chithanh
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Reasoning behind dual Micro HDMI, and potential future alternatives?

Wed Jun 26, 2019 11:26 am

The product page of the Raspberry Pi 4 says:
We've listened to your feedback: with Raspberry Pi 4, you can run two monitors at once — and in 4K, too!
I think that allowing dual displays is commendable and it makes the Raspberry Pi 4 much more useful for many desktop users. However, the decision to go with dual Micro HDMI struck me as odd, and I would like to ask about the reasoning why this option was chosen over the alternatives, and if the possibility exists (given enough feedback) to revert this decision for future Pi 4 hardware revisions.

The problem I see with the Micro HDMI is that it is a relatively obscure connector, and cables/adapters are hard to obtain in some emerging markets, the connector is much more flimsy, and requires to carry an adapter around everywhere you bring the Pi 4 because existing infrastructure is rarely prepared for Micro HDMI.

The obvious alternative to two micro HDMI ports would have been a full-size dual deck HDMI connector, or two upright full-size HDMI connectors. Was this alternative considered at all? And was it rejected due to cost, connector height, PCB layout difficulties, or for other reasons?

The other alternative would have been using a full-size HDMI connector for one output, and routing the second display output via USB-C DisplayPort Alternate mode. This would only be feasible if the BCM2711 supports DisplayPort at all, and it would increase cost due to the requirement of an extra Alt mode switch. The advantage is that USB-C single cable docking is spreading rapidly everywhere, and you can fall back to normal HDMI without an adapter where the infrastructure is not yet prepared for USB-C. Was cost the primary reason for not going the USB-C route, or the BCM2711 feature set?

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Re: Reasoning behind dual Micro HDMI, and potential future alternatives?

Wed Jun 26, 2019 11:56 am

There's no facility in the SoC for display over USB, so that is a non-starter.

With regards to dual height connectors, they would apply too much torque to the attachment point

The micro-HDMI are the right size, are robust (I've been plugging in and out for months with no problems), cheap, and we now sell very cost effective cables (or use an adapter) to go with them.

I doubt we will be moving back to the larger connectors on dual capable devices.
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Re: Reasoning behind dual Micro HDMI, and potential future alternatives?

Wed Jun 26, 2019 12:08 pm

Various reasons, not all of which I'll mention.

All the dual-deck HDMI images that show up in that search have noticeable screw mounts next to the upper connector, designed to take the stresses of plugging and unplugging the top connector. Those don't work if you don't have a case to mount the thing in, therefore are all out.

Output via USB-C is not supported by the SoC, therefore not an option.

And the design of the Pi is very cost sensitive.

The cost difference for micro HDMI to HDMI leads vs standard HDMI isn't huge. A quick search on Ebay shows leads from £1.70. The lead we sell is £4 (+VAT), which still isn't that expensive. Any sensible distributor selling the Pi will also be selling the leads to go with it, so lack of availability is a bit of an odd claim.
People complained over the mini-HDMI on the PiZero too, but again it's not that big a deal.

The design of the Pi4 is as it is. What happens in future will happen.
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RichardUK
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Re: Reasoning behind dual Micro HDMI, and potential future alternatives?

Wed Jun 26, 2019 12:55 pm

Is that a Matrix quote at the end? :D

I think you're brave doing this dual display / micro hdmi, I think if I was involved I would have argued for retaining one normal size HDMI. Let the compute daughter board developers create a 'work station desktop computer' solution.

But I am sure a lot would say it's good as I was not involved. :D

For the Zero, I would hazard a guess that most never got connected to a display. Also the zeros connector was already being use on GFX cards. I have not seen this micro connector in the wild.

I would like to thank you for your hard work and the support on the forums. Way beyond any other SBC products gets. :)

dsyleixa123
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Re: Reasoning behind dual Micro HDMI, and potential future alternatives?

