HD404459 Datasheet, PDF (91/122 Pages) Renesas Technology Corp – 4-bit HMCS400-series microcomputers

HD404459 Series
Registers for Serial Interface
The serial interface operation is selected, and serial data is read and written by the following registers.
 Serial mode register A (SMRA: $005)
 Serial mode register B (SMRB: $028)
 Serial data register (SRL: $006, SRU: $007)
 Port mode register A (PMRA: $004)
 Miscellaneous register (MIS: $00C)
Serial Mode Register A (SMRA: $005): This register has the following functions (figure 71).
• R4₁/SCK pin function selection
• Transmit clock selection
• Prescaler division ratio selection
• Serial interface initialization
Serial mode register A (SMRA: $005) is a 4-bit write-only register. It is reset to $0 by MCU reset.
A write signal input to serial mode register A (SMRA: $005) discontinues the input of the transmit clock to
the serial data register (SRL: $006, SRU: $007) and octal counter, and the octal counter is reset to 000.
Therefore, if a write is performed during data transfer, the serial interrupt request flag (IFS: $023, bit 0) is
set.
Written data is valid from the second instruction execution cycle after the write operation, so the STS
instruction must be executed at least two cycles after that.
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