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PIC16F1789 数据表(PDF) 21 Page - Microchip Technology |
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PIC16F1789 数据表(HTML) 21 Page - Microchip Technology |
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21 / 450 page  2011-2012 Microchip Technology Inc. Preliminary DS41579D-page 21 PIC16(L)F1782/3 3.1.1 READING PROGRAM MEMORY AS DATA There are two methods of accessing constants in program memory. The first method is to use tables of RETLW instructions. The second method is to set an FSR to point to the program memory. 3.1.1.1 RETLW Instruction The RETLW instruction can be used to provide access to tables of constants. The recommended way to create such a table is shown in Example 3-1. EXAMPLE 3-1: RETLW INSTRUCTION The BRW instruction makes this type of table very simple to implement. If your code must remain portable with previous generations of microcontrollers, then the BRW instruction is not available so the older table read method must be used. 3.1.1.2 Indirect Read with FSR The program memory can be accessed as data by setting bit 7 of the FSRxH register and reading the matching INDFx register. The MOVIW instruction will place the lower 8 bits of the addressed word in the W register. Writes to the program memory cannot be performed via the INDF registers. Instructions that access the program memory via the FSR require one extra instruction cycle to complete. Example 3-2 demonstrates accessing the program memory via an FSR. The high directive will set bit<7> if a label points to a location in program memory. EXAMPLE 3-2: ACCESSING PROGRAM MEMORY VIA FSR constants BRW ;Add Index in W to ;program counter to ;select data RETLW DATA0 ;Index0 data RETLW DATA1 ;Index1 data RETLW DATA2 RETLW DATA3 my_function ;… LOTS OF CODE… MOVLW DATA_INDEX call constants ;… THE CONSTANT IS IN W constants RETLW DATA0 ;Index0 data RETLW DATA1 ;Index1 data RETLW DATA2 RETLW DATA3 my_function ;… LOTS OF CODE… MOVLW LOW constants MOVWF FSR1L MOVLW HIGH constants MOVWF FSR1H MOVIW 0[FSR1] ;THE PROGRAM MEMORY IS IN W |
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