Places to order parts

In my previous post I referenced Futurlec as a place where you can order parts. Note that they are slow, but they are inexpensive and they have some very unique items. I really like their "LCD8X2", which is the smallest LCD display I have found so far that can still display 16 characters on two lines, and is controlled by a normal HD44780.

If I needs parts quickly, I usually go to Jameco, which is around the corner of my work place. They are more expensive, but they have very good service: you can just walk in, and walk out with your parts within 15 minutes. If you order them over the phone, they can even make them ready for pick up.

For the actual PIC microcontrollers I used to use Glitchbuster, but unfortunately they disappeared. I currently still have a large amount of them from winning the grand prize of the Microchip innovation competition but dread the moment that I would actually have to order more, as none of the websites I mentioned so far carry a good assortment. I might just order them directly of Microchip's website.

Finally, for all sorts of odds and ends, like soldering irons, solder, wirewrap wire, and various PCB, I tend to visit the local Radioshack. Do not buy parts here, as they are very expensive, and inferior in quality. Just look out for those things that will make your life better.

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.

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

Microcontroller LCD interface using HD44780

This is doubtless the most simple way of hooking up a character based LCD display with a built-in HD44780 controller (and almost all character LCD displays are like this). The Vcc and Vdd are connected as usual, with the 0.1uF decoupling capacitor. For the contrast voltage a 10k potentiometer is used. Normally this has to be calibrated once, so a tiny one that you set with a screwdriver is good. The R/W is grounded, ensuring that the connection is write only (do NOT use a resistor for this. Quite some current goes through this pin, and not grounding it will cause the I/O ports to occasionally go into read mode, which will destroy your microcontroller, even if it is a PIC). The RS and E(nable) lines are hooked up to arbitrary pins on the 16F628A, and the D0-D3 are left disconnected. D4-D7 are connected to half a port of the microcontroller, in this case PORTA (RA0-RA3). Pins 15 and 16 are sometimes absent and are used to control the backlight. Regardless, they are left disconnected too.

Controlling this uses the 4-bit interface with maximum delays (it is not possible to use shorter delays, as it is a write only connection and there is no way of knowing whether the sent nybble has arrived). The software should show how it works. The initialization sequence is crucial, and I recommend copying it from the code, as it is easy to get wrong. When you see no characters appear, also fiddle with the contrast setting, as this could be set such that nothing displays.