How to drive to identical displays with the same parallel signal? - Page 1
Forum section: electronics / projects

author: steinerlein
author_rank: Newbie
timestamp: September 27, 2018, 02:30:28 pm
content: Hey all, this is my first post here - I hope I'm not doing anything wrong yet!I am looking to drive to identical displays over DPI from a raspberry pi zero. The displays are not that big with a DTCLK of 33Mhz. I suppose it is advisable to use a digital buffer for the signals and connect them in parallel after that. I could use this for example:http://www.ti.com/product/sn74lvth162244In addition to that, should I use a dedicated buffer for the clock? I have found this fan-out buffer which looks like it's a good match:http://www.ti.com/product/cdclvc1102My question is: Do I need to separate the displays further? Do I need to add diodes? Is my approach advisable at all?Thanks for any input provided!

author: nihial
author_rank: Regular Contributor
timestamp: September 27, 2018, 07:33:54 pm
content: Hello steinerlein,A buffer is always a good idea. I guess you don't have very long cables so I think that with your frequency it should work without too much pain. What would be your cable length to the screens ?I'd say the clock buffer is mandatory as reflections from the super stub could cause double clocking (and from my experience in your case it can happen), and you don't want that it will mess up everything.Put resistors in series with the driver both for data and clock, if it's included it might be ok to not put them in general but in your case if it's not too hard I'd put them.If you can, chose cables with the same length (for this topology it's better to have same length in the two branches overall pcb included of course).I haven't checked your chips if I read another datasheet today I'll die, in general just check that:- The voltage level is good- They can achieve your frequencies- If you use different chips check that the difference in propagation delay will not skew your signals too much (should hardly be a problem at those frequencies)- If you use same chips check that the part to part skew will not skew your signals too much (should hardly be a problem at those frequencies)Why would you add some diodes ?

author: DaJMasta
author_rank: Super Contributor
timestamp: September 27, 2018, 08:14:52 pm
content: A noninverting buffer with sufficient bandwidth should be plenty, as mentioned there can be issues with propagation delay if you use different kinds of chips, and since the clock is well within modern TTL speeds, it would be simpler just to use jellybean buffers. Something like the 74 series 541 or 245 in HCT or LVC flavors would likely be good choices, and at 8 bits per chip you need a max of 4 chips (if you're only using 6 bit color, 3 chips).

author: steinerlein
author_rank: Newbie
timestamp: September 28, 2018, 07:55:53 am
content: Thanks for the replies so far, it seems to me I'm on the right path. A couple of questions though:Quote from: nihial on September 27, 2018, 07:33:54 pmAll components will be on one pcb with the screens going plugged in with their FPC directly.Quote from: nihial on September 27, 2018, 07:33:54 pmWhat do you mean by super stub?Quote from: nihial on September 27, 2018, 07:33:54 pmI was thinking that maybe I need to isolate the two signals to the screens on the output side of the buffer. Doesn't make much sense now that I think about it, though.

author: nihial
author_rank: Regular Contributor
timestamp: September 28, 2018, 09:17:51 am
content: Quote from: steinerlein on September 28, 2018, 07:55:53 amQuote from: steinerlein on September 28, 2018, 07:55:53 amSo the FPC length.Quote from: steinerlein on September 28, 2018, 07:55:53 amLet's say you have screen A and B. Your signal travels through both paths to reach them. For A, the path to get to B from when the signal split is a stub and the same goes for B. And this is quite a long stub it's at least the length of the FPC cable.Quote from: steinerlein on September 28, 2018, 07:55:53 amIf you want to isolate them, use a fanout buffer also for the data. This is imho the best and safest way for it to work if done correctly but your solution should be viable too for your case.However it's still a risk (I think it should be ok but I could miss something), if it's for yourself and you don't mind a redesign, go for it you might learn a lot from this either if it works or not.

author: steinerlein
author_rank: Newbie
timestamp: September 28, 2018, 11:45:13 am
content: Quote from: nihial on September 28, 2018, 09:17:51 amGot it. I think I will got with the inititally mentioned approach of a fanout buffer for the clock and a simple non-inverting buffer with 8 or 16 bits for the data. I have not found an IC that has the right no. of channels and fanout ratio. This way I would have to use a lot of ICs which is not very appealing to me.Thank you very much for your opinion on this!

author: nihial
author_rank: Regular Contributor
timestamp: September 29, 2018, 09:06:28 pm
content: Quote from: steinerlein on September 28, 2018, 11:45:13 amYes that would require a shitload of IC's and it's probably not worth the time. Good luck