The integrated CAN solution contains a variety ofI/O, memory, and other system features. One key point of the CAN protocol isthat, although a set of basic features must be implemented in any CAN device,a number of extended features may also be implemented in silicon, dependingon the intended target application. The implications must be understood wheninteroperability and interchangeability are being considered. A number ofsemiconductor manufacturers have designed and offer several solutions. In fact,the broad product availability is among the reasons that industrial users arechoosing CAN [7].An example of the CAN protocol implemented in an MCU is providedby Motorola’s M68HC05 microcontroller family. This CAN implementationconforms to the CAN 2.0 part A specification. All Motorola CAN (MCAN)devices include automatic bus arbitration, collision resolution, transmissionretry, digital noise filtering, message ID filtering, frame generation and checking,CRC generation and checking, as well as single transmission and multiplereceive frame buffers [20]. As shown in Figure 6.9, the MCAN module containsfull message transmit and receive buffers and limited message filteringcapability [21]. The integrated bus interface includes input comparators andCMOS output drivers. However, an external transceiver may be necessary,depending on the user’s application.The CAN module is totally CPU independent and can be integrated witha CPU core, other MCU hardware, and I/O functions into a single chip.Figure 6.10 shows the block diagram of a 32-bit reduced instruction set computer(RISC) microcontroller with two integrated CAN 2.0 B (Toucan) modules[22]. The chip has two third-generation timer processing units that canhandle fuel and spark computations without interrupting the main processor.Each unit has its own 32-bit MicroRISC engine, capable of processing 20 millioninstructions per second, and features its own onboard scheduler and 16timer channels. Additional peripherals include two queued ADC modules, aqueued serial multichannel module with two high-speed universal asynchronousreceiver transmitters and a queued serial port, an 18-channel modularinput/output subsystem (MIOS1), and a system-bus interface control blockwith general-purpose I/O support. This complex chip was designed specificallyfor automotive powertrain control.For automakers to achieve complete system functionality, a range ofproducts must be available. Table 6.6 lists other lower cost 8-bit MCUs witha variety of features [23]. Example software drivers available for these units