Transfer Bit Order Switching Circuit - NEC 78K0 User Manual

(c) Switching MSB/LSB as start bit
Figure 17-12 shows the configuration of serial I/O shift register 0 (SIOA0) and the internal bus. As shown
in the figure, MSB/LSB can be read/written in reverse form.
Switching MSB/LSB as the start bit can be specified using bit 1 (DIR0) of serial operation mode
specification register 0 (CSIMA0).
Figure 17-12. Transfer Bit Order Switching Circuit
7
6
Internal bus
1
0
LSB-first
MSB-first
SIA0
SOA0
SCKA0
Start bit switching is realized by switching the bit order for data written to SIOA0. The SIOA0 shift order
remains unchanged.
Thus, switching between MSB-first and LSB-first must be performed before writing data to the shift
register.
(d) Communication start
Serial communication is started by setting communication data to serial I/O shift register 0 (SIOA0) when
the following two conditions are satisfied.
• Serial interface CSIA0 operation control bit (CSIAE0) = 1
• Serial communication is not in progress
Caution If CSIAE0 is set to 1 after data is written to SIOA0, communication does not start.
Upon termination of 8-bit communication, serial communication automatically stops and the interrupt
request flag (ACSIIF) is set.
CHAPTER 17 SERIAL INTERFACE CSIA0
Read/write gate
Shift register 0 (SIOA0)
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User's Manual U15947EJ2V0UD
Read/write gate
SOA0 latch
D Q
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