Wed Jun 26, 2019 1:03 pm

I had voted for an enlarged board form factor feat. 2 standard HDMI ports, 2 more USB ports, a 7-12V DC/DC adapter jack, and a 2nd GPIO header for more digital and analog GPIOs, just my 2 ct... :?

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Re: Reasoning behind dual Micro HDMI, and potential future alternatives?

Wed Jun 26, 2019 1:23 pm

dsyleixa123 wrote:
Wed Jun 26, 2019 1:03 pm
I had voted for an enlarged board form factor feat. 2 standard HDMI ports, 2 more USB ports, a 7-12V DC/DC adapter jack, and a 2nd GPIO header for more digital and analog GPIOs, just my 2 ct... :?

Everyone is an expert when it is not their time and money being spent :lol:
Retired disgracefully.....

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Re: Reasoning behind dual Micro HDMI, and potential future alternatives?

Wed Jun 26, 2019 1:51 pm

I wish there was an official micro hdmi to DVI-D cable for those of us with cheap monitors!

I can get one on Ebay for £3 but will it work?...

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Re: Reasoning behind dual Micro HDMI, and potential future alternatives?

Wed Jun 26, 2019 2:25 pm

jbudd wrote:
Wed Jun 26, 2019 1:51 pm
I wish there was an official micro hdmi to DVI-D cable for those of us with cheap monitors!

I can get one on Ebay for £3 but will it work?...
For £3 I'd try it and see. Less than a pint of beer, my standard measure of cost.
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Re: Reasoning behind dual Micro HDMI, and potential future alternatives?

Wed Jun 26, 2019 2:26 pm

RichardUK wrote:
Wed Jun 26, 2019 12:55 pm
Is that a Matrix quote at the end? :D

I think you're brave doing this dual display / micro hdmi, I think if I was involved I would have argued for retaining one normal size HDMI. Let the compute daughter board developers create a 'work station desktop computer' solution.

But I am sure a lot would say it's good as I was not involved. :D

For the Zero, I would hazard a guess that most never got connected to a display. Also the zeros connector was already being use on GFX cards. I have not seen this micro connector in the wild.

I would like to thank you for your hard work and the support on the forums. Way beyond any other SBC products gets. :)
2 different connectors would just mean you need two different sorts of cable - the worst of both worlds.
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Re: Reasoning behind dual Micro HDMI, and potential future alternatives?

Wed Jun 26, 2019 2:48 pm

Coming back to the OP's post, I'm bemused by the claim that
The problem I see with the Micro HDMI is that it is a relatively obscure connector,
It is an HDMI standard connector, and looking at many of the tablets that have HDMI, the majority have micro HDMI, with a couple having mini HDMI. https://www.alphachooser.com/tablet_com ... _computers

In fact I bought an Asus UX31 laptop about 6 years ago that had micro HDMI for external monitors, as does the UX330 that replaced it. In fact the original Tesco Hudl had it, so it's not that new and obscure.
USB-C for video is the newcomer here, and requires a fair amount of support within the chipset.
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chithanh
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Re: Reasoning behind dual Micro HDMI, and potential future alternatives?

Wed Jun 26, 2019 3:02 pm

james, 6by9, thank you for your replies and explanations.
6by9 wrote: The cost difference for micro HDMI to HDMI leads vs standard HDMI isn't huge. A quick search on Ebay shows leads from £1.70. The lead we sell is £4 (+VAT), which still isn't that expensive. Any sensible distributor selling the Pi will also be selling the leads to go with it, so lack of availability is a bit of an odd claim.
jamesh wrote: The micro-HDMI are the right size, are robust (I've been plugging in and out for months with no problems), cheap, and we now sell very cost effective cables (or use an adapter) to go with them.
About the availability of Micro HDMI accessories: You can go into any almost place that sells electronics, worldwide, be it a large store or small store or bazaar, and buy HDMI cables.

