ARTICLE UNDER CONSTRUCTION

Things you will need:

• Instruction-set for PIC16F628
• PIC16F628 PDF Data Sheet
• BlankF628.asm
• Chip Programmer - PICkit2 from Modtronix
• MPASM (and MPLAB) comes with PICkit2
• PIC16F628.inc
• Library of Routines:
  • A-E
  • E-P
  • P-Z
• Notepad++ or VS Code

• Hex Addresses
• The .asm listing for the 15x7 Display
• The .hex code for the 15x7 Display

This project is what you have been waiting for.
Remember the scrolling signs you see in shops and on advertising billboards?
Now you can program your own moving sign with all the effects you can think of.
We have provided a blank canvas in the form of a scanning screen 15 LEDs by 7 LEDs and on this you can put anything you like.
Of course this is not a two-day job. You have to learn how to program and how to get the effects. But when you get something on the screen, you will be able to classify yourself as a PROGRAMMER.
Writing a program is a bit like writing a book. You have to think about a lot of things and actually invent many features. For instance, the design of each number and letter has to be determined. Then you have to decide how they are going to be stored and accessed. We have used one approach to displaying characters on the screen, but there are many ways to achieve the same result and you can start a-new or use our sub-routines.
The switches can be re-assigned at any stage and they can have more than one function.
It’s all up to you.
The first thing to do is go through the program and determine how each sub-routine operates. Then you can add your own ideas.
But remember this important point: Only add a very small step at a time.
Don’t expect a complex routine to work the first time.
Divide everything into small parts and make sure it works, before adding the next small step.
This is the only way to avoid frustration. You will learn a lot more about how to go about this process in this article and you can come out a successful programmer.
All the big signs are just an advancement on this project and once you have the concepts in place, you can tackle almost anything.

A point to note:

THE VOLTAGE

The voltage on the PIC chip is critical and the author had some random faults develop on the screen when the voltage dropped below 4.5v. As soon as any unusual fault develops, the first thing to do is check (replace) the batteries. The author also had a problem with the PIC chip when the supply was above 5.6v (in another project). So, be warned!

THE PC BOARD

This project is provided as a “prototyping project” as no PC board has been produced. The photographs show the project built on a matrix board using surface-mount components and very fine wire.
The final design will require a double-sided board with plate-through holes, especially for the matrix of LEDs, but for the moment you can refer to the photographs and construct the circuit yourself.
Don’t be put off by the appearance of the circuit. It is really just a duplication of a very simple circuit driving a LED that can be built on a matrix board with 0.1” spacing. Most of the surface-mount components can be positioned so the legs will reach a solder-land and you won’t need a special surface-mount board. But you will need good eyesight and a fine tipped soldering iron. Simply follow the layout and use the photographs as reference.

Let’s start:

THE CIRCUIT

The circuit looks very complex but it consists of just two chips, 105 LEDs, a set of driver (buffer) transistors and 6 tactile switches. All the transistors and resistors seem to disappear when surface-mount components are used and the whole project can be constructed on matrix board or on a double-sided plate-through hole PC board.
Here is the circuit:

The 4017 is a “decade counter” and this means it has 10 outputs, starting with output pin 3, only one output is HIGH at a time. The 10th output is pin 11 and then the counter starts at pin 3.
The program starts by clocking the 4017 until pin 9 is HIGH. This pin is connected to the input of the PIC chip and the program now knows the state of the 4017.
This is called SYNCHRONISING. See more on this below.
The ‘628 has two ports, Port A and Port B. Port A has 7 output lines (plus one input-only line) and Port B has 8 output lines. This creates our 15 lines for the screen. The screen is laid out as 15 LEDs “across” and 7 LEDs “down.” The data (or information) is fed in at the top of the screen from the PIC chip to the 15 LEDs and the LEDs are connected to the 0v rail via a transistor that is driven from the 4107 chip.
Each LED has an 82R resistor in series with the drive-line from the PIC chip and when you work out the voltage available and the value of the series resistance, you will find the current is about 30mA.
Since there are 7 rows of LEDs, each row is only turned on for 1/7th of the total time and this is why you need to drive the LEDs with a high current, as they are only getting a pulse for a very short period of time. Pulsing the LEDs like this produces a much brighter result than a constant current as your eye has a delay (or an extension) in seeing the illumination - called PERSISTENCE OF VISION and the end result is a very bright screen.

