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HT45R37 Datasheet(PDF) 11 Page - Holtek Semiconductor Inc |
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HT45R37 Datasheet(HTML) 11 Page - Holtek Semiconductor Inc |
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11 / 100 page  HT45R37 Rev. 1.10 11 January 29, 2010 Stack This is a special part of the memory which is used to save the contents of the Program Counter only. The stack has multiple levels depending upon the device and is neither part of the data nor part of the program space, and is neither readable nor writeable. The acti- vated level is indexed by the Stack Pointer, SP, and is neither readable nor writeable. At a subroutine call or in- terrupt acknowledge signal, the contents of the Program Counter are pushed onto the stack. At the end of a sub- routine or an interrupt routine, signaled by a return in- struction, RET or RETI, the Program Counter is restored to its previous value from the stack. After a device reset, the Stack Pointer will point to the top of the stack. If the stack is full and an enabled interrupt takes place, the interrupt request flag will be recorded but the ac- knowledge signal will be inhibited. When the Stack Pointer is decremented, by RET or RETI, the interrupt will be serviced. This feature prevents stack overflow al- lowing the programmer to use the structure more easily. However, when the stack is full, a CALL subroutine in- struction can still be executed which will result in a stack overflow. Precautions should be taken to avoid such cases which might cause unpredictable program branching. Arithmetic and Logic Unit - ALU The arithmetic-logic unit or ALU is a critical area of the microcontroller that carries out arithmetic and logic oper- ations of the instruction set. Connected to the main microcontroller data bus, the ALU receives related in- struction codes and performs the required arithmetic or logical operations after which the result will be placed in the specified register. As these ALU calculation or oper- ations may result in carry, borrow or other status changes, the status register will be correspondingly up- dated to reflect these changes. The ALU supports the following functions: · Arithmetic operations: ADD, ADDM, ADC, ADCM, SUB, SUBM, SBC, SBCM, DAA · Logic operations: AND, OR, XOR, ANDM, ORM, XORM, CPL, CPLA · Rotation RRA, RR, RRCA, RRC, RLA, RL, RLCA, RLC · Increment and Decrement INCA, INC, DECA, DEC · Branch decision, JMP, SZ, SZA, SNZ, SIZ, SDZ, SIZA, SDZA, CALL, RET, RETI Program Memory The Program Memory is the location where the user code or program is stored. For these device the Pro- gram Memory is an OTP type, which means it can be programmed only one time. By using the appropriate programming tools, this OTP memory device offer users the flexibility to conveniently debug and develop their applications while also offering a means of field pro- gramming. Structure The Program Memory has a capacity of 4K by 15 bits. The Program Memory is addressed by the Program Counter and also contains data, table information and interrupt entries. Table data, which can be setup in any location within the Program Memory, is addressed by a separate table pointer register. Special Vectors Within the Program Memory, certain locations are re- served for special usage such as reset and interrupts. · Location 000H This vector is reserved for use by the device reset for program initialisation. After a device reset is initiated, the program will jump to this location and begin execution. · Location 004H This vector is used by the external interrupt 0. If the external interrupt pin receives an active edge, the pro- gram will jump to this location and begin execution if the external interrupt is enabled and the stack is not full. · Location 008H This vector is used by the external interrupt 1. If the external interrupt pin receives an active edge, the pro- gram will jump to this location and begin execution if the external interrupt is enabled and the stack is not full. · Location 00CH This internal vector is used by the Timer/Event Coun- ter 0. If a Timer/Event Counter 0 overflow occurs, the program will jump to this location and begin execution if the timer/event counter interrupt is enabled and the stack is not full. P r o g r a m C o u n t e r S t a c k L e v e l 1 S t a c k L e v e l 2 S t a c k L e v e l 3 S t a c k L e v e l 6 P r o g r a m M e m o r y T o p o f S t a c k S t a c k P o i n t e r B o t t o m o f S t a c k |
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