Raspberry Pi 2 B  -  LED Matrix Driver

7 - 23 - 2015

Documenting a Raspberry Pi 2 B build, to run an LED matrix system that is 8 x 112 super bright LED's.

LED Pannel Photo

There are 8 LED driver chips on each of 7 LED boards. Toshiba Bi-Cmos TB6270BN/BF  24 pin DIPS. Each chip drives one row with 16 LEDs and has a 16 bit internal latch with serial input chain that can go up to 20 MHz.
On the right end are 3 driver chips [TTL 74-240] for the Latch clocking and Output enable leads. The Logic is all 5 volt, and the LEDs use 3.3 volts from the PC power supply.
Brightness control can be achieved with an external resister [presently 1k] on each chip to set the LED constant current supply, and also by using a strobe sequencer on the 8 Output Enable leads.

The buss interface uses 8 data in pins, 8 output enable pins for strobing brightness, 1 clock pin, 1 Latch enable pin, and two blank display pins.

If we want to use the same control chips, we will need a total of 20 pins used in our feeder cable [40 pin cable].
We must remember to blank the display on power up, so a delay circuit may be required for that on our interface board.

A ground is carried on every other pin on the 40 pin connector to the LED panel.
There is also a power cable with plug to the PC power board.



 
This was a high school senior project in 2005, and since then the controller board has lost it's brains. It was designed and built with good documentation, but cannot be accessed anymore with our newer computer ports for troubleshooting or control.


Inside the Box Photo

You can see the display boards are hard wired, as is the controller board, and all chips were socketed for fast replacement.

The older controller board was accessed via a parallel printer interface, and none exist on new computers.
I will toss out the old controller board and replace it with the Pi, adding wireless network connection, wireless keyboard and mouse.
The only cable leaving the LED panel will be a monitor cable for temporary programming and the PC power supply cord.
I am hoping the Pi 2 can be powered from the 3.3v and / or the 5v supply.



The Raspberry Pi 2 is a computer that uses Linux software. It has a 40 pin IO port for experimenters. The hard drive is a compact micro SD chip with 8 GB of storage.
I am hoping I can write a driver to run the panel, and possibly build an interface board between the LED drivers and the Pi using only a few chips.
I will have to use logic level shifting to move from the 3.3v outputs of the Pi to 5v TTL logic of the LED driver board. The two ports on the Pi that are attractive are the I2C and the SPI, both serial ports.

I will try to list most of the parts I will be using with URL's from Amazon.com
or other vendors below.

Boot Screen Photo

Showing the boot screen to command line editing mode, for Linux access.



Showing the Pi 2 B assembled from the kit.

Pi 2 B photo

Amazon.com
Eleduino Raspberry Pi 2 (1GB) Super Starter Kit

This will be used to design the basic interface boards. Another bare Pi 2 B was ordered to go inside the LED sign box. It will not need a cover.
 

Showing the Pi Windows screen mode and all the beginning components of the project.

Windows Screen Photo

Amazon.com   Wireless Keyboard Mouse
Logitech Wireless Combo MK520

Dell Monitor  1920 x 1080  with USB hub and DVI or VGA input.
 Keyboard dongle is plugged into a USB on the monitor.

Monitor Cable
Gefen DVI to HDMI Cable 6 feet Male-Male
This conversion cable works with the Pi 2 B to drive the Dell monitor with a DVI port from it's HDMI port.
As I understand, just any cable may not work.

Raspberry Pi 2 C

I was quite amazed, that for it's small size, all the features the Pi 2 B offers. Standard PC components all interface very easily, including the wifi, wireless keyboard, and mouse.
There are many online tutorials for the Pi that can get you up and working. Here is the starting point, from the people who designed this system board.

https://www.raspberrypi.org/products/raspberry-pi-2-model-b/

On the down side, I will have a large learning curve in mastering the Raspbain Linux command line interface, and creating the software driver for this project.
I am fluent in C ++, but from a Windows Operating System environment.



Interface Board

Old Controller board was removed.
The new Pi with copper heat sink, is mounted on the back Plexiglass wall, and the new interface board on the floor below, with lots of room for circuit development.
First order of business to get the power feed to the Pi and bring it up.


Powering the PI



Note that the old hard disk connector provides 4 wires, yellow +12,  2 each Black grounds,  red + 5.
The old sata drive power connectors however have an orange lead that provides +3.3 vdc.

I opted to do a splice, orange to yellow, dropping the +12 and delivering the +3.3 to the board on the yellow connector wire.

The Raspberry uses 5 volts and then generates it's own 3 volt supply to run the logic on the board.
The logic will have to be converted to 5 volt for the interface that drives the LED display logic.
I will have to use level shifters to accomplish this, and they will be fed from the 3.3 on the interface board, rather then from the small regulator on the Pi.

Also note the de-spiking capacitors on both power lines to the ground buss and one near the Pi interface cable. This is to eliminate noise from the power crossing between chips, and is a good digital chip practice.
I now have power on the Pi from the older type PC power supply and it comes up, as you can see from the lit LED on the Pi!


