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M68HC12 SCI operation
SCI operation consists of 3 distinct procedures:
- system initialization
- character transmission, and
- character reception.
SCI system initialization consists of
- setting the Baud rate in SCxBDH and SCxBDL,
- setting the data format in SCxCR1,
- configuring parameters using SCxCR1 and SCxCR2, and
- clearing the TDRE (Transmit Data Register Empty) flag.
- To clear TDRE, first read from SCxSR1, then
- write to data register low byte.
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.