Файл: Microcontroller Programming. Thi Micro Chip PIC (Julio Sanchez, 2007).pdf

invoked. For example, the following macros make the corresponding bank selections in a mid-range PIC with four banks.

; Macros to select the register banks

Once the bank switching macros have been defined, the application can change banks simply by calling the macro name; for example, if we know that the ADCON1 register is in bank 1 we can select the bank by calling:

Bank1

At this point in the code the macro expansion inserts the corresponding operations to make the switch.

Which method to use when switching banks is a matter of personal preference and program constraints. Setting and clearing the RP1/RP0 bits is simple enough, but can be error-prone. Using the banksel directive is convenient since we do not need to know in which bank the item is located. The objection to using banksel is that some unnecessary bank changes may take place. For example, if the program is already in bank 1 and the banksel directive appears with a register file in that same bank, bank switching is generated.

The use of bank selection macros seems like a suitable method for most conditions. One advantage of the macro approach is that programs for different PICs can have their own banking macros. This way code can be easily ported to a different architecture.

Deprecated Banking Instructions

Several instructions in the mid-range instruction set have been deprecated and are no longer recommended by Microchip. These instructions are tris and option. Microchip’s reason for not recommending these instructions is to maintain compatibility with future mid-range products. From a programmer’s viewpoint, it is difficult to see why using these instructions may be undesirable. In the unlikely case that code using tris or option is ported to a future device that does not support them, it will be easy enough to modify.

The tris and option instructions are convenient since they allow loading the contents of the w register to the OPTION, TRISA, and TRISB registers directly, without bank concerns. For example, the following code fragment sets port line 1 to input and all others to output:

tris PORTA

We continue to use the deprecated instructions in programs in which there is no concern about future consequences. In programs in which portability is an issue, we use the banking macros discussed previously.

9.4.4 Processor and Configuration Controls

PIC programs must define the processor to be used by the development software. The processor directive assembler (and also the list directive) allows defining the PIC type. For example, a program for the 16F877 would contain the following line:

processor 16f877

Configuration Bits

The PIC microcontrollers contain a special register called the configuration register. The bits in this register allow customizing certain processor features. These bits are mapped to program memory location 0x2007. This memory location can be accessed only during the programming mode, so the bits cannot be changed during normal program operation. The configuration bits cannot be read at runtime.

Microchip recommends that the configuration bits be set by means of the __config directive. The bits are mapped as follows:

CP1:CP0: Code Protection bits

11 = Code protection off

10 = See device data sheet

01 = See device data sheet

00 = All memory is code protected

Some devices use different numbers of bits to determine the level of code protection. Some use a single bit. In this case, the encoding is as follows:

1 = Code protection off

0 = Code protection on

DP: Data EEPROM Memory Code Protection bit

1 = Code protection off

0 = Data EEPROM Memory is code protected

BODEN: Brown-Out Reset Enable bit

1 = BOR enabled

0 = BOR disabled

Enabling Brown-out Reset automatically enables PWRT (the Power-up Timer) regardless of the value of bit PWRTE. The Power-up Timer must be enabled any time that the Brown-out Reset is enabled.

PWRTE: Power-up Timer Enable bit 1 = PWRT disabled

0 = PWRT enabled

See note about the BODEN bit.

MCLRE: MCLR Pin Function Select bit

FOSC1:FOSC0: Oscillator Selection bits 11 = RC oscillator

10 = HS oscillator

01 = XT oscillator

00 = LP oscillator

FOSC2:FOSC0: Oscillator Selection bits 111 = EXTRC oscillator, with CLKOUT 110 = EXTRC oscillator

101 = INTRC oscillator, with CLKOUT

100 = INTRC oscillator

011 = Reserved

010 = HS oscillator

001 = XT oscillator

000 = LP oscillator

The __config directive is used to embed configuration data in the source file. Alternatively, the configuration bits can be set at the time the PIC is blown. The following code fragment shows setting the configuration bits for a 16F877 PIC:

; Switches used in __config directive:

;_PWRTE_OFF

;_PWRTE_OFF

;* _CPD_OFF

;OSCILLATOR CONFIGURATIONS:

; |_____ * indicates setup values presently selected

__CONFIG _CP_OFF & _WDT_OFF & _BODEN_OFF & _PWRTE_ON & _HS_OSC & _WDT_OFF & _LVP_OFF & _CPD_OFF

;——- INTCON Bits —————————————————————————

;——- EECON1 Bits —————————————————————————

;====================================================================

;

; RAM Definition

;

;====================================================================

__MAXRAM H’CF’

__BADRAM H’07’, H’50’-H’7F’, H’87’