Summary of How to use an LED Array Module using AVR
This article explains how to use a 5x7 or 8x8 LED array module with a microcontroller to display characters and simple graphics. It details the hardware connection using two eight-bit ports, describes the method for encoding column patterns into hexadecimal values for firmware, and outlines the process of creating a custom font set by mapping illuminated LEDs to binary data.
Parts used in the LED Array Module Project:
- LED Array Module (5x7 or 8x8)
- Microcontroller
- Port A (5 lines)
- Port C (7 lines)
- Hexadecimal pattern values
With a single LED you can indicate the state of something: on or off. That might be a little boring. With an array of LEDs you can display characters or even some simple blocky graphics. That might add a little pizzaz to a small microcontroller project.
You could use individual LEDs wired into a grid of rows and columns, or you can make use of a module that has the LEDs conveniently placed into a single package. Who wants to wrangle with a pile of LEDs? That is a LOT of leads to solder.
Remember the old dot-matrix printers? CGA displays? No? Well old dudes like myself sure do. Many devices like those displayed or printed their characters as an array of 7 rows of 5 pixels/dots. This project will help you learn how to use a 5 by 7 LED array.
As an alternate you can make use of an 8 by 8 LED array.
This Instructable will cover how the LED Array Module works and how to design a character set to display. I will put together a very simple microcontroller circuit to drive the display, and list the development environment and some of the firmware needed to do the task.
Step 1 An array of LEDs
“Encoding column patterns” sounds like something an android would do as a leisure activity. For this Instructable it is converting a pattern of illuminated LEDs into a hex value that can be used in the microcontroller source code.
The microcontroller will connect to the LED Array Module using two eight-bit ports. The 5 columns are connected to 5 lines from port A of the microcontroller. The 7 rows will be connected to 7 lines from port C.
The 5 column lines correspond to port A bits b0 to b4. Bit b0 is column 1, b4 is column 5. The microcontroller firmware turns on one column at a time, cycling through this hex pattern over and over:
0x01
0x02
0x04
0x08
0x10
The 7 row lines correspond to port C bits b0 to b6. Port C bit b0 is row 1 while b6 is row 7. The microcontroller firmware sends a pattern to the row unique to each column.
To encode the pattern for a column, the character pattern – font if you want to call it that – was drawn out for each ASCII character. Then each column was converted into a hex byte.
Rows 1-4 are b0-b3. This is the lower nibble of the column pattern. Rows 5-7 are b6-b4 – the upper nibble. If you want LED C1 R1 to be on, then the encoded value for the first byte of the character becomes 0x01.
You might note that sending a 0x01 to port C would turn on all of the LEDs in the row except for C1 R1. This is because an on led is encoded as a 1 bit, while the off LEDs are encoded as 0 bits. The firmware simply inverts the encoded patterns before sending them to Port C.
It is simpler to understand the patterns when 1 = on.
Step 3 Pattern guide
This is a page from my notes. It shows a column from the LED Array Module. A penciled-in square represents an LED that is on. Immediately under it is the hex value for that nibble.
If these LEDs are on:
C1 R1
C1 R3
C1 R5
C1 R7
Then the lower nibble and the upper nibble pattern would translate into the hex value of 0x55.
Each character has 5 columns, so there would be 5 bytes of data needed to display any one character.
Since we are talking about 5 here, the bytes needed to display the 5 character are:
0x27
0x45
0x45
0x45
0x37
This was determined by drawing out the five in a 5 by 7 grid of boxes and encoding each column into a number.
See note1.zip for a copy of the notes.
Step 4 Creating a font for the display
To make a font for the 5 by 7 characters, I filled in a 5 by 7 grid in my notes.
After a while – about the time I got to the ampersand – my hand cramped up from coloring in little boxes. Coloring is the type of thing you want to have a kid do.
Luckily for me about a decade and a half ago I decided to have a kid just to do things like this for me.
So I had my son create this font. In the pictures below are my notes – and by “my” I mean his – that show all of the character patterns as well as the encoded bytes needed to display each column of the character.
See note2.zip for a copy of the notes.
For more Detail: How to use an LED Array Module using AVR
- How does the microcontroller connect to the LED Array Module?
The microcontroller connects using two eight-bit ports where five columns link to Port A and seven rows link to Port C. - What is the purpose of converting LED patterns into hex values?
Converting patterns into hex values allows the microcontroller source code to drive the specific illumination of LEDs in the array. - Which bits on Port A correspond to the columns?
Bits b0 to b4 on Port A correspond to columns 1 through 5 respectively. - How are row patterns encoded in the firmware?
Firmware sends a unique pattern to the row for each column, cycling through hex patterns like 0x01, 0x02, 0x04, 0x08, and 0x10. - Why does sending 0x01 to Port C turn off most LEDs except one?
An on LED is encoded as a 1 bit while off LEDs are 0 bits, so the firmware must invert the encoded patterns before sending them to Port C. - How many bytes of data are needed to display a single character?
Five bytes of data are needed because each character consists of five columns that require individual encoding. - What is the hex value for a column with LEDs on at rows 1, 3, 5, and 7?
The lower and upper nibble patterns for those specific rows translate into the hex value of 0x55. - Can an 8x8 LED array be used instead of a 5x7 array?
Yes, the article states you can make use of an 8 by 8 LED array as an alternate option.
