HT46R221 Holtek Semiconductor Inc, HT46R221 Datasheet - Page 9

• Manufacturer Part Number: HT46R221
• Description: 8-Bit A/D Type MCU
• Manufacturer: Holtek Semiconductor Inc

Indirect addressing register
Location 00H is an indirect addressing register that is
not physically implemented. Any read/write operation of
[00H] accesses data memory pointed to by MP (01H).
Reading location 00H itself indirectly will return the re-
sult 00H. Writing indirectly results in no operation.
The memory pointer register MP (01H) is a 7-bit register.
The bit 7 of MP is undefined and reading will return the
result 1 . Any writing operation to MP will only transfer
the lower 7-bit data to MP.
Accumulator
The accumulator is closely related to ALU operations. It
is also mapped to location 05H of the data memory and
can carry out immediate data operations. The data
movement between two data memory locations must
pass through the accumulator.
Arithmetic and logic unit - ALU
This circuit performs 8-bit arithmetic and logic operations.
The ALU provides the following functions:
The ALU not only saves the results of a data operation but
also changes the status register.
Status register - STATUS
This 8-bit register (0AH) contains the 0 flag (Z), carry
flag (C), auxiliary carry flag (AC), overflow flag (OV),
power down flag (PD), and watchdog time-out flag (TO).
It also records the status information and controls the
operation sequence.
With the exception of the TO and PD flags, bits in the
status register can be altered by instructions like
Rev. 1.10
C
AC
Z
OV
PD
TO
Labels
Arithmetic operations (ADD, ADC, SUB, SBC, DAA)
Logic operations (AND, OR, XOR, CPL)
Rotation (RL, RR, RLC, RRC)
Increment and Decrement (INC, DEC)
Branch decision (SZ, SNZ, SIZ, SDZ ....)
Bits
6, 7
0
1
2
3
4
5
C is set if the operation results in a carry during an addition operation or if a borrow does not take
place during a subtraction operation; otherwise C is cleared. C is also affected by a rotate
through carry instruction.
AC is set if the operation results in a carry out of the low nibbles in addition or no borrow from the
high nibble into the low nibble in subtraction; otherwise AC is cleared.
Z is set if the result of an arithmetic or logic operation is 0; otherwise Z is cleared.
OV is set if the operation results in a carry into the highest-order bit but not a carry out of the
highest-order bit, or vice versa; otherwise OV is cleared.
PD is cleared by system power-up or executing the CLR WDT instruction. PD is set by execut-
ing the HALT instruction.
TO is cleared by system power-up or executing the CLR WDT or HALT instruction. TO is set
by a WDT time-out.
Unused bit, read as 0
Status register
9
most other registers. Any data written into the status
register will not change the TO or PD flag. In addition
operations related to the status register may give dif-
ferent results from those intended. The TO flag can
be affected only by system power-up, a WDT
time-out or executing the CLR WDT or HALT in-
struction. The PD flag can be affected only by exe-
cuting the HALT or CLR WDT instruction or a
system power-up.
The Z, OV, AC and C flags generally reflect the status of
the latest operations.
In addition, on entering the interrupt sequence or exe-
cuting the subroutine call, the status register will not be
pushed onto the stack automatically. If the contents of
the status are important and if the subroutine can cor-
rupt the status register, precautions must be taken to
save it properly.
Interrupt
The device provides an external interrupt, an internal
timer/event counter interrupt, the A/D converter interrupt
and the I
0 (INTC0;0BH) and interrupt control register 1
(INTC1;1EH) contains the interrupt control bits to set the
enable or disable and the interrupt request flags.
Once an interrupt subroutine is serviced, all the other in-
terrupts will be blocked (by clearing the EMI bit). This
scheme may prevent any further interrupt nesting. Other
interrupt requests may happen during this interval but
only the interrupt request flag is recorded. If a certain in-
terrupt requires servicing within the service routine, the
EMI bit and the corresponding bit of INTC0 and INTC1
may be set to allow interrupt nesting. If the stack is full,
the interrupt request will not be acknowledged, even if the
related interrupt is enabled, until the SP is decremented.
If immediate service is desired, the stack must be pre-
vented from becoming full.
Function
2
C BUS interrupts. The interrupt control register
HT46R22/HT46C22
October 2, 2002