The top row is turned on for a short period of time as determined by a delay routine and then the drive-lines are turned off.
The information for the next line is then placed on the drive-line (port A and B) and the outputted to the screen. This also clocks the 4017 so the second row of LEDs is illuminated.
The program does not know which row of LEDs is being illuminated during the scanning process and that is why it is necessary to synchronize the two chips when the project is turned on, via a short sub-routine.
For the remainder of the running, the program assumes the two chips remain in synchronization. There is no reason why this will not be maintained, but it is not checked or verified at any stage. More on this under “SYNCHRONISING ” below.
This assumption is not allowed in an emergency environment but is perfectly suitable in our case.

Here is a photo of the project, built by S. Sandhu:

Here are some updates of the project by S. Sandhu:

See below for another photo of the project, built by a reader.

THE SUPPLY

The project is powered from 4 AA cells (6v) and when the cells are fresh, the voltage is slightly too high for reliable operation of the chip.
A power diode is used to drop the voltage to 5v4 and this is the rail voltage we have shown as 5v on the circuit diagram.

DESIGNING ON MATRIX BOARD

As you can see in the photos, we have built the 15x7 project on a matrix board.
This is a special PC board, designed by Talking Electronics. It has xx holes by xx holes at a spacing of 0.1” . This is the normal spacing for through-hole components but many of the surface-mount components can also be fitted by placing them so they touch at least one of the lands.
At first appearance, the project may look to be very messy, but after 30 years of designing circuits, this type of board is by the best for this type of circuit.
When you are designing a circuit, it is very important to have all the components and wiring on the same side of the board so you can see everything at the same time.
It is very frustrating to be turning the board over to follow the wiring from one place to another, especially when a fault develops.
Our type of layout reduces frustration and allows modifications to be easily implemented.
You can also place the components exactly as they will appear in the final design and this makes laying out the artwork a lot easier.
By using very fine enamelled wire to join the components, the wiring almost disappears and the surface-mount components are so small they take up almost no board-space at all.
Due to the complex wiring of the LEDs for the display, the final PC board will have to be double-sided, plate-through hole.
This will allow the tracks for the LEDs to run up and down the board to connect the columns and from left to right on the underside to pick up the rows.
A double-sided board also allows long tracks to be run to the switches etc.
When designing new projects, you need a range of prototyping boards to suit the circuit and components. Talking Electronics has designed boards for both through-hole components and surface-mount, including boards for surface-mount chips.
The most important feature of a prototyping board is the ease of building and modifying the circuit.
Depending on the complexity and the amount of modifying you will be carrying out, it is always easier to place both the components and wiring on the top-side so that everything is visible.
Sometimes a circuit has to be modified many times and the removal of a component that has been soldered through the board is awkward and eventually damages the board.

RECAPPING

This project uses 105 LEDs to produce a screen 15 LEDs long, by 7 LEDs High for “Running Messages.”
This is one of the best projects you can build as it has the widest application - - in shops, outdoor advertising, roadside information and lots more - larger signs are just an extension of the technology you learn in this project.
Larger displays can show more letters but this is the largest display that can be produced with a single PIC16F628 micro.
The LEDs are scanned by turning on the 15 LEDs along the top row from the PIC micro and sinking them via a transistor connected to the 4017 chip.
Each of the 15 LEDs is buffered by a transistor so the brightness of the screen can be a maximum.
After the 7 rows of LEDs, the 4017 accesses the 5 switches and the scan repeats.
By keeping the scan-rate high, no flicker is detected and the whole screen can be illuminated to a very high intensity as high-brightness LEDs are used.
When-ever you create a project as complex as this, there are a lot of things you have to decide upon and “invent.”
After deciding on the direction-of-scan, the letters and numbers have to be created and placed in a table.
The table (or tables) cannot be longer than a total of 256 bytes for all the tables as it is very awkward to access the second page of memory. The first page is actually called “page 0” and occupies program memory from 000 to 0FFh. You will notice we have only put two instructions before the tables, so they can be maximum length.
The next clever feature of the program is detecting the end of each character. Instead of placing a “marker” such as 0FFh, we have incorporated the marker with the last column (last byte) of data.
Refer to the following image showing the letters A to Z and see the last value for each column is increased by a value of 08h.

