E202075_HD404889 Datasheet, PDF (39/202 Pages) Renesas Technology Corp – Low-Voltage AS Microcomputers with On-Chip LCD Circuit

HD404889/HD404899/HD404878/HD404868 Series
W register (W), X register (X) and Y register (Y):
The W register is a 2-bit register, and the X and Y registers are 4-bit registers, used for RAM register
indirect addressing. The Y register is also used for D port addressing.
SPX register (SPX) and SPY register (SPY):
The SPX and SPY registers are 4-bit registers used as X register and Y register auxiliary registers,
respectively.
Carry flag (CA):
This flag holds ALU overflow when an arithmetic/logic instruction is executed. It is also affected by the
SEC, REC, ROTL, and ROTR instructions. The contents of the carry flag are saved to the stack when
interrupt handling is performed, and are restored from the stack by the RTNI instruction (but are not
affected by the RTN instruction).
Status flag (ST):
This flag holds ALU overflow when an arithmetic/logic or compare instruction is executed, and the result
of an ALU non-zero or bit test instruction. It is used as the branch condition for the BR, BRL, CAL, and
CALL instructions. The status flag is a latch-type flag, and does not change until the next arithmetic/logic,
compare, or bit test instruction is executed. After a BR, BRL, CAL, or CALL instruction, the status flag is
set to 1 regardless of whether the instruction is executed or skipped. The contents of the status flag are
saved to the stack when interrupt handling is performed, and are restored from the stack by the RTNI
instruction (but are not affected by the RTN instruction).
Program counter (PC):
This is a 14-bit binary counter that holds ROM address information.
Stack pointer (SP):
The stack pointer is a 10-bit register that holds the address of the next save space in the stack area. The
stack pointer is initialized to $3FF by an MCU reset. The stack pointer is decremented by 4 each time data
is saved, and incremented by 4 each time data is restored. The upper 4 bits of the stack pointer are fixed at
1111, so that a maximum of 16 stack levels can be used.
There are two ways in which the stack pointer is initialized to $3FF: by an MCU reset as mentioned above,
or by resetting the RSP bit with the REM or REMD instruction.
Reset
An MCU reset is performed by driving the RESET pin low. At power-on, and when subactive mode,
watch mode, or stop mode is cleared, RESET should be input for at least tRC to provide the oscillation
settling time for the oscillator.In other cases, the MCU is reset by inputting RESET for at least two
instruction cycles.
Table 1 shows the areas initialized by an MCU reset, and their initial values.
37