The Registers

Zoltan Padrah edited this page Aug 22, 2012 · 1 revision
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First thing you will notice is that it is split into two - Bank 0 and Bank 1. Bank 1 is used to control the actual operation of the PIC, for example to tell the PIC which bits of Port A are input and which are output. Bank 0 is used to manipulate the data. An example is as follows: Let us say we want to make one bit on Port A high. First we need to go to Bank 1 to set the particular bit, or pin, on Port A as an output. We then come back to Bank 0 and send a logic 1 (bit 1) to that pin. The most common registers in Bank 1 we are going to use are STATUS, TRISA and TRISB. The first allows us to come back to Bank 0, TRISA allows us to select which pins on Port A are output and which are input, TRISB allows us to select which pins on Port B are output and which are input. The SELECT register in Bank 0 allows us to switch to Bank 1. Let us take a closer look at these three registers,

Table of Contents

Status

To change from Bank 0 to Bank 1 we tell the STAUS register. We do this by setting bit 5 of the STATUS register to 1. To switch back to Bank 0, we set bit 5 of the STATUS register to 0.

TRISA and TRISB

These are located at addresses 85h and 86h respectively. To program a pin to be an output or an input, we simply send a 0 or a 1 to the relevant bit in the register. Now, this can either be done in binary, or hex. I personally use both, as the binary does help visualize the port. If you are not conversant with converting from binary to hex and vice versa, then use a scientific calculator. So, on Port A we have 5 pins, and hence 5 bits. If I wanted to set one of the pins to input, I send a 1 to the relevant bit If I wanted to set one of the pins to an output, I set the relevant bit to 0. The bits are arranges in exactly the same way as the pins, in other words bit 0 is RA0, bit 1 is RA1, bit 2 is RA2 and so on. Lets take an example. If I wanted to set RA0, RA3 and RA4 as outputs, and RA1 and RA2 as inputs, I send this: 00110 (06h). Note that bit zero is on the right, as shown:

PORTA Pins RA4 RA3 RA2 RA1 RA0
Bit Number 4 3 2 1 0
Binary 0 0 1 1 1

The same goes for TRISB

PORTB Pins RB7 RB6 RB5 RB4 RB3 RB2 RB1 RB0
Bit Number 7 6 5 4 3 2 1 0

PORTA and PORTB

To send one of our output pins high, we simply send a 1 to the corresponding bit in our PORTA or PORTB register. The same format follows as for the TRISA and TRISB registers. To read if a pin is high or low on our port pins, we can perform a check to see if the particular corresponding bit is set to high (1) or set to low (0). Before I give an example code, I need to explain just two more register w and f.

W

The W register is a general register in which you can put any value that you wish. Once you have assigned a value to W, you can add it to another value, or move it. If you assign another value to W, its contents are overwritten.