The numbers and letters above have been created by the author but if you want to change the appearance of any of them, refer to the values in table 2 and change the appropriate byte. (some of the formats of the characters can be improved.) The two tables do not use the full 0FFh bytes of the first page of memory. There are twenty six bytes available for your own use.

DETECTING

end of character etc etc etc
When writing a program, you have to know where the micro is at any given time.
The micro can only carry out one operation at a time but since it is executing instructions at one million per second, it can produce very impressive results.
Two of the things needed for a table for characters is: (1) the end-value for each character and (2) the end-marker for the table.
All the characters are placed in a long table and the end-value for character is simply a value that is not identical to any column-value. The program can then work out that the value is an end-value. Since 7 lines are used for the columns, all the values from 01h to 7Fh have been used. The only values available for a marker are 80h to 0FFh. This involves bit 7, the highest bit in the byte.
In this way it is easy to produce a program to go up or down the table and produce the letters and numbers. This could be done with a single table but reversing up the list of characters would require complex programming.
It is also necessary to know when the end of the table is reached so the up and down buttons do not pass the end or start of the list.
The third item that needs to be detected is a SPACE.

SYNCHRONISING

The first output of the 4017 must be active at the time when the PIC chip is outputting the first row of data and this means the 407 must be “clocked” so that it is turning on the first output at the beginning of the scanning cycle.
Since we do not have any spare outputs from the PIC chip to reset the 4017, we have to find another way to achieve this.
We have used the first output of the PIC chip to clock the 4017 and the input-only line, RA5 to detect when the 4017 is at the 9th output. The following sub-routine is used to clock the 4017 until the PIC chip detects a HIGH on input line RA5:

Set4017   bsf    status,rp0
          movlw  b'00100000'  ;Set the TRIS for A in/out
          movwf  05h
          bcf    status,rp0
Set_      movlw  b'00000001'  ;(turn off RA4) Make RA0 HIGH
          movwf  05h
          call   _10uS
          movlw  b'00000000'  ;Make RA0 LOW
          movwf  05h
          call   _10uS        ;clock 4017
          btfsc  05h,5        ;see if 4017 is at 9th output
          retlw  00
          goto   Set_

The program now knows the 4017 has output 9 HIGH and one clock of the 4017 via the following sub-routine will make the 10th output HIGH:

clock   movlw  b'00000001' ;clock the 4017
        movwf  05h
        call   _10uS
        movlw  b'00000000'
        movwf  05h
        call   _10uS
        retlw  00

The actual clocking of the 4017 is done via RA0, by taking it HIGH then LOW and the 4017 is now clocked by the sub-routine SCAN, which scans the 7 rows. In fact it is the presence of data on line RA0 that clocks the 4017 and if line RA0 does not contain data (it will be LOW), the sub-routine calls the clock routine above.
This is how the 4017 is synchronized with the PIC chip.
For the remainder of the program it is assumed the two chips are “in step” and there is no reason why this does not occur, if the 4017 receives its correct number of clocking pulses.

We have now clocked the 4017 without using any dedicated output lines from the PIC chip. This allows us to use all the outputs of the PIC to control 15 columns of LEDs on the display.
The next feature of the program is called a “set up” feature.
We need to have a “ghost” screen, equal to or larger, than the display so that whatever pixel we activate on the ghost display will be displayed on the actual screen.
This allows us to produce a routine that will easily move the characters left or right or up and down and whatever is on the ghost screen, will be displayed without any difficulty.
All the setting-up of the ghost screen and looking at the switches is done so fast that it does not interfere with the scanning of the display.