For Micro HDMI, not so much. Yes large central stores and international mail-order are options. But many local places are not, especially in less developed countries. Using Micro HDMI requires planning ahead. HDMI is ubiquitous. USB-C is well on its way to ubiquity. This does place considerable additional burden on users in poor and less developed countries, including having to buy extra Micro HDMI equipment to prepare for future/unforeseen situations, or accidental breakage.

About the flimsiness of Micro HDMI, this is what I read from other users who posted about the topic. I have only ever owned one such device, an Amazon Kindle Fire HD 7 (tate), and I rarely used the Micro HDMI, but the physical connection seemed indeed less stable than HDMI. But I will wait for Pi 4 user reports before I make further comments on that.
6by9 wrote: People complained over the mini-HDMI on the PiZero too, but again it's not that big a deal.
I think the Pi Zero is much more of a niche device for fixed installations, and not so much a personal computing device. So the Mini HDMI isn't an issue for most users I imagine (if they connect displays at all).
jamesh wrote: With regards to dual height connectors, they would apply too much torque to the attachment point
6by9 wrote:All the dual-deck HDMI images that show up in that search have noticeable screw mounts next to the upper connector
There are ones who don't, like this one from Aliexpress.

I understand that the mechanical stress situation on dual deck and upright connectors is worse. But OTOH they are still quite popular where horizontal space is tight, in the case of upright connectors even in very low-cost devices.
jamesh wrote: There's no facility in the SoC for display over USB, so that is a non-starter.
6by9 wrote: Output via USB-C is not supported by the SoC, therefore not an option.
I think almost no SoCs support output of display directly via USB-C, that is why Alternate mode switches are in use that mux the display and USB signals. But yes, these cost money...
6by9 wrote: And the design of the Pi is very cost sensitive.
...and that is why I guessed it was not an option, at least for now. But if the BCM2711 supports DisplayPort output (I haven't found an answer to this yet) it would at least be possible in a future revision.
6by9 wrote: The design of the Pi4 is as it is. What happens in future will happen.
Sure. I'm not holding my breath. Certainly you will need to evaluate feedback from Pi 4 users first before evaluating the next steps, and see if Micro HDMI troubles are even a blip on the radar.

Edit:
6by9 wrote:
Wed Jun 26, 2019 2:48 pm
Coming back to the OP's post, I'm bemused by the claim that
The problem I see with the Micro HDMI is that it is a relatively obscure connector,
It is an HDMI standard connector, and looking at many of the tablets that have HDMI, the majority have micro HDMI, with a couple having mini HDMI.
Yes, it is part of the HDMI standard. Yet it is rarely used, mostly on tablets where people don't usually connect external displays. And it has been almost completely replaced by USB-C in new devices.

You know what was part of the DVI standard? Mini-DVI. But almost nobody except Apple used it... Micro HDMI is not quite as obscure as Mini-DVI, but not far away either.
6by9 wrote:
Wed Jun 26, 2019 2:48 pm
USB-C for video is the newcomer here, and requires a fair amount of support within the chipset.
It is quite new, but the trajectory if you look at smartphones and even ultra-cheap Chromebooks is clear. It may not be as ubiquitous as HDMI and won't be for some time, but still lots of places will have USB-C infrastructure in the future.
Last edited by chithanh on Wed Jun 26, 2019 3:10 pm, edited 1 time in total.

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Re: Reasoning behind dual Micro HDMI, and potential future alternatives?

Wed Jun 26, 2019 3:10 pm

chithanh wrote:
Wed Jun 26, 2019 3:02 pm
I think almost no SoCs support output of display directly via USB-C, that is why Alternate mode switches are in use that mux the display and USB signals. But yes, these cost money...
Not quite that easy. To get full speed output you need connections inside the SoC that move the pixels from the graphics compositor (HVS) to the output - we call these pixelvalves - and there simply isn't a route from there to the USB system (Or PCIe which is what connects to the USB). It could probably be done with the intervention of one of the processing cores, via RAM, but that would absolutely HAMMER the SDRAM bandwidth and slow the system down, and use a load of CPU time. That's a recipe for overheating if nothing else!
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Re: Reasoning behind dual Micro HDMI, and potential future alternatives?

