Servo - June 2010

Formalizing the Variable List

To finish off the first round of design, it is helpful to make some concrete decisions about how data will be stored. After the most basic features are prototyped, you should have a fairly good idea of how well your assumptions have played out, and whether or not you need to add any major variables. I like to lay out specifically where each variable should be placed in data memory. This is especially important in a microcontroller with banked memory, because switching banks takes extra time and memory. In some microcontrollers, you may not be able to copy data from one bank to another without using temporary variables in the non-banked (or shadowed) memory — unless you switch banks between each item.

I then lay out the memory in a spreadsheet in the same format as the data sheet’s register map (Figure 2). I use one cell for each memory location, and type in the variable names that I want in each place. This makes them easy to rearrange. When writing your program later, you can easily see which bank holds each variable. To minimize bank switching, the shadowed memory should hold most variables that are used frequently or used by multiple parts of the program. It is also useful to place temporary variables there, so they can be accessed no matter which bank you are using at the time.

00

00h INDF 01h TMR0

02h PCL 03h STATUS 04h FSR 05h OSCCAL 06h PORTB 07h PORTC 08h Delay0

09h Delay1

0Ah Scratch0

0Bh Scratch1

0Ch Scratch2

0Dh Scratch3

0Eh Scratch4

0Fh Scratch5

10h Display0

11h Display1

12h Display2

13h Display3

14h Display4

15h Display5

16h Display6

17h Display7

18h Display8

19h AppState 1Ah SwState0Prev 1Bh SwState0Curr 1Ch SwState1Prev 1Dh SwState1Curr 1Eh GameOptions 1Fh OptionCounter FIGURE 2. Data memory diagram with variable names.

Bank bits 01 10 || || || || || || || Shadowed Shadowed || || || || || || || || MasksL0 MasksH0

MasksL1 MasksH1

MasksL2 MasksH2

MasksL3 MasksH3

MasksL4 MasksH4

MasksL5 MasksH5

MasksL6 MasksH6

MasksL7 MasksH7

MasksL8 MasksH8

Random1

Random2

Random3

Random4

Random5

Random6

Random7

Random8

Random9

PUZZLE BOX PARTS LIST

Specific Parts, Supplier & Part
Microcontroller
Microchip – PIC16F505
Pushbutton switches ( 9)
RadioShack 275-1547 or 275-1571
LEDs, D0-D8
RadioShack 276-1622 (assorted)

Common Parts
Power switch, SPST
0.1 µF capacitor, any type
Resistors R1-R3, 220 ohms
Diodes D9-D11, general-purpose such
as 1N4001
Single-row six-pin header for ICSP
(optional)

Download files that accompany this article are available at www.servomagazine.com.

Additional Analysis

Analysis and design tend to blend together, and that is okay. It would be hard to avoid making any decisions on the design while analyzing the requirements or the results of testing your design. Forcing yourself to stop and analyze where you stand is a good way to check if you are really designing the software the best way you can.

In the prototype, I had stored the entire value I read from PORTB as a variable to use each time I ran through the main loop. This wastes some memory, but more importantly it makes it harder to work with the switch values. I noticed at this point that it would be easier to store the previous and current values of each switch as a single bit in two separate variables. If these variables are XORed together, a bit is set for each switch that changes states.

The display and switch reading code in the prototype worked well, so I determined there was nothing major to change there. I tweaked the variable list (adding the new previous/current variables for the switch states) and after reviewing the variable list again, everything looked good, so no additional analysis was needed.