Issues with LCD

Discuss PIC and electronic related things

Moderators: David Barker, Jerry Messina

Post Reply
maxwell
Posts: 7
Joined: Sun Mar 09, 2008 12:31 pm
Location: Cambridge, UK

Issues with LCD

Post by maxwell » Sat Apr 26, 2008 8:05 pm

Hey guys finaly got my project working from development board to pcb however i use a ribbon to connect the lcd to my project its quite long but no more than a meter this stops the whole pic from operating when i mount the lcd onto the board its fine. I/ve checked the ribbons continuity and it seems fine. Im also defined an RS i was wondering if this could make a difference maby i should just ground it out and set the timings my self. Also what is the longest an lcd ribbon should be if any should be used at all.

Many thanks
Ben

maxwell
Posts: 7
Joined: Sun Mar 09, 2008 12:31 pm
Location: Cambridge, UK

Post by maxwell » Sun Apr 27, 2008 10:27 am

Its ok solved my own problem 2 of the data pins were twisted however now the chip just keeps resetting its self grrrrr! one day my little project will work hehe.

Many thanks
Ben

User avatar
RadioT
Registered User
Registered User
Posts: 157
Joined: Tue Nov 27, 2007 12:50 pm
Location: Winnipeg, Canada

Post by RadioT » Sun Apr 27, 2008 1:00 pm

What do you mean by RS, display reset?

If I had a design that needed to use a ribbon cable for a display I would change the design so the display mounts to the same board as the controller interfacing it! If you need to use a ribbon cable for a display I would keep it SHORT - a couple inches, maximum. It is also good practice to intersperse the lines with grounds to reduce crosstalk. Have the display on a board where it is mechanically sound and then the connector mounts to the board. For a one-off project that should be fine.

Background
Ribbon cables are horribly notorious for picking up noise, especially if every other line is not a ground. That's why you rarely see them in built-up systems, and when you do, they often have a ground in every other line (such as hard drive connection ribbon cables) and are always as short as possible.

Also be aware that for any cable you use, it takes at least a nanosecond for a pulse to move one foot or about 30 cm. The math is simple - take the speed of light, 300000 km/sec, and divide it by 1,000,000,000 - you get about 29 centimetres, or about 1 foot.

Then you have to factor in any effects of the speed of a pulse caused by the the pulse impedence and capacitance of the cable. Cheap PCV ribbon cable capacitance is quite high, and a rule of thumb is to use a maximum of 10 feet in any situation, and that is with grounds on every other line. For something with timing as critical as a display, assuming you are directly driving parallel lines, it's probably way too long. Round trip for a signal and it's response is at least 6 nS on your cable, without considering the time for the processor in the display to respond, impedence matching at the connectors and cable capacitance. At that length, I would also expect significant crosstalk-scope it at the other end to see what the signals look like. If there is anything in your display requiring bidirectional control on the order of 15 nS or less, the cable is probably imposing too much delay. If there's a lot of crosstalk, it just won't work.

Some graphic LEDs have timings on that order, although I don't know what your display is. Assuming it's running in parallel mode, it would be even more susceptible. In that case, you would have a lot of signals that have to synchronize so if there is enough noise and crosstalk on there it will disrupt operation.

73's,
de Tom

Post Reply