Features and Functions of PIC Series Microcontroller Data Memory (1)

Although there are many varieties of PIC series MCUs, the data memory settings of internal hardware resources of each product are still very regular. The author takes PIC16C71A and PIC16C63/65/65A as examples to view the structure of their on-chip data memory, find out their characteristics and explain the main functions of some registers for users to quickly program. Table 1 and Table 2 are the resource tables of the on-chip data memory of PIC16C71A and PIC16C63/65/65A respectively. The resources of the on-chip data memory structure of other series products are similar to those of Table 1 and Table 2. The difference is only the slice. The types and numbers of internal functional components are different (the PIC16C57/58 microcontroller has 4 banks). The author introduces the characteristics and main functions of Table 1 and Table 2 from the following aspects.

1 Unified Addressing PIC Series MCUs All types of data memory are operated and addressed in register mode. The special registers include the timing register TMRO, the select register OPTION (also known as the item selection register), the program counter PCL, the status register STATUS, the indirect addressing registers INDF and FSR, the port I/O registers (such as PORTA, PORTB...) and phases. Corresponding port I/O control registers (also known as port I/O data direction registers such as TRIAS, TRISB...), holding register PCLATH, and interrupt control register INTCON. The above special registers are common to the PIC16C63/65/65A and PIC16C71A. They are not only the register names and functions, but also the registers. If you look at other PIC microcontrollers, such as PIC16C62/62A/64/64A, PIC16C71/72/73/73A/74/74A, PIC16C8X... their special register names are the same as above, and their addresses are exactly the same as above. Although there are many varieties of PIC series MCUs, it is not difficult to learn after mastering their laws.

For PIC microcontrollers with different models, the internal resources of the data memory are only the types of functions and how many different. For example, the PIC16C71A type has a pin of 18 pins. The main function is an A/D conversion unit with 8 bits. There are 4 A/D channel analog inputs, so the special registers related to its A/D conversion unit are shown in Table 1. ADRES (for storing the numerical result of A/D conversion), A/D control register ADCON0 (for controlling the operation of the A/D converter), and A/D control register ADCON1 (for controlling the selection of the A/D pin) Function) and so on. For the PIC16C65/65A type, its pin is 40-pin, its function is stronger than the PIC16C71A microcontroller, so the type of special register in Table 2 of the data memory is much larger than that of Table 1.

Each register unit of a special register has a corresponding fixed purpose, and they can be divided into two categories: one for CPU operation (such as INDF and FSR, STATUS, PCL, etc.); the other is used to control peripheral function chips. Operation.

When learning the PIC microcontroller data memory, you should not only understand the function of each register unit, but also call them to complete the programming purpose when programming. Below I will explain their use with programming examples.

2  Indirect Addressing Registers INDF and FSR The indirect addressing register INDF at the top of the PIC microcontroller data memory, address 00 unit (address code minimum) is an empty register. It only has an address code and is not physically a real register. Its function often works in conjunction with the register FSR (also known as the register select register) for indirect addressing purposes. When learning the special registers INDF and FSR, it is helpful to remember that the following logic is helpful for programming: any instruction that uses the register INDF is logically accessing the RAM pointed to by the register FSR, ie, for INDF (self) Indirect addressing (access), the read should be FSR content. The following simple program is an example of clearing the contents of the RAM register 20h to 2Fh unit registers by indirect addressing.

MOVLW 0x20 ; 20h → w, pointer to the RAM unit

;initialization

MOVWF FSR; 20h → FSR, FSR points to RAM

LOOP CLRF INDF ; Clear INDF, clear the FSR content

; unit 20h→2Fh

INCF FSR; (pointer) FSR content plus 1

BTFSS FSR, 4; discriminate (instruction) D3 bit of FSR, if zero

; execute the next loop instruction; if it is 1 hop

;carried out.

GOTO LOOP; jump to LOOP (loop)

CONTINUE... ; completed function, continue to execute the program

It can be seen from the above instructions that the register clearing of the RAM addresses 20h to 2Fh is achieved due to the cooperation of the registers INDF and FSR. Since the number of instructions to perform the above functions is small, this simplifies the instruction system and simplifies the instruction set of the PIC microcontroller.

Note: The above instructions are easy to understand, so there is no need to repeat them, but one of the discriminating instructions "BTFSS FSR, 4" is more critical. This instruction is to ensure that the upper limit of the RAM address unit 2Fh is selected in the design. The corresponding binary number is 00101111B, and the fourth bit of the FSR is exactly 1. Therefore, a judgment instruction is used in the above instruction; BTFSS FSR, 4, determines whether the D3 bit value of the FSR is 1, and if it is not 1 and is 0, the next loop instruction GOTO LOOP is executed, so that the address in the FSR is continuously incremented by one. Until the D3 bit of the register FSR is 1, then its contents represent the RAM address exactly 2Fh.

It can be seen that when learning the special registers in the data memory of the PIC microcontroller, it is not necessary to learn the special registers of each product. It is only necessary to learn their common points first, and then select a dedicated register of a product model for detailed analysis. When you perform the necessary related instruction operations, you can fully grasp the microcontroller technology.

The A/D conversion register is an excerpt from the source program part of the PIC16F877 microcontroller chip (with flash memory) that is downloaded from the Internet to complete the A/D conversion, and it is used to explain the usage of the A/D conversion register in the instruction. Here we first refer to some source programs. The comments in the source program are the comments added by the author in accordance with the role of the instructions in the program (not the direct function of an instruction). This is something that beginners should pay attention to when reading the following instructions. The list of source programs for the A/D converter is as follows:

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