Wed Jun 26, 2019 3:27 pm

dsyleixa123 wrote:
Wed Jun 26, 2019 1:03 pm
I had voted for an enlarged board form factor feat. 2 standard HDMI ports, 2 more USB ports, a 7-12V DC/DC adapter jack, and a 2nd GPIO header for more digital and analog GPIOs, just my 2 ct... :?
I like the form factor just as it is. At one point, I picked up Another Board and it looked absolutely enormous, even though it isn't all that big. I was just so used to Pis...

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Re: Reasoning behind dual Micro HDMI, and potential future alternatives?

Wed Jun 26, 2019 3:28 pm

jbudd wrote:
Wed Jun 26, 2019 1:51 pm
I wish there was an official micro hdmi to DVI-D cable for those of us with cheap monitors!

I can get one on Ebay for £3 but will it work?...
Yeah... I checked my standard source for cables (Monoprice) and...nothing. I"ll probably get a fistful of the RPF adapters for the cables I have and consider getting a bunch more HDMI to DVI adapters to go with RPF cables going forward.

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Re: Reasoning behind dual Micro HDMI, and potential future alternatives?

Wed Jun 26, 2019 3:29 pm

jamesh wrote:
Wed Jun 26, 2019 3:10 pm
Not quite that easy. To get full speed output you need connections inside the SoC that move the pixels from the graphics compositor (HVS) to the output - we call these pixelvalves - and there simply isn't a route from there to the USB system
I must admit that this statement surprises me.
There exist external boards and adapters that are have USB3 + DisplayPort inputs connectors and will mux them into an USB-C (with DisplayPort Alternate Mode). There is no special support required from USB host nor from DisplayPort source. One example is the Wacom Link Adapter for MobileStudio Pro, also some evaluation boards like the CY4541 EZ-PD CCG4.

There exist also PCIe x1 cards for normal PCs which contain USB3 controller + DisplayPort connector + Alt mode switch, like the Sunix UPD2018. Again, no special host support required.

(Yes, I know these are all prohibitively expensive for the Pi.)

But are you saying that these all will not work at full speed? I'm still not convinced that for USB-C with Display you need more than DisplayPort source and USB 3 host and an Alt mode switch.

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Re: Reasoning behind dual Micro HDMI, and potential future alternatives?

Wed Jun 26, 2019 3:37 pm

Totally aside from all of the reasons that have been given for not using the USB-C connector for video output, the elephant in the room is that it is the power connector. Use it for video and you have to find some other way to supply power to the board, probably either through the GPIO pins or by adding the PoE HAT. I don't think you want a lot of rank beginners going either route as they won't have the skills and wariness to use the GPIO pins without a disaster, nor will they want to add $20 for the HAT, plus whatever it costs to get a PoE switch.

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Re: Reasoning behind dual Micro HDMI, and potential future alternatives?

Wed Jun 26, 2019 3:48 pm

Yes, and using a single USB-C for all of power/data/display is becoming the norm. You can today buy mid-range monitors that have USB-C connector for all of these at once, and incorporate USB hubs, and sometimes even Ethernet. For easy standards-based single cable docking on smartphones and notebooks.

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Re: Reasoning behind dual Micro HDMI, and potential future alternatives?

Wed Jun 26, 2019 4:01 pm

chithanh wrote:
Wed Jun 26, 2019 3:29 pm
jamesh wrote:
Wed Jun 26, 2019 3:10 pm
Not quite that easy. To get full speed output you need connections inside the SoC that move the pixels from the graphics compositor (HVS) to the output - we call these pixelvalves - and there simply isn't a route from there to the USB system
I must admit that this statement surprises me.
There exist external boards and adapters that are have USB3 + DisplayPort inputs connectors and will mux them into an USB-C (with DisplayPort Alternate Mode). There is no special support required from USB host nor from DisplayPort source. One example is the Wacom Link Adapter for MobileStudio Pro, also some evaluation boards like the CY4541 EZ-PD CCG4.

