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M68HC12 Serial Interface
The serial interface of the M68HC12 consists of two independent serial I/O subsystems.
The pins for serial I/O are shared with port S.
The SCI is compatible with RS-232. It contains
- internal baudrate generation circuitry,
- SCI transmitter, and
- SCI receiver.
The SPI is used for communication with peripheral devices and other processors.
- The SPI system can operate either as master or slave.
- The system can be used in a multiple master system.
When copying a set of data from one memory (or I/O) location to another memory (or I/O) location, the microprocessor needs to read the data from the source location into the CPU and then write the data to the destination location. Since executing each read or write instruction may take several clock cycles, the data transfer is slow.
A DMA chip is a special processor that just does read and write operations. Each read or write takes only one clock cycle.
There are some clock cycles that the CPU does not need the buses. DMA can use these bus cycles to transfer data.