Where to connect unused Data Bus pins on LCD display? - Page 1
Forum section: electronics / projects

author: mcinque
author_rank: Unknown
timestamp: June 14, 2014, 05:41:49 pm
content: I'm using data bus pins from D4 to D7 on a "not so cheap" LCD display. Since it should be soldered with a standard strip onto a PCB, it should be safe to connect the unused D0-D3 pins to ground to avoid any antenna effect.If it was a cheap LCD I would surely tried without asking. In this case, it's better to hear some advices..What do you suggest?

author: SeanB
author_rank: Super Contributor
timestamp: June 14, 2014, 05:51:21 pm
content: Connect them each to ground using a resistor between 1 and 10k, whatever resistor value you have around in bulk. that way they all will have a defined low level and will not have any problems with spikes.

author: free_electron
author_rank: Super Contributor
timestamp: June 14, 2014, 06:19:53 pm
content: NEVER connect the unused pins directly to ground or vcc . during powerup the display starts in 8 bit mode... while initialising onto 4 bit mode these pins may become output temproarily , frying the controller.leave em open they have internal pull resistors.

author: mcinque
author_rank: Unknown
timestamp: June 14, 2014, 07:18:47 pm
content: hmmm... my objecive is to avoid any EMI that could be irradiated from the floating pins, maybe it's better to use a couple of bypass caps?

author: dannyf
author_rank: Super Contributor
timestamp: June 14, 2014, 08:18:25 pm
content: QuoteSince they are input pins, they are unlikely to "radiate" anything.And since they are unused, their logic states aren't going to be of any concern for the correct operation of the lcd.Preferably, you should tie them to ground or Vcc via some resistors (10k - 100k for example).I would just leave them floating. If you are making a pcb, it is wise to make available traces / space so you can tie them to gnd or vcc if you want later.

author: bktemp
author_rank: Super Contributor
timestamp: June 14, 2014, 08:39:17 pm
content: Do not pull the lines to ground using pulldowns! The original 44780 and almost all clones have internal pullups. Pullup + pulldown is a very bad combination...If you want to add something, use pullups, but in my opionion it is not necessary. I have seen and build a lot of circuits using 44780 lcds, none had external pullups.I have never experienced any problems. If you do not connect D0-3, these inputs have the shortest possible wires connected and therefore are least sensitive.A good solution is to use a periodic reinitialization to restore all lcd controller settings to a known state.

author: mcinque
author_rank: Unknown
timestamp: June 14, 2014, 10:39:42 pm
content: Quote from: dannyf on June 14, 2014, 08:18:25 pmthis could be right, even if many suggest to avoid floating pins at all, butQuote from: free_electron on June 14, 2014, 06:19:53 pmso they could radiate something during the initialization sequence.My goal is to pass the EMC and I was considering all the floating pins as they may become potentially antennas.Quote from: bktemp on June 14, 2014, 08:39:17 pmIt's a good advice.Quote from: bktemp on June 14, 2014, 08:39:17 pmOf course they work fine. I've a lot of LCD with D0-D3 unconnected andI've never considered to connect them until now that I'm considering that damn EMI compliance. I was wondering if it's EMI safe to leave them floating or not; I've not the hardware to do any radiated pre-EMI compliance test

author: Rerouter
author_rank: Super Contributor
timestamp: June 15, 2014, 01:00:31 am
content: Another agreeing to tie them low, equally if your not running it against the speed limits, possibly some resistance on your active data lines to slow down your edges (ground plane has capacitance making an RC pole)

author: T3sl4co1l
author_rank: Super Contributor
timestamp: June 15, 2014, 02:10:04 am
content: Another vote for pullups or open. Caveat: check if they have internal pullups, if so, don't bother.Pay more attention to common mode currents (and signal quality) along the cable connecting to the display.Tim

author: mcinque
author_rank: Unknown
timestamp: June 15, 2014, 08:41:06 am
content: Thank you all for your advices!

author: dannyf
author_rank: Super Contributor
timestamp: June 15, 2014, 10:16:22 am
content: QuoteThose pins are the least of your problems from an emi perspective. Their "input resistance" is on the tone of 5v/50ua.

author: mcinque
author_rank: Unknown
timestamp: June 15, 2014, 01:10:19 pm
content: Quote from: dannyf on June 15, 2014, 10:16:22 amGood to know!

author: free_electron
author_rank: Super Contributor
timestamp: June 15, 2014, 04:35:07 pm
content: It is a non issue for emc. The pins could only radiate during initialisation, which is in the order of microseconds. They won't toggle arbitrarily (they all go high) and they will only toggle oi the display is read (most lcd routines out there never read from the display, he k most even dont check the status bit !)There is no way the emc measurement will capture that event. First of all they run multiple sweeps and average them over time.In short : it is a non event.Things to worry about are switching regulators, unterminated power planes and massive powergungry digital crap that bounces around.The only thing you will see in the spectrum from that lcd is the scanning of the pixel array. And you'd have to stick an antenna in physical contact to detect that...Don't go looking for problems... They are perfectly capable of finding you all by themselves.

author: mcinque
author_rank: Unknown
timestamp: June 15, 2014, 08:34:51 pm
content: Quote from: free_electron on June 15, 2014, 04:35:07 pmfantastic and sadly true...!

author: Neilm
author_rank: Super Contributor
timestamp: June 16, 2014, 06:06:17 pm
content: If you are that worried, put pads to allow both pull ups and pull downs / caps and don't fit the components. That way if you do need to connect them to some where, the pads for the components are already there.