THE RASTER

When the project is turned on, the screen will be blank but it is actually being scanned at more than 100 frames per second but none of LEDs are activated. The scanning starts at the top of the screen and moves down, with each row being illuminated in turn.
This scanning is the same as the production of the white screen on a TV or monitor and is produced by a separate sub-routine that we will not be concerned with.
If the scan-rate is reduced, you will be able to see the rows being turned on in sequence. This can be done by increasing the length of the delay routine associated with the scan sub-routine and you should do this to see exactly what is happening on the screen.
If we did not scan the screen like this, we would need 105 drive-lines to drive each LED individually. However LEDs are ideally suited to being turned on and off very quickly and their performance improves with this “pulse” approach. The concept is called POV (Persistence Of Vision) and is explained elsewhere in this article.

When the project is turned on, the program moves the data in files 50h - 5Fh (called the GHOST files) to 70h - 7Fh (called the SCREEN files). If there is no data in the ghost files, the screen is blank.
The reason why ghost files are needed is simple. Any of the information we want to put onto the display, must be contained a set of files.
We now have to move the data from the files so it can be shown on the display. To show a complete screen of date requires 15 files.
How do we take the information from the 15 files and show it on the screen? Of course you can take each bit for each file and individually load it into port A and B and display it, but this will take a lot of programming.
The easiest way is to shift the bits in a file so that the end bit goes into a location called the “carry.” When another file is shifted, the bit in the carry is loaded into the file. This is a very clever way to transfer a bit from one file to another.
When this is done with 15 files, the data for the top row is available.
Also the sub-routine for the 15 files can be used again for the second row etc and thus the lines of code can be kept to a minimum.
This is how the end-bits of 8 files are loaded into a single temporary file and 7 bits are loaded into another temp file. These two files are then transferred to port A and Port B. Line RA5 (of port A) is an in-only pin and since it cannot output to the display, an extra shift is provided in the program to close the gap in the temporary file by the non-appearance of data from RA5.
It is much easier to work on ghost files when animations are required, however it is not essential, and you can try manipulating the screen files, as a challenge.
As you can see from the illustration below, the next letter (letter L) to be displayed is placed in a set of files that are outside the Ghost file area and it is very easy to shift the letter into position via a simple sub-routine.
If you can think of a simpler way to carry out any of these tasks, you can produce the code and send it in for inclusion in the article. This is what this article is all about.
After all, some of the smartest software in the world has been produced by very young, talented, programmers and I am not going to miss any opportunity to improve the project.
In this world, the more you know, the more you realise you don’t know.

BURNING THE CHIP

The PIC chip used in this project is a PIC16F628. This is an 18 pin device with 16 port-lines plus a pin for 5v and a pin for ground. It has two ports, called port A and port B. Each port has 8 lines but port A has RA4 as a sinking-line only and RA5 as input only.
This microcontroller chip comes BLANK from the manufacturer and you have to program it by “burning” a program into its memory. The “burning” process is called “programming” or simply “writing to the chip.” We have provided some details on this process in the section: Elektor,EPE,Silicon Chip
The program for the project can be viewed as a .asm file and each line of code is explained fully.
When you want to burn the chip, the .hex file is loaded into PICkit2 software called PICkit2 Programmer, that comes with the small plastic box called “Microchip PICkit2,” containing lots of surface-mount components to take the signal on the USB port and produce clock and data information to burn the chip you have placed in a socket that is connected to the output of the programmer.
See photo for chip being programmed:

THE SKY’s THE LIMIT

We have provided the basis for all sorts of visual effects.
If you have ever wanted to create your own programs to simulate animated displays, running signs and traffic control boards, now is your opportunity.
The screen may be small but it is sufficient to produce all sorts of visual effects and we have provided some basic sub-routines to guide you.

EFFECTS

The “effects” button is the pathway to the effects. By pushing this button you can re-name all the other buttons and produce any effect you like.
All “effects” take the screen data and modify it in some way.
We have already learnt, the data that will be shown on the screen is stored in a set of files 50h to 68h and these files are called GHOST FILES.
You will appreciate the concept of storing data in this way, as the files are easy to manipulate.
This has already been shown with the “running effect” where the letters are moved across the screen so words and sentences can be displayed.
The sub-routine “shift” creates this effect.
The next simplest effect is called COMPLIMENT.
This effect reverses the color of the letters, similar to white writing on black background, in printing.
The sub-routine “comp” changes all “1’s” into “0’s” and “0’s” into “1’s.”
In this sub-routine, all the illuminated LEDs are turned off and and the non-illuminated LEDs are turned on.
We do this by taking each file in turn and complementing it.
By calling the “run” sub-routine and adding the complement feature, we have produced a new effect.
As each column of data is placed in file 5Fh, it is complemented and “shift” displays it on the screen as reverse lettering.