Interface board Progress


Initial Layout

I have set up two interface chips to expand the I/O ports of the Pi, using level shifters to drive the 5 volt logic of the chips from the 3v SPI buss on the Pi.
I have placed two 40 pin connectors on the experimental board, so there is room to build two different circuits. One will use the high speed SPI buss, on the left side, the other will access an I2C module from the Pi expansion stack to be added later when the 20 pin connector cables arrive.
I want to compare the speed of the two different buss's and have option to drive the LED board with either one by moving the cable between these two connectors.

A driver chip will be added at the bottom in the black 14 pin socket, as the CLK clock, and LE Latch Enable leads have to drive 8 inputs each on the LED display board.
The data and chip enable leads will only have to drive one input to the display board, so the I/O expanders should be able to handle that without any aid.

MCP23S17-E/SS  SPI interface chips  and SSOIC adapter boards - Amazon.com
Much difficulty was encountered in soldering the small 28 pin chips to the SS0IC boards. I ruined the first 3 chips and hope the last two are good, after learning how to solder these tiny surface mount chips from videos on youtube.
We shall see if they work. If possible I would order larger chips next time, but these were not available anywhere I could locate.

SparkFun  parts
BOB-12009 Level shifter boards - 2 each - 8 shifters
PRT-13054 Raspberry PI GPIO Shrouded Connectors - 2 each
CAB-13028 Raspberry Pi GPIO Ribbon Cable 40 pin 6"


Proto board was from my "save it storage box" when we first built the unit 10 years ago.


Interface board bottom view

Bottom of the proto board showing all battery and grounds now run in. Orange wire is the 3 volt supply for the level shifters. Bare wires are gounds to the outer foil, that is grounded all the way around the board.
Red leads are the +5. I used 24 gauge for all the power and ground runs.
Note on the cable connectors for the LED display board one side of all the pins are a ground to eliminate noise on the cable. This leaves 20 active pins.
The two I/O expander chips will give us 32 outputs to choose from.



LED Board Pinout Detail

iopiplus photo
This board was purchased for the i2c interface and plugs onto the pi itslef.
2 each 20 pin ribbon cables are then used to extend the ports down onto the IO interface board.
Pinnouts are layed out below in the next diagram, showing cross connects.

Raspberry Pi 2 model B IO Pi Plus


IO Plus photo



Interconnect

Programming Reference

Port 0x20  Bits GPA 0-7

                 Screen Data, vertical line 8 bits  on = Hi

Port 0x20  Bits  GPB 0-7

                 Unused

Port 0x21  Bits GPA 0-7

                Strobe Leads, Horizontal Line 8 bits  on = Low

Port 0x21  Bits GPB 0-7

               Bit 7  Blank Display - Hi = Display on          10000000
               Bit 6  Clock               Lo = Edge Trigger      01000000
               Bit 5   Latch Enable - Lo = Active                00100000
               Bits 0-4 unused                                            00011111     


Getting the software drivers for C language control of the i2c I/O ports

sudo raspi-config [or set up system config from NOOBS startup menu]
set I2C bus active
set enable ssd for remote connection
set keyboard and language to en-us, delete uk
set local time zone and time

User:   pi
Password:  raspberry  "change this one"
Host Name LED-Sign

Local Router Setup
LED-Sign   Raspberry     7c:dd:90:8c:ba:0f       192.168.1.120   [locked in local internet router as DHCP for this hardware address]

startx: Set up ip access to my wireless router in the startx overlay on upper right of screen. Programs the wireless lan usb for you and connects you to the internet.


i2c Tools and Missing Config File Text

sudo apt-get update
sudo apt-get install i2c-tools

The NOOBS leaves out some necessary i2c changes to other files, that were hard to find, to get it working.
They are in bold below and had to be added to these files manually for the GPIO port and i2c testing to complete.

https://learn.adafruit.com/adafruits-raspberry-pi-lesson-4-gpio-setup/configuring-i2c
Installing Kernel Support (Manually) nano is a text editor.
Any lines missing must be added.

sudo nano /etc/modules

i2c-bcm2708
i2c-dev
Control-X Y <return> to save changes

sudo nano /boot/config.txt
dtparam=i2c1=on
dtparam=i2c_arm=on

sudo reboot

Testing the port access :

sudo i2cdetect -y 1

Port 20 21 are 0x hexidecimal values, and show up in the detection chart.
Previous to these additions all I got was "dev file does not exist."

I wish to work on the system remotely now.

Remote connection - Windows PC on same network
sudo raspi-config    [enable ssd remote connection]
sudo apt-get install xrdp
sudo service xrdp stop
sudo service xrdp start
sudo service xrdp restart
sudo reboot
 
Once the service is active on the raspberry, now set up a Remote Desktop Connection on your PC to 192.168.1.120 and then log in.


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