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CPU - Motorola 68HC12 layout diagram
The above slide shows the pin layout diagram for a Motorola 68HC12 microcontroller. Microcontroller is an integrated circuit that contains all CPU, memory and I/O components. The three buses are also included in the chip. If the memory size and functionality of a microcontroller are adequate for an application, the whole system hardware can consist of only the microcontroller and a few supporting chips. Since all components are inside the chip, address pins, data pins and control pins for the information transfer between memory and CPU are not needed. Therefore, most pins of the microcontroller can be used to support I/O, analog to digital conversion, or serial communication. In case that the internal memory size is not adequate, microcontroller can be configured as in microprocessor mode so that it is act like a regular microprocessor.
In microprocessor mode, ports A and B carries the address information, port B also carries data information, port E carries control information.