HD404849 Datasheet, PDF (15/125 Pages) Hitachi Semiconductor – 4-Bit Single-Chip Microcomputer

HD404849 Series
SPX Register (SPX), SPY Register (SPY): Four-bit registers used to supplement the X and Y registers.
Carry Flag (CA): One-bit flag that stores any ALU overflow generated by an arithmetic operation. CA is
affected by the SEC, REC, ROTL, and ROTR instructions. A carry is pushed onto the stack during an
interrupt and popped from the stack by the RTNI instruction—but not by the RTN instruction.
Status Flag (ST): One-bit flag that latches any overflow generated by an arithmetic or compare
instruction, not-zero decision from the ALU, or result of a bit test. ST is used as a branch condition of the
BR, BRL, CAL, and CALL instructions. The contents of ST remain unchanged until the next arithmetic,
compare, or bit test instruction is executed, but become 1 after the BR, BRL, CAL, or CALL instruction is
read, regardless of whether the instruction is executed or skipped. The contents of ST are pushed onto the
stack during an interrupt and popped from the stack by the RTNI instruction—but not by the RTN
instruction.
Program Counter (PC): 14-bit binary counter that points to the ROM address of the instruction being
executed.
Stack Pointer (SP): Ten-bit pointer that contains the address of the stack area to be used next. The SP is
initialized to $3FF by MCU reset. It is decremented by 4 when data is pushed onto the stack, and
incremented by 4 when data is popped from the stack. The top four bits of the SP are fixed at 1111, so a
stack can be used up to 16 levels.
The SP can be initialized to $3FF in another way: by resetting the RSP bit with the REM or REMD
instruction.
Reset
The MCU is reset by inputting a low-level voltage to the RESET pin. At power-on or when stop mode is
cancelled, RESET must be low for at least one t_RC to enable the oscillator to stabilize. During operation,
RESET must be low for at least two instruction cycles.
Initial values after MCU reset are listed in table 1.
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