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Using Registers (SFRs) and reading the Microchip Data Sheet


  • Using Registers (SFRs) and reading the Microchip Data Sheet

    The following excerpt from the ME Labs Trainer Board and Tutorial is provided to help users create a deeper understanding of SFRs and how to utilize information from the Microchip Data Sheet to write to a specific register. Here we are using an excerpt from the Microchip PIC16F1937 Data Sheet.

    Using registers (SFRs) and reading the Microchip Data Sheet

    The Microchip Data Sheet holds a lot of information. The intent is for it to hold all the information you will ever need
    when developing program code for the microcontroller.

    In reality, you only need a small part of this information at any given time. Most of the time, you will be using the Data
    Sheet to learn the function of bits in a specific register (SFR) and setting the register to achieve the desired effect.

    What follows are some clues to help you make sense of the register diagrams and notes that are found in the Data

    Each register in the microcontroller is detailed with a diagram similar to the following. I've detailed the different parts
    of the diagram and its description below. The register shown below is called "ANSELB". As you can see, Microchip
    calls it the "PORTB Analog Select Register".

    Click image for larger version  Name:	register12-9.jpg Views:	1 Size:	98.4 KB ID:	6670

    (1) Bit information: This bar gives us clues about the nature of each individual bit in the register. The meanings
    of the codes are detailed in the legend (item-4). In this example, "U" means that the bit is unimplemented and
    "R/W" means that the bit is "read-write" (it can be both read and written). The numbers indicate the state of
    the bit after a reset or power-cycle. In this SFR, the unimplemented bits will always read zero. The other bits
    will be set to "1" after all types of reset.
    (2) Bit names: This bar lists the names of the individual bits. These names may be used elsewhere in the Data
    Sheet to refer to these bits.
    (3) Bit numbers: This bar shows how the bits are numbered. In this case, and in most cases, it only shows the
    leftmost bit-number (7) and the rightmost bit-number (0). The intent is for you to fill in the blanks. In reality,
    the bits are numbered | 7 | 6 | 5 | 4 | 3 | 2 | 1 | 0 |.
    (4) Legend: As mentioned previously, this block supplies the information needed to decode the information in the
    bit-information block (item-1).
    (5) Bit function description: This section lists all the bits and describes briefly what happens when the bits are set
    to "1" or "0". If a bit is a read-only indicator, the description denotes the meaning behind the state of the bit.
    (6) Notes: Lastly, Microchip provides notes that are important to know when using the register.
    Here are some practical examples showing different ways (there are many) to write to the ANSELB register. On our
    Trainer board, we need PORTB bits 4 and 5 (ANSB4 and ANSB5) to be analog inputs, and bits 0-3 to be digital I/O.
    The following examples will all accomplish this.

    ANSELB = %00110000 ' PortB.4 and B.5 analog inputs

    The example above writes values to all the bits in ANSELB. This works when you know what the all the bits need to
    be (sometimes you won't know). The percent sign signifies that you are writing a binary number. The order of the bits
    is the same as the order in the register diagram. Bit-7 is on the left and bit-0 is the on the right.

    ANSELB = $30 ' PortB.4 and B.5 analog inputs (hexadecimal)
    ANSELB = 48 ' PortB.4 and B.5 analog inputs (decimal)

    This example is functionally identical to the previous one, but I wrote the number in hexadecimal format ($30) and
    decimal format (48). Since the microcontroller only understands binary, the numbers in each case will be converted by
    the compiler. In reality, they are all the same number: %00110000 is equal to $30 which is equal to 48. Note that the
    percent sign (%) and the dollar sign ($) are specific to PICBASIC PRO. Other compilers may use different characters
    to specify the format of the number. It is your choice which format to use.

    ANSELB.0 = 0 ' PortB.0 digital I/O
    ANSELB.1 = 0 ' PortB.1 digital I/O
    ANSELB.2 = 0 ' PortB.2 digital I/O
    ANSELB.3 = 0 ' PortB.3 digital I/O

    The method above works because (as shown in the register diagram) all the implemented bits in ANSELB default to a
    value of "1" when power is applied. All we have to do is clear the bits that correspond to the pins that we want to be
    digital I/O. All other bits remain in their default states.
    Last edited by SteveC; 06-28-2017, 01:52 PM.
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