HDMI display problem notes

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I have recently invested in a HDMI touchscreen from Amazon, and a Raspberry Pi 4, For the touchscreen to work it must be connected via GPIO, however my pi cannot supply enough power to it from my usb power bank which can provide enough power for the pi but not the display. As the display has a usb port for power as well as GPIO, I attempted powering the display from usb, however this caused the pi to boot up, hinting that there is some connection causing power to go into the pi from GPIO, thus I question whether it is safe to power the pi from GPIO and USB-C simultaneously?

Update (6/05/20): I am asking this question as when I power it from the USB-C on the Pi alone it gives me a yellow lightning bolt in the top right-hand corner.

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    This question has little to do with the Pi. What you are asking is it safe to connect 2 switch mode supplies together – to which the answer is NO – Milliways yesterday
  • Hi @Big Bob, Welcome and nice to meet you. Ah, let me see. Your Rpi screen looks good, as summarized below: (A) “Amazon Landzo 5 Inch Touch Display for Raspberry Pi “: amazon.com.au/LANDZO-Touch-Display-Raspberry-Banana/dp/… Features: (1) Hardware Interface -HDMI, USB, (2) Connector Type – USB, HDMI, (3) Date First Available -17 July 2019, (4) Customer Reviews – 5.0 out of 5 stars 1 customer rating, (5) Amazon Bestsellers Rank 1,427 in Computers #23 in Monitors. / to continue, … – tlfong01 23 hours ago   
  • Now let me look at your power bank, as summarized below: (B) “OfficeWorks Keji 15000mAh Powerbank – US$30”: officeworks.com.au/shop/officeworks/p/…. Features: (1) Single USB port to charge smart device like a tablet or phone, (2) 15000mAh, can connect device for charging using micro-B USB cable, (3) Not fast charge compatible, (4) 2 USB Charge Ports, (5) Max crrent 2.4 A. (6) Voltage Provided 5 V. / to continue, … – tlfong01 23 hours ago   
  • Your screen spec is not clear. I usually read user manual of a similar product from a reputable vendor, eg AdaFruit, SparkFun, or WaveShare. Now I casually googled the following: (C) :Waveshare Raspberry Pi IPS 1024×600 7inch HDMI LCD”: amazon.com.au/Waveshare-HDMI-Capacitive-LCD-Toughened/dp/…. / to continue, … – tlfong01 22 hours ago   
  • Now that we have three devices – 2 screens and 1 power bank, to refer to for discussion. Ah, locking own lunch time now. See you late this afternoon or evening. Cheers. – tlfong01 22 hours ago   
  • Before I go, 2 questions: (1) Your screen spec says it is compatible to Rpi 2 and 3, but NO MENTION ABOUT RPI 4, (2) It also says that you NEED TO INSTALL A DRIVER FROM CD. Have you done that? (3) Considering (1) and (2) together, you might be in trouble. Anyway, you might comment and suggest something. Cheers, – tlfong01 22 hours ago   
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    @tlfong01, Thank you for your response! The screen should be compatible with the Raspberry Pi 4 as it is HDMI and likely uses ‘Raspberry Pi’ as a form of marketing, the screen does work with the Pi 4 (touchscreen aswell), so in response to (1) it works and in response to (2) I have installed the necessary drivers. The question is more so related to power concerns, as when I power it from the USB-C on the Pi alone it gives me a yellow lightning bolt in the top right-hand corner. I’ll update the question to include this information. – Big Bob 21 hours ago
  • Ah, if your HDMI touch screen works with Rpi4B (with or without installing the driver from the CD), then you don’t have any Rpi3/4 incompatibility problem. I agree with you that the “driver” might not be any driver, because HDMI is universal. What the “driver” does is perhaps just configuring the display parameters by changing the /boot/config.txt. Anyway, I agree with you that it is now only the “Yellow Lightning” voltage too low problem. I guess the first thing you should do is to make sure the power supply for you Rpi4B should be powerful: 5.25V, 3A. Otherwise anything weird would happen. – tlfong01 20 hours ago   
  • And there is one more catch. Please read the following manual (D) “7inch HDMI LCD (C) User Manual“: waveshare.com/w/upload/c/cc/…. (4) Page 7 Rpi Instructions Point 3 – Connect USB Port of LCD to USB Port of PC. This PC/Rpi USB to Screen USB connection is for both WinPC and Rpi. I need to check the schematic if the Screen USB is NOT for power only, but also for Touch Screen signal transfer. But I am not very sure. Perhaps I should check out later. – tlfong01 20 hours ago    
  • PS – The spec does say USB and HDMI are for “interface”, but no mention that USB is for power only! 🙂 – tlfong01 20 hours ago   
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    @tlfong01, The “driver” for my display is install though this file (Shell Script), perhaps it might reveal something as to what the “driver” does (looks like it just updates /boot/config.txt), this might mean that it can receive touch signal over USB alone? It is frustrating how poorly documented some of these products are. – Big Bob 18 hours ago
