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PIC12F683 Datasheet(PDF) 19 Page - Microchip Technology |
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PIC12F683 Datasheet(HTML) 19 Page - Microchip Technology |
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19 / 148 page  2004 Microchip Technology Inc. Preliminary DS41211B-page 17 PIC12F683 2.3 PCL and PCLATH The Program Counter (PC) is 13 bits wide. The low byte comes from the PCL register, which is a readable and writable register. The high byte (PC<12:8>) is not directly readable or writable and comes from PCLATH. On any Reset, the PC is cleared. Figure 2-3 shows the two situations for the loading of the PC. The upper example in Figure 2-3 shows how the PC is loaded on a write to PCL (PCLATH<4:0> → PCH). The lower exam- ple in Figure 2-3 shows how the PC is loaded during a CALL or GOTO instruction (PCLATH<4:3> → PCH). FIGURE 2-3: LOADING OF PC IN DIFFERENT SITUATIONS 2.3.1 COMPUTED GOTO A computed GOTO is accomplished by adding an offset to the program counter (ADDWF PCL). When perform- ing a table read using a computed GOTO method, care should be exercised if the table location crosses a PCL memory boundary (each 256-byte block). Refer to the Application Note AN556, “Implementing a Table Read” (DS00556). 2.3.2 STACK The PIC12F683 family has an 8-level x 13-bit wide hardware stack (see Figure 2-1). The stack space is not part of either program or data space and the stack pointer is not readable or writable. The PC is PUSHed onto the stack when a CALL instruction is executed or an interrupt causes a branch. The stack is POPed in the event of a RETURN, RETLW or a RETFIE instruction execution. PCLATH is not affected by a PUSH or POP operation. The stack operates as a circular buffer. This means that after the stack has been PUSHed eight times, the ninth push overwrites the value that was stored from the first push. The tenth push overwrites the second push (and so on). 2.4 Indirect Addressing, INDF and FSR Registers The INDF register is not a physical register. Addressing the INDF register will cause indirect addressing. Indirect addressing is possible by using the INDF register. Any instruction using the INDF register actually accesses data pointed to by the File Select Register (FSR). Reading INDF itself indirectly will produce 00h. Writing to the INDF register indirectly results in a no operation (although Status bits may be affected). An effective 9-bit address is obtained by concatenating the 8-bit FSR register and the IRP bit (Status<7>), as shown in Figure 2-4. A simple program to clear RAM location 20h-2Fh using indirect addressing is shown in Example 2-1. EXAMPLE 2-1: INDIRECT ADDRESSING PC 12 8 7 0 5 PCLATH<4:0> PCLATH Instruction with ALU Result GOTO, CALL OPCODE<10:0> 8 PC 12 11 10 0 11 PCLATH<4:3> PCH PCL 87 2 PCLATH PCH PCL PCL as Destination Note 1: There are no Status bits to indicate stack overflow or stack underflow conditions. 2: There are no instructions/mnemonics called PUSH or POP. These are actions that occur from the execution of the CALL, RETURN, RETLW and RETFIE instructions or the vectoring to an interrupt address. MOVLW 0x20 ;initialize pointer MOVWF FSR ;to RAM NEXT CLRF INDF ;clear INDF register INCF FSR ;inc pointer BTFSS FSR,4 ;all done? GOTO NEXT ;no clear next CONTINUE ;yes continue |
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