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M68HC12 SCI data format
SCI data format requires:
- Idle line goes high before transmission or reception of message.
- Start bit (0) indicates start of a character.
- Data is transmitted LSB first.
- Stop bit (1) indicates end of frame.
So the frame consists of:
- Start bit,
- Character (either 8 or 9 bits as configured),
- Stop bit.
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.