RSS MOVING DISPLAY Contest number NXP 3835 This is a report concerning the revival of an old 5x7led 24 digit message display board, where the controls are removed and the display functionality is completely absorbed by the Mbed module. With the new controller extra features are possible like internet connectivity. This enables us to add RSS‐display functionality and set the real time clock by a time server. It is also possible to “twitter” messages to the message board. This all should be worked out in short time, because the contest is shutting down end February. Goal: To make an old message display working again with the new Mbed module. Goal is to replace all functionality by the Mbed. Extra feature is to add internet connectivity and display RSS‐messages generated by websites or an own database RSSfeed generator. Introduction: From a colleague I got an old message display with 24 5x7led digits. In first instance I wanted to use it for my work. On the search of the brand of the display there raised the problem that it was a custom made industrial display board without any references to the type or model. The only indication on the display PCB’s was the marking code DL210‐04. The control board had marking sign: DATA DISPLAY SHIP SM1 D97 7So the software was not available anymore and I didn’t want to ask around and begging for the software which was probably from the last century. After opening the cover, it turned out that the design of the display was from 1995 and had an old fashioned controller board. Microprocessor, EPROM chip, Ram chip, real time clock with gold cap it was all provided as separate chips. This was the moment I thought that it would be nice to give the message board a new life with Mbed. Removing the old hardware control board, and replacing it by the Mbed module would be a nice challenge to work on. Also adding an internet connection would be great. With that and using a time server for time setting and RSS‐feed reading these were nice possibilities. Picture 1: The old 1995 technology above, the new 2011 technology under. FIRST ATTEMPTS TO SEE THE LIGHT I was hoping to reach in a few days the point that I could start writing software to control the display. My first attempt was very hope full, I tried to display 1234567890AB on one of the two cascaded PCB display. The result wasn’t what I hoped for, but it was stable and I could understand what was wrong. The character table codes were already found on the internet, I found some projects where the whole character table was available, so why invent the wheel again, copy and paste…. First sign of life of the Mbed module connected with display board. Visually you could see that the picture is upside down, and row 1 and 7 were mixed. It took 5 minutes to find the problem and fix it with the right software control. I had a timing problem with row 7 and 1, and I reversed the row counting sequence. After this I tried to see the time real time counting on the display. For that I used the real time clock on the Mbed and I tried to display it. This took not a long time, but already I started to think on a way how to synchronize the clock time. With the helpful Mbed website it took not very long to use an internet connection with a time server. My first attempt in the clock display was a success and the next picture shows the result. First clock display attempt: it worked and time was accurate. FUNCTIONAL DESCRIPTION OF HARDWARE I had to reverse engineer the old hardware circuits. Took me some time. After that, I made a block diagram of the display driver hardware and peripherals. In the block diagram below, the connection with the Mbed is already drawn into the picture. In the old casing this was the old controller board. The connection between both parts was made via a flat cable with two female 14 pins header connectors. The detailed drawings are in PDF format, attached with this document. MAINS
power
supply
+7V/3A
Voltage
Regulator
24 DISPLAY SEGMENTS CSM 57221
7 x drivers
+5V
Mbed NXP LPC1768
7 x ROW CONTROL
Data IN
Clock IN
CLR
120 bit SHIFT REGISTER
Block diagram principal functionality The circuit exists of a 120bits shift register with driver for steering the 120 led columns. Rows are been driven by a 7 times identical circuit with power transistor driver. The led display modules are 5x7 led display with joined anode and cathode connections. In this way every led can be driven separately, although for a message display there should be multiplexing row by row. How the display should work: First shift in 120 bits for one of the 7 horizontal led rows, to display the marked led’s in the bit stream. Set data for led1 ready, give a clock pulse, set data ready for the next led, clock pulse. The CLR line acts as a clear all 120 bits in shift register to zero. It also acts as an output disable, because 0 as control value means led off. One row update can be done in less than 200µs. Select the row to light up for some time (in my case 2.87ms). After this shift in the next 120 bits for the next row and select this row, and again wait some time. This should be done in such a way that the display doesn’t flicker for the human eye. One repeat sequence lasts for 20ms. Our eyes have the strange behavior called Persistence of view (POV). One could drive a led with high intensity for 20% of time, and the rest of the time the led is switched off. We humans see this as a 100% time high light intensity. Therefore the display seems to light up at high intensity with “low” led current. By displaying row by row the shifted data, we can build up a display with seems to light up in one time. The refresh rate should be high enough to overcome flicker effect. In this application I chose for 20ms for one “frame” update. If 7 rows are passed we have made a complete frame. U16
1
2
3
IRpulse
GND
Vcc
S1
3V3
time of f set
GND
HIM602
S2
*place near
RJ45
connector
f eed select
S3
U15
GND
+5V
1
3
5
7
9
11
13
2
4
6
8
10
12
14
2
4
6
8
10
12
14
20
19
18
17
16
15
14
p20
p19
p18
p17
p16
p15
p14
CONN FLEX 14
13
12
2
11
1
spare
p26
p27
p28
p29
27
28
29
GND
J2
RD+
RD3v 3
p13
p12
Vin
p11
26
TD+
TDVB
35
36
40
1
2
3
4
5
6
7
8
33
34
3
GND
Mbed NXP LPC1768
RJ45
internet
connector
TYCHO
1
JF2
1
3
5
7
9
11
13
6605759-1
BT1
2
CR2032
3V3
GND
C5
100nF
GND
3V3
3V3
*
*
GND
GND
GND
C6
100nF
C7
100nF