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PIC16F84A Datasheet(PDF) 18 Page - Microchip Technology |
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PIC16F84A Datasheet(HTML) 18 Page - Microchip Technology |
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18 / 124 page  PIC16F8X DS30430C-page 18 © 1998 Microchip Technology Inc. 4.3 Program Counter: PCL and PCLATH The Program Counter (PC) is 13-bits wide. The low byte is the PCL register, which is a readable and writable register. The high byte of the PC (PC<12:8>) is not directly readable nor writable and comes from the PCLATH register. The PCLATH (PC latch high) register is a holding register for PC<12:8>. The contents of PCLATH are transferred to the upper byte of the program counter when the PC is loaded with a new value. This occurs during a CALL, GOTO or a write to PCL. The high bits of PC are loaded from PCLATH as shown in Figure 4-1. FIGURE 4-1: LOADING OF PC IN DIFFERENT SITUATIONS 4.3.1 COMPUTED GOTO A computed GOTO is accomplished by adding an offset to the program counter (ADDWF PCL). When doing a table read using a computed GOTO method, care should be exercised if the table location crosses a PCL memory boundary (each 256 word block). Refer to the application note “Implementing a Table Read” (AN556). 4.3.2 PROGRAM MEMORY PAGING The PIC16F83 and PIC16CR83 have 512 words of pro- gram memory. The PIC16F84 and PIC16CR84 have 1K of program memory. The CALL and GOTO instruc- tions have an 11-bit address range. This 11-bit address range allows a branch within a 2K program memory page size. For future PIC16F8X program memory expansion, there must be another two bits to specify the program memory page. These paging bits come from the PCLATH<4:3> bits (Figure 4-1). When doing a CALL or a GOTO instruction, the user must ensure that these page bits (PCLATH<4:3>) are programmed to the desired program memory page. If a CALL instruc- tion (or interrupt) is executed, the entire 13-bit PC is “pushed” onto the stack (see next section). Therefore, manipulation of the PCLATH<4:3> is not required for the return instructions (which “pops” the PC from the stack). 4.4 Stack The PIC16FXX has an 8 deep x 13-bit wide hardware stack (Figure 4-1). The stack space is not part of either program or data space and the stack pointer is not readable or writable. The entire 13-bit PC is “pushed” onto the stack when a CALL instruction is executed or an interrupt is acknowl- edged. The stack is “popped” in the event of a RETURN, RETLW or a RETFIE instruction execution. PCLATH is not affected by a push or a pop operation. The stack operates as a circular buffer. That is, after the stack has been pushed eight times, the ninth push over- writes the value that was stored from the first push. The tenth push overwrites the second push (and so on). If the stack is effectively popped nine times, the PC value is the same as the value from the first pop. PC 12 8 7 0 5 PCLATH<4:0> PCLATH INST with PCL as dest ALU result GOTO, CALL Opcode <10:0> 8 PC 12 11 10 0 11 PCLATH<4:3> PCH PCL 8 7 2 PCLATH PCH PCL Note: The PIC16F8X ignores the PCLATH<4:3> bits, which are used for program memory pages 1, 2 and 3 (0800h - 1FFFh). The use of PCLATH<4:3> as general purpose R/W bits is not recommended since this may affect upward compatibility with future products. Note: There are no instruction mnemonics called push or pop. These are actions that occur from the execution of the CALL, RETURN, RETLW, and RETFIE instruc- tions, or the vectoring to an interrupt address. Note: There are no status bits to indicate stack overflow or stack underflow conditions. |
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