USING THE PROJECT

When the project is complete and tested, the first thing you will want to do is produce something on the screen.
The 6 buttons on the project are labeled UP, DOWN, STORE, RUN, DELETE, and EFFECTS.
These buttons are used to create and scroll a message of up to 7Fh characters. Any message is immediately saved in EEPROM and thus it is not lost when the project is turned off. To write a new message, the old message must be deleted by pressing the DELETE button as explained below.

Here is the function of each button:

• UP - pressing this button will produce letters A - Z and numbers 1 - 9 then the number zero. The next character is a set of dots on the lower part of the screen to indicate a space.
• DOWN - pressing this button will produce letters in the reverse order. It can be used to go back to a missed letter or get to a character quickly without having to advance through the whole list.
• STORE - When the required letter appears on the right side of the screen, the STORE button is pressed. This will store the character in EEPROM and move the character to the left side of the screen and add a set of dots on the right side to indicate a place for the next character. If these dots are STORed, they will show when in the scroll mode. The space character is explained below.
• RUN - When the required number of characters has been stored, the RUN button is pressed and the message will scroll across the screen. You can store up to 7Fh characters.
• DELETE - pressing this button will remove the last character from the message when the message is scrolling across the screen. (To scroll the message, the RUN button is pressed).
• EFFECTS - This button will produce all type of effects as produced by you and other readers, as well as new effects, as they become available.

WRITING A NEW MESSAGE

To write a new message, the old message must be deleted by pressing the DELETE button until no character scrolls across the screen.
Start by viewing the old message. Turn the project ON and press RUN. You will see characters scroll across the screen. Now press the DELETE button until no character scrolls across the screen.
The project is now turned OFF then ON and is ready for you to create a new message.

To store a BLANK, press UP to get the letter A, then DOWN to get the row of 5 dots. This character is then stored as a SPACE. The Space character (called a BLANK) is located after the numbers and after the number zero. The 5 dots will not appear in the scroll-mode - just a empty space.

WRITING YOUR OWN SUB-ROUTINES

Before adding anything to the program, you need to look through what has been written and understand how it works.
Although this project is not very complex, you may want to see some of the simpler projects and get a grounding on how to write lines of code. See the left index for other PIC projects.
Each line of code has a comment to let you know what the instruction does. The layout of the program is alphabetical, with the Main sub-routine at the end of the program and the delay routines at the beginning. In this way you will know where to find things.
You can start by getting the “Effects” button to perform a function.
It can send the micro to a new area of the program in which all the buttons have a new function or the Effects button can display numbers on the screen to represent a list of new effects.
You can make the button perform anything at all. You can even create a sub-routine to scan from bottom-to-top or create a slow-scan to show how the scanning operation is carried out.

Firstly, look at the some of the signs in your local neighborhood. See the effects of flashing, scrolling up, and individual letters running across the screen. Many of these effects take a long time to view and the motorist has already passed, before the message has been delivered.
Consequently, some effects are counter-productive and fail to add to the delivery of the message.
Since our display is very small, only things like reversal,

Here is the 15x7 Display produced by one of our readers, Emilo Konishi:
e-mail: emilio@w3serigrafica.com.br
W3 Serigrafica www.w3serigrafica.com.br
Rua Canindé, 86-Jacaré- Rio de Janeiro- RJ- ZIP Code- 20975-010- Brazil
Phone: + 55 21 3125 3333
Mobile phone: +55 21 8852 7846

More details of the board can be obtained from Emilo.

Things to come:
Photos of the project showing close-ups of the components

Photos of the programmer, the interface board and the board containing the IC socket for programming the chip.