There exist also PCIe x1 cards for normal PCs which contain USB3 controller + DisplayPort connector + Alt mode switch, like the Sunix UPD2018. Again, no special host support required.

(Yes, I know these are all prohibitively expensive for the Pi.)

But are you saying that these all will not work at full speed? I'm still not convinced that for USB-C with Display you need more than DisplayPort source and USB 3 host and an Alt mode switch.
If you have a chip that takes display port as input, then that would be possible, if the SoC had display port as an output (it doesn't AFAIK). I was talking about direct connections rather that all that conversion. ie direct output from the HVS in the SoC to the video output device, ie USB-C. Adding the extra chip is expensive, in chip cost and board real estate. This is really the sort of thing you need inside the SOC itself to get the Pi levels of cheapness.

Took a quick look at that Sunix board. It's bigger than the entire Pi, and look at the the associated circuitry on the PCB!! And at £79 that not even remotely close to the level of cost required. About two orders of magnitude out, if not more.
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Re: Reasoning behind dual Micro HDMI, and potential future alternatives?

Wed Jun 26, 2019 5:11 pm

I'm not getting the hate for Micro HDMI. I have 4 devices that use it :

2 x Cheap Windows Atom tablets - both have Micro HDMI for external displays.
1 x ODroid C1 which has a Micro HDMI
1 x Motorola Lapdock that uses Micro HDMI for video input (they are used with a LOT of Raspberry Pis)

Converters and cables are quite easy to find.

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Re: Reasoning behind dual Micro HDMI, and potential future alternatives?

Wed Jun 26, 2019 5:32 pm

Thank you for bringing amazing Raspberry Pi 4 ;)
I respect your decision, but I must say, using Micro HDMI is not a good idea in any aspect: durability, universality, etc...

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Re: Reasoning behind dual Micro HDMI, and potential future alternatives?

Wed Jun 26, 2019 5:36 pm

chithanh wrote:
Wed Jun 26, 2019 3:48 pm
Yes, and using a single USB-C for all of power/data/display is becoming the norm. You can today buy mid-range monitors that have USB-C connector for all of these at once, and incorporate USB hubs, and sometimes even Ethernet. For easy standards-based single cable docking on smartphones and notebooks.
But will those ports reliably supply a solid 3A at 5v?

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Re: Reasoning behind dual Micro HDMI, and potential future alternatives?

Wed Jun 26, 2019 5:38 pm

chithanh wrote:
Wed Jun 26, 2019 3:29 pm
There exist external boards and adapters that are have USB3 + DisplayPort inputs connectors and will mux them into an USB-C (with DisplayPort Alternate Mode). There is no special support required from USB host nor from DisplayPort source. One example is the Wacom Link Adapter for MobileStudio Pro, also some evaluation boards like the CY4541 EZ-PD CCG4.
There is a major, major problem with your contention with regard to the USB-C connector. Its internal connection is to the "legacy" SoC USB port...which is USB 2.

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Re: Reasoning behind dual Micro HDMI, and potential future alternatives?

Wed Jun 26, 2019 6:41 pm

RichardUK wrote:
Wed Jun 26, 2019 12:55 pm
... I have not seen this micro connector in the wild.
It's not uncommon on Ultrabooks and tablets. I have some micro HDMI cables from a previous Ultrabook (Lenovo Yoga 2 Pro), so it won't be a problem for me... assuming I can find them (we've moved since then).

6by9 wrote:
Wed Jun 26, 2019 2:48 pm
In fact I bought an Asus UX31 laptop about 6 years ago that had micro HDMI for external monitors...
Same here. My old Yoga 2 Pro convertible ultrabook from 2013 used micro HDMI. That was a great computer. I miss it sometimes (gave it to my niece when her new Dell crapped out for the 2nd time in as many months and she returned it).

