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ICDR is an 8-bit readable/writable register that is used as a transmit data register when
transmitting and a receive data register when receiving. ICDR is divided internally into a shift
register (ICDRS), receive buffer (ICDRR), and transmit buffer (ICDRT). ICDRS cannot be read
or written by the CPU, ICDRR is read-only, and ICDRT is write-only. Data transfers among the
three registers are performed automatically in coordination with changes in the bus state, and
affect the status of internal flags such as TDRE and RDRF.
After transmission/reception of one frame of data using ICDRS, if the I
2
C bus is in transmit
mode and the next data is in ICDRT (the TDRE flag is 0), data is transferred automatically from
ICDRT to ICDRS. After transmission/reception of one frame of data using ICDRS, if the I
2
C
bus is in receive mode and no previous data remains in ICDRR (the RDRF flag is 0), data is
transferred automatically from ICDRS to ICDRR.
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