NEC uPD78P078 - Manual (Page 384)

384
CHAPTER 18 SERIAL INTERFACE CHANNEL 0 (
µ
PD78078Y Subseries)
(4) MSB/LSB switching as the start bit
The 3-wire serial I/O mode enables to select transfer to start from MSB or LSB.
Figure 18-9 shows the configuration of the serial I/O shift register 0 (SIO0) and internal bus. As shown in
the figure, MSB/LSB can be read/written in reverse form.
MSB/LSB switching as the start bit can be specified with bit 2 (CSIM02) of the serial operating mode register
0 (CSIM0).
Figure 18-9. Circuit of Switching in Transfer Bit Order
Start bit switching is realized by switching the bit order for data write to SIO0. The SIO0 shift order remains
unchanged.
Thus, MSB-first and LSB-first must be switched before writing data to the shift register.
(5) Transfer start
Serial transfer is started by setting transfer data to the serial I/O shift register 0 (SIO0) when the following
two conditions are satisfied.
• Serial interface channel 0 operation control bit (CSIE0) = 1.
• Internal serial clock is stopped or SCK0 is a high level after 8-bit serial transfer.
Caution If CSIE0 is set to “1” after data write to SIO0, transfer does not start.
Upon termination of 8-bit transfer, serial transfer automatically stops and the interrupt request flag (CSIIF0)
is set.
7
6
Internal Bus
1
0
LSB-first
MSB-first Read/Write Gate
SI0 Shift Register 0 (SIO0)
Read/Write Gate
SO0
SCK0
D Q
SO0 Latch