There is a nice feature in the instruction set that allows you to use a data table. A data table is simply a list of data values, where each one is read depending on some criteria. For example, you might have a circuit that uses a PIC where it counts the number of times an input pin goes high in 1 second. You can then display the number on a 7 segment display. Once the timing has started, the PIC counts the number of times the pin goes high. After 1 second it goes to the table and looks up the information it needs to display the number on the display that corresponds to the number of times the pin went high. This is useful, because we don’t know what the number will be until the PIC has completed its count. By using a table, we can let the PIC decide which number to display.
Now, before I carry on to explain how the data table works, I have to explain how the PIC keeps track of whereabouts in the program it is when the program is running. It helps if you have done some programming in BASIC. If not, don’t worry, you should still be able to see the concept.
Imagine we have a BASIC program like the one shown below:
10 LET K=0
12 IF K>10 THEN GOTO 20 ELSE GOTO 11
20 PRINT K
The program starts at line 10. Once K is set to 0, it then proceeds to line 11. After we have added 1 to K we then move on to line 12. Here we are asking if K is greater than 10. If it is, then we go to line 20, if not we go back to line 11. Line 20 prints the value of K, and line 21 ends the program. BASIC uses line numbers to help the programmer keep track of where things are, as labels are not allowed.
The PIC uses labels to jump between locations – or does it? We use the labels so that we know where things are, and also so that we can tell the PIC in an easy way where to go. What actually happens is the PIC uses an internal line counter called a Program Counter. The Program Counter (abbreviated to PC) keeps track of the memory location of where the current instruction is. When we tell the PIC to go to a particular label, it know the memory location and hence increase the PC until it reads that memory location. This is exactly the same way as we read the BASIC program above. Below is a section of code, with the memory locations, or the contents of the PC, next to each instruction:
0000 movlw 03
0001 movwf 0C
0002 Loop decfsc 0C
0003 goto Loop
In the example above, I have set the PC to 0000. At this location we have the instruction movlw 03. When the PIC has executed this instruction, it increments the PC so that the next instruction is read. Here the PIC sees movwf 0C. The PC is incremented again. Now the PIC reads decfsc 0C. If the contents of 0C are not 0, then the PC is incremented by 1, and the next instruction, goto Loop, tells the PC to go back to location 0003, which is where we have said Loop. If the contents of 0C is 0, then the PC is told to increment by 2, in other words skip the next instruction. This puts the PC at location 0004, where the program ends. The locations are set by the assembler, and we don’t normally need to worry what the PC is doing. Until, that is we need to control it like we are about to do when using data tables.
The best way to explain how a data table works, is to start off with an example.
PC equ 02
table addwf PC
The first instruction is assigning the label PC with the address of the Program Counter (02h). We are then placing the value of 03h into the w register. We then make a call to table. The first line in the subroutine table adds the contents of the W register (03h) to the program counter. This causes the program counter to increase by 3, or to put it another way, causes the program counter to move down 3 lines. When the counter reaches 3 lines down it the PIC sees the instruction retlw. This command passes the value following it into the W register, and then returns from the subroutine. RETLW actually means Return, Literal to W. Notice I put a comma after the word Return. As we are in a subroutine, we need a Return instruction to come out of it. Hence the RET in the instruction. After the RETLW instruction is a number, and this is what is placed in the W register. In this case it is the number 3.
We can assign any number to the W register, as long as when this number is added to the Program Counter in the table subroutine, we will find a retlw instruction. In the above example this means we can have any number from 1 to 7. If we go past the subroutine, we could end up executing another part of the program. Because of this, it is always a good idea to put the data table right at the end of the PIC program, so if we do overshoot then we will reach the end of the program anyway.