EDIT:
As far as the future is concerned, I'm hoping this dual micro HDMI form factor will be the future for Raspberry Pi, since I've already pre-ordered the new Flirc case. :D
My mind is like a browser. 27 tabs are open, 9 aren't responding,
lots of pop-ups...and where is that annoying music coming from?

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Re: Reasoning behind dual Micro HDMI, and potential future alternatives?

Wed Jun 26, 2019 9:16 pm

noggin wrote:
Wed Jun 26, 2019 5:11 pm
Converters and cables are quite easy to find.
That completely depends on where you live and which markets you can access. Sure, if you live in an urban area in a developed country, then you can just walk into the next electronics mega-store and buy one.

One of the really, really great things about the Raspberry Pi 1-3 is that you could take the bare board basically any place in the world that has electricity, hook it up using technology that is omnipresent (microUSB for power, HDMI for display, SD/microSD cards for storage, USB for keyboard/mouse) and get going.
jamesh wrote:
Wed Jun 26, 2019 4:01 pm
If you have a chip that takes display port as input, then that would be possible, if the SoC had display port as an output (it doesn't AFAIK).
Ok, if the BCM2711 has no DisplayPort, then display via USB-C can't reasonably happen for the Raspberry Pi 4 revisions. Thanks for clearing that up.
jamesh wrote:
Wed Jun 26, 2019 4:01 pm
Took a quick look at that Sunix board. It's bigger than the entire Pi, and look at the the associated circuitry on the PCB!! And at £79 that not even remotely close to the level of cost required. About two orders of magnitude out, if not more.
Yes, the Sunix PCIe card is big. But the Wacom adapter is quite compact (and even more expensive).
W. H. Heydt wrote:
Wed Jun 26, 2019 5:36 pm
chithanh wrote:
Wed Jun 26, 2019 3:48 pm
Yes, and using a single USB-C for all of power/data/display is becoming the norm. You can today buy mid-range monitors that have USB-C connector for all of these at once, and incorporate USB hubs, and sometimes even Ethernet. For easy standards-based single cable docking on smartphones and notebooks.
But will those ports reliably supply a solid 3A at 5v?
Yes, these ports support USB power delivery standard, typically 45 W or 65 W. They are intended to charge your notebook while docked.
W. H. Heydt wrote:
Wed Jun 26, 2019 5:38 pm
There is a major, major problem with your contention with regard to the USB-C connector. Its internal connection is to the "legacy" SoC USB port...which is USB 2.
That was a choice by the board designer. Changing the board layout and connecting USB-C to USB3 would be possible. But as the BCM2711 appears to not support DisplayPort (see above), we will never see display via USB-C in this generation, so routing one of the two USB 3 ports via the USB-C connector would be a waste.
HawaiianPi wrote:
Wed Jun 26, 2019 6:41 pm
RichardUK wrote:
Wed Jun 26, 2019 12:55 pm
... I have not seen this micro connector in the wild.
It's not uncommon on Ultrabooks and tablets.
It is orders of magnitude less common than HDMI.
And for new personal computing devices (basically anything produced in the last 3-4 years), it is a lot less common than USB-C.

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Re: Reasoning behind dual Micro HDMI, and potential future alternatives?

Thu Jun 27, 2019 12:17 am

jamesh wrote:
Wed Jun 26, 2019 11:56 am
There's no facility in the SoC for display over USB, so that is a non-starter.

With regards to dual height connectors, they would apply too much torque to the attachment point

The micro-HDMI are the right size, are robust (I've been plugging in and out for months with no problems), cheap, and we now sell very cost effective cables (or use an adapter) to go with them.

I doubt we will be moving back to the larger connectors on dual capable devices.

What functions does the USB C connector provide besides 5 volts?
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