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HT49R30A-1 Datasheet(PDF) 11 Page - Holtek Semiconductor Inc |
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HT49R30A-1 Datasheet(HTML) 11 Page - Holtek Semiconductor Inc |
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11 / 41 page  HT49R30A-1/HT49C30-1/HT49C30L Rev. 2.10 11 July 30, 2012 The ALU not only saves the results of a data operation but also changes the status register. Status Register - STATUS The status register (0AH) is of 8 bits wide and contains, a carry flag (C), an auxiliary carry flag (AC), a zero flag (Z), an overflow flag (OV), a power down flag (PDF), and a watchdog time-out flag (TO). It also records the status information and controls the operation sequence. Except the TO and PDF flags, bits in the status regis- ter can be altered by instructions similar to other regis- ters. Data written into the status register does not alter the TO or PDF flags. Operations related to the status register, however, may yield different results from those intended. The TO and PDF flags can only be changed by a Watchdog Timer overflow, chip power-up, or clear- ing the Watchdog Timer and executing the ²HALT² in- struction. The Z, OV, AC, and C flags reflect the status of the latest operations. On entering the interrupt sequence or executing the subroutine call, the status register will not be automati- cally pushed onto the stack. If the contents of the status is important, and if the subroutine is likely to corrupt the status register, the programmer should take precautions and save it properly. Interrupts The device provides two external interrupts, an internal timer/event counter interrupt, an internal time base in- terrupt, and an internal real time clock interrupt. The in- terrupt control register 0 (INTC0;0BH) and interrupt control register 1 (INTC1;1EH) both contain the interrupt control bits that are used to set the enable/disable status and interrupt request flags. Once an interrupt subroutine is serviced, other inter- rupts are all blocked (by clearing the EMI bit). This scheme may prevent any further interrupt nesting. Other interrupt requests may take place during this interval, but only the interrupt request flag will be recorded. If a certain interrupt requires servicing within the service routine, the EMI bit and the corresponding bit of the INTC0 or of INTC1 may be set in order to allow interrupt nesting. Once the stack is full, the interrupt request will not be acknowledged, even if the related interrupt is en- abled, until the SP is decremented. If immediate service is desired, the stack should be prevented from becom- ing full. All these interrupts can support a wake-up function. As an interrupt is serviced, a control transfer occurs by pushing the contents of the program counter onto the stack followed by a branch to a subroutine at the speci- fied location in the ROM. Only the contents of the pro- gram counter is pushed onto the stack. If the contents of the register or of the status register (STATUS) is altered by the interrupt service program which corrupts the de- sired control sequence, the contents should be saved in advance. External interrupts are triggered by a high to low transi- tion of INT0 or INT1, and the related interrupt request flag (EIF0;bit 4 of INTC0, EIF1;bit 5 of INTC0) is set as well. After the interrupt is enabled, the stack is not full, and the external interrupt is active, a subroutine call to location 04H or 08H occurs. The interrupt request flag (EIF0 or EIF1) and EMI bits are all cleared to disable other interrupts. The internal timer/event counter interrupt is initialized by setting the timer/event counter interrupt request flag (TF;bit 6 of INTC0), which is normally caused by a timer overflow. After the interrupt is enabled, and the stack is not full, and the TF bit is set, a subroutine call to location 0CH occurs. The related interrupt request flag (TF) is re- set, and the EMI bit is cleared to disable further inter- rupts. Bit No. Label Function 0C 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 ro- tate through carry instruction. 1AC 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. 2 Z Z is set if the result of an arithmetic or logic operation is zero; otherwise Z is cleared. 3OV 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. 4 PDF PDF is cleared by either a system power-up or executing the ²CLR WDT² instruction. PDF is set by executing the ²HALT² instruction. 5TO TO is cleared by a system power-up or executing the ²CLR WDT² or ²HALT² instruction. TO is set by a WDT time-out. 6, 7 ¾ Unused bit, read as ²0² Status (0AH) Register |
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