Free Project - 245 LED Display

[Bronx68]

Please explain the use of D3 through D7 do make an 'or gate' for reset. Why 100K resistors?
Why not use Q0 from each of the CD4017 counters?

[falingtrea]

Please explain the use of D3 through D7 do make an 'or gate' for reset. Why 100K resistors?
Why not use Q0 from each of the CD4017 counters?

The diodes are used to create a master reset from the PIC and to isolate each 4017's reset from each other. Diodes are usually a lot cheaper than logic for something as simple as this. The 100k is probably on the conservative side as far as a value, unless the designer was trying to reduce race conditions or output glitches by creating a simple RC filter.

[T.Jackson]

The diodes are used to create a master reset from the PIC and to isolate each 4017's reset from each other.

Correct!

Further explanation -- without the diodes intact, Q0 lines on the counters wouldn't have control over their designated MR line. You simply can't tie all the Q0 lines together with anything but a semiconductor for this. As for the 100K resistors -- these ensure that there's never going to be a short between RB5 on the PIC and associated Q0 lines on the counters. When RB5 swings high, any of the Q0 lines on the counters could be either high or low. This is CMOS we're talking about here, we could even go as high as a mega ohm.

[inspired4real]

HELLO T JACKSON, YOUR PROJECT IS INSPIRING AS IF TO SAY CHALLENGING SO MANY PEOPLE AROUND THE WORLD. I WANT TO BUILD THIS PROJECT AND MAGUS SITE TENDS NOT TO OPEN AND IT IS LIKE IT IS NOT IN ENGLISH(SORRY TO MAGU IF I AM WRONG). I AM THOUGH A NEWBIE TO PIC IN SOME SENSE BUT HAVE BEEEN PLAYING WITH ANALOG ELECTRONICS FOR SOMETIME. WHAT I DONT REALLY UNDERSTAND IS THE EEPROM ASPECT OF YOUR PROJECT, I WANT TO KNOW HOW YOU LOAD THE ASCII CHARACTER SET INTO THE EEPROM FROM YOUR PROGRAM, OR ARE YOU NOT USING THAT METHOD. I WAS THINKING OF DEFINING EEPROM ADDRESSES FOR ASCII CHARACTER AND THEN LOADING EACH CHARACTER IN AN ADRESS THAT I WILL LATER READ TO DISPLAY CHARACTER. DO YOU HAVE ANY DETAIL MATERIALS ON HOW TO USE EERPOM LIKE THE ONE YOU USED IN THE PROJECT? HOW FAST REALLY DO YOU FETCH DATA FROM THIS EEPROM . I AM THINKING OF BUILDING A BIGGER DISPLAY USING 3mm ULTRABRIGHT LED WITH BUFFER AND SUPPLYING ABOUT 50ma OF CURRENT WITH MULTIPLEXING I DONT THINK THE LEDS WILL GET DAMAGED? SORRY REALLY IF MY QUESTION MAY BE TOO SIMPLE FOR YOU TO REPLY TO?
IS THERE ANYWAY I CAN GET MAGU'S LINK AND DOWNLOAD HIS STUFF. THANKS.

[T.Jackson]

The ASCII characters are custom, specific only to this project. They are programmed into the eeprom one byte at a time, whereas each byte represents data for the column in the character. Each column of data for a character is assigned a unique address in the eeprom and all the columns are collated together to form the character. It is impossible to explain the entire inner workings of the system in just a few paragraphs. I don't recommend this project for anyone who is new to pics / electronics.

Regards,

Trent Jackson

[Bronx68]

Trent,
I am also designing a similar scrolling sign. I am storing the entire character set in the PIC program memory. I then plan to fetch the characters out by switches to build a buffer that contains the message. Any insight on this approach? Did you look at this approach?

[T.Jackson]

Give me a 100 engineers and I'll give you 100 different solutions to the problem. This kind of project is really quite abstract, my solution is far from the best and or the cheapest. I haven't really explored other possibilities well enough to make any suggestions. Using single sided PCBs was a silly thing to do (this is why it wasn't published)

Regards,

Trent Jackson