  • @Big Bob, Many thanks for clarifying about your driver. This is exactly what I have guessed. Almost always SPI/8/16 bit parallel interface LCD display drivers are low level stuff written in C/C++ and must be compiled to become drivers/kernel modules to insert into OS using DT tree tricks. And they are usually OS variant, ie, Rpi3 stretch drivers cannot be used in Rpi4 buster. But as you say, your display is HDMI, which is sort of universal, so you only need to set the display (scanning lines etc) parameter using bash script level. / to continue, … – tlfong01 18 hours ago   
  • Coming back to question of whether “USB to USB” cable is used to interface touch screen signals, I need to google harder to see what is going on. My touch screen experience is very limited and always get stuck now and then, here and there. You might like to skim the following post to get an idea of the complexity: (1) raspberrypi.stackexchange.com/questions/98549/…, (2) raspberrypi.stackexchange.com/questions/104600/…, / to cont, … – tlfong01 18 hours ago   
  • (3) raspberrypi.stackexchange.com/questions/99187/…. Ah, jogging time. See you later. Cheers. – tlfong01 18 hours ago    
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    @tlfong01, Thanks for your help on this so far, I found some form of documentation/user manual on the CD which came with it, I put it into Google Translate and found this: ” USB power supply interface: USB power supply input (5V), as shown in figure ④ The female base has been connected to the Raspberry Pi for power supply, then this USB can not be connected”, which confirms that the USB connection is in fact not for touch signal, thus I need to work out which pins it actually uses for the touch interface so I can run some jumpers to it. Which I gather are the SPI pins. Thank you once again. – Big Bob 17 hours ago
  • (1) I am glad that you confirmed that the USB connector at the HDMI screen is only for “power interface” NOT for “signal interface”. You might also agree that the enggr guy writing the spec speaks Chinese but not technical English, therefore a “bit” misleading. (2) The spec mentions only two interfaces: USB and HDMI. Please confirm that there is indeed another 3/4 wire SPI connector at the bottom side of the HDMI screen to connect to Rpi’s SPI pins. In that case, you might find my references above useful for reference, because touch screen chip I played is popular and used in many SPI LCDs. – tlfong01 2 hours ago   
  • By the way, the Rpi HDMI connector is a bit complicated: (a) it has a small power regulator, to power the HDMI hardware on the other side (even not powered on at that side), to check out their configuration, (2) the RPi’s HDMI connector actually has I2C interface pins/wiring to control the HDMI screen. But that is all I know, after scratching the surface, and I have not tested further the Rpi’s I2C stuff at the HDMI connector. The SPI based LCD hardware/software has already been scratching hard on my little head. Good luck and cheers. – tlfong01 1 hour ago   
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    @tlfong01, Thanks for you replies, It says on the bottom of the screen that it is a XPT2046 Touch Controller, The Female header on the underside of the display takes up the majority of the pi’s header, yet has the following pins labelled: 5V, 5V, JP4, RQ, CS, M1 (or MI), M0 (or MO), SCK, GND, would it then be correct to assume that these are the only pins it uses, so I should be able to run some wires between the GPIO pins on the Pi and the header? Thanks – Big Bob 1 hour ago
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    That is, without connecting the 5V and GND pins, as this is already compensated for by the USB cable into the screen. Or does the GND need to be connected to the Pi for SPI? – Big Bob 7 mins ago
  • (1) OMG, so your HDMI screen, beside the two HMDI and USB connector, has a third connector SPI for touch screen XPT2046. This is the one and only one touch screen chip I once played with (See Fig 6 of my Ref #1 above). The good news is that, as I said, this chip is very popular, so you easily googled examples (WaveShare is the place I started searching.) . / to continue, … – tlfong01 7 mins ago    
  • (2) About SPI, my reference above is the most messy, with 3 SPI connectors: (1) TFT LCD ILI9341, (2) Touch screen XPT2046, (c) SPI SD card for storing images. Standard SPI has 4 wires: CLK, MOSI, MISO, CS. Non standard SPI 3 wire version combines MOSI and MISO. Now that because you already have your fee wet, (1) there is no harm digging the SPI stuff a bit deeper, (2) XPT2048 seems to be a good place to start, because it popular, (3) The learning curve of SPI is not so steep as I2C, so it is not a bad choice. Anyway, good luck and cheers. – tlfong01 8 mins ago   
  • Ah two more things about Grounding. The HDMNI display expects you to use the USB connector that comes with Rpi, NOT another external smart phone charger. You smart phone charger might be 5.5V or even higher, so might cause problem clashing with the 5V power for Rpi (causing current flowing in “unwanted” direction. (2) All SPI connectors should have their ground points “connected”, which si usually called the “common ground” for reference. You might like to google or wiki “common grounding” for more details. Cheers. – tlfong01 just now   Edit   

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