Calculator software

No diagram of how the PIC16F877A was actually connected: the only special things were the ceramic resonator that I used to keep it at a specific frequency (the PIC16F877A unfortunately does not have an internal clock) and the diode that I used to share pins with the ICSP circuitry.

This article concerns the code for the calculator I built.
By now a lot of features of my code should be recognizable. The initialization of the system is generally the same, both of the LCD display as well as the rest of the system. The code starts defining icons used on the display: the HD44780 allows user definable characters, and here they are defined and read. Then there is the keymap, which maps the scanlines of the keyboard to actual values. After initialization, the following steps happen:

1. Day and month are read by sending their locations in the DS1302 and calling TIMERead which retrieves that value.
2. In the birthday loop ROMretrieveword is used to retrieve the next birthday, which is then compared with the current date to see if the name associated with this birthday should be displayed.
3. After this done it waits for a random key to be pressed, indicating that the birthdays have been viewed. After this the birthdays are not displayed until the calculator is turned off and on again.
4. At this point it reaches the MainLoop where the menu is displayed and the user can use a key to select an item from the menu. The menu has groups of 3, representing the lines on the keypad. This interface has failed in all my user tests, but it works for me. You might want to find a better organization, though. The menu options are 1 to display the time, 2 to set the time, 3 to use the calculator and 4 to use the address book.

The addressbook starts at MainContacts:
1. The LCD display is cleared.
2. ROMretrieveword is used to skip the birthday.
3. If this was the last contact, go back to the main menu.
4. Display the first line of the contact (which is the name).
5. Wait for a key press.
6. If it is 2, skip the next parts of the address and continue at 2.
7. Show rest of address and return to main menu.

The calculator starts at MainCalculator. The menu works the same as the main menu. Some of the keys on the keyboard (backspace, +, +/- and =) have short cuts. At any point # can be used to access a menu with additional features. The most remarkable aspect of the calculator is the method of storage: the numbers are stored as they would be displayed, in the decimal system. All calculations are done using the decimal system as well, using the mechanisms that are taught to children. Not efficient in either memory and space, but quite readable and very easy to convert between keyboard input and display, which was my biggest concern.

A few other routines that require mentioning are the keypad read routine, which always checks for *. If * is pressed, the system jumps to the main menu, regardless of what state it is in. This is a very useful shortcut for the user, but is a good example of how code should not be written. Another aspect of the keypad that is interesting is how it debounces. The other interesting aspect is the way data is stored. The data is not stored in an external memory, like with the GPS system, but
uses the main memory of the PIC16F877A. It has 8K of 14 bit wide memory, and using clever tricks we can store 12K of address data and birthdays in this memory. I wrote a separate JAVA program to cover this address compression.

Keeping time and date

Keeping time and date is relatively simple if one uses an external component. It is in principle possible to use the PIC microcontroller itself, moving it into sleep mode, giving it a 32.768 kHz crystal, et cetera. However, the head aches are not worth it if you have the space for a 8 pin DIP, like the DS1302. The DS1307 is another option if you wish to use I2C. This circuit was meant for my calculator, mostly to keep the date so it would tell me of birthdays that I might otherwise miss.

Connecting a DS1302 is pretty simple: just connect the 3 wires to the PIC microcontroller, connect the X1 and X2 to an appropriate crystal (these crystals have very small pins, so if a socket is used for the DS1302, something I would recommend, you can add the crystal to the same socket, and not solder it) and connect the power. Do not forget the 0.1uF decoupling capacitor, although in this case it might be less important, because the DS1302 is supposed to work using a backup battery. The DS1302 can charge this backup battery, so the ideal choice is a super capacitor. The value in the circuit is no joke, I used a 1F capacitor for this purpose. They are slightly bigger than a watch battery and can NOT handle voltages larger than 5V. However, most of the time it will not charge over 2V, which is still sufficient for the DS1302 to work. It has never lost time in my application, even with the power being off for weeks. The hardest part of the DS1302 is the software, but fortunately you can just copy the pieces of code that I wrote.

Calculator switchpad

Today I got my first comment on this blog, and I appreciate it a lot. Hopefully more comments follow.

The switchpad of the calculator came from Jameco, and was basically 16 switches in a grid, as displayed in this schematic. To connect them to the PIC16F877A was simple: on the read side the voltage is pulled up to a known voltage (5V makes perfect sense) using a resistor network. The values of these resistors are arbitrary. I like 100k Ohm because this reduces power consumption. Speed does not really matter as they are switches anyway, and their fastest response time is 10ms. On the scan side you just connect the pins to the PIC16F877A. To read it is a matter of setting a particular line to 0V and see if any of the receiving ports detect it. If this is the case, that particular switch was pressed.

The software is slightly more complicated, because switches tend to "bounce" a bit, and it needs to be debounced, preferably over a 20ms range. So if there are changes faster than 20ms, these should be ignored, and the system should wait until the signal is stable for over 20ms. This is relatively easy to code as well, though.

My calculator

On the piclist I recently posted that I would publish my calculator here. So here is the first installment, with picture.

As you can see, the calculator uses a weird keyboard that I found in the catalog of Jameco. It used an even smaller one first (with only 12 keys) and later I replaced it with the current one. It uses a PIC16F877A microcontroller to do basic arithmic operations, has an HD44780 display and works using 2 AAA batteries (and has worked on the same batteries since 2006). There is a LED to indicate it is on, as well as an on/off switch. I've started making a cover out of it (using a cat litter container) but this is not finished yet. The software for the calculator is available.

Features of the calculator are:
* Keeps time and date
* Alerts of birthdays
* Stores addresses
* Basic arithmetic (multiply, divide, addition, subtraction, random number)
* 0.00000001 to 99999999