Powering the LED of the clock

Obviously the hardest part of the clock was all the LED. For a display of 21x15 you already have 315 LED that need to be powered, and even the large 16F877A has only 40 pins. The first step to simplify things is to group the LED in groups of 7, with a shared cathode. At that point you can select a group of LED by setting the cathode to 0V, and light a specific LED by setting its anode to 5V. LED burn very brightly at 5V, but can handle this only for a short duration of time (the so-called duty interval). For the LED I used the duty interval was 1/16th, but because of the amount of LED I stayed well below that. To select a group of LED I used the 74LS154, which demultiplexes a 4-bit input into 16 separate lines that can be individually pulled low. Attaching these to each of the cathodes would allow the control of 16 groups of LED. As they are grouped in groups of 7, this meant approximately 1/3rd of the LED could be controlled this way. Using 3 74LS154 the entire board could be controlled (in the schematic not all LED are drawn, one has to imagine the missing LED in between). To make things easy the 74LS154 allows you to individually select which 74LS154 is active at a given time, so with only 7 lines you can control all cathodes, and with 7 additional lines all anodes. With 45 cathodes, this means the duty cycle is, at best, 1/45th. However, it turned out that the drain on the 74LS154 is not high enough to power 7 LED at the same time, and it has to be split in 2 groups (one of 3 LED, one of 4 LED). This means the duty cycle became 1/90th. It turns out that even at 1/90th the LED burn quite brightly and are well visible.

The anodes could obviously be powered directly from the PIC16F877A, as it can output 25mA per line. However, it can only output 100mA in total, and it is therefore recommend to add the 74LS125 buffer chip, which can output 35mA per line. Do not forget to add the 0.1uF decoupling capacitors at every IC used: in this case there are 3 74LS154, 2 74LS125 and 1 16F877A that need decoupling. The result is a display that can be used for many purposes, but was used, in this case, for a clock. Alternatively the LED could be all put in a long line, making a scroll text display.

Clock

Another interesting project I completed last year was a clock, that uses 5x7 segment displays in a 21x15 configuration to form a display, on which a clock is displayed. The clock also measures temperature, keeps graphs over time and has a scroll text explaining its function. It is controlled by two buttons. On the picture you can see the normal display, with the clock set to approximately 5:30 PM (the LED in the corner indicates the PM). It also shows the current temperature in Celcius as well as the date (13th). The clock uses a PIC16F877A, mostly because of the large number of outputs that is needed to control all the LED. Most time was the soldering of all these connections, I'll include a picture of the back later, which shows how this was done. There are some IC to ensure proper power to the LED (although it is still lacking, occasionally). It uses the DS1307 for the time. The DS1302 with capacitor would've been better, but unfortunately I had none available, so I used the DS1307 with a separate battery.

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.

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