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Driving 2X RA8875 off of one microcontroller, can it be done
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Re: Driving 2X RA8875 off of one microcontroller, can it be

by paulstoffregen on Wed Jan 21, 2015 5:11 pm

Sure would be nice if RAIO had documented this before Adafruit designed their breakout board.

Then again, it goes to show the need for every company making breakout boards and Arduino shields, like Adafruit, Sparkfun, Seeed etc, to do this simple 2 resistor MISO test during their prototype or product testing phase. It's so very easy, just connect 2 equal pullup and pulldown resistors on MISO, and check for "floating" voltage to be 1/2 of Vcc when the chip is inactive.

I wish I knew how to reach more of the decision makers inside those other companies. Most aren't nearly as conscientious as Adafruit.

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Re: Driving 2X RA8875 off of one microcontroller, can it be

by brian49 on Thu Jan 22, 2015 6:02 pm

I must say RAIO did screw up badly with SDO not tri-state when not selected. RAIO datasheet shows the signal is tri-state in the timing diagrams for both 3-wire and 4-wire buses. Imagine what if Apple found out the issue after they had selected the device for their next gen iphone LCD display; sh*** will hit the fan.

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Re: Driving 2X RA8875 off of one microcontroller, can it be

by eph_raim on Sat Aug 08, 2015 9:39 pm

Good Day .. i would like to ask if the RA 8875 driver board Product ID #1590 is compatible with the 5.0" 40-pin 800x480 TFT Display without Touchscreen
PRODUCT ID: 1680 ?? your response will be highly aprreciated thankyou !

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Re: Driving 2X RA8875 off of one microcontroller, can it be

by local_dani_21 on Sun Aug 09, 2015 2:24 am

They are!

Regards, Dani

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Re: Driving 2X RA8875 off of one microcontroller, can it be

by eph_raim on Sun Aug 09, 2015 6:16 am

Sir dani , the 5.0 inch no touchscreen lcd can be driven by the ra 8875 driver board even if the driverboard is a touch driver board ?? Thankyou

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Re: Driving 2X RA8875 off of one microcontroller, can it be

by local_dani_21 on Sun Aug 09, 2015 6:24 am

Yes! I'm running 7 of these displays without touchscreen on a teensy 3.1.
Best regards, Dani

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Re: Driving 2X RA8875 off of one microcontroller, can it be

by eph_raim on Sun Aug 09, 2015 8:24 am

so i will be able to use two (2pcs) ra 8875 driver board with 2 pcs 5.0 notouch display on arduino mega ?

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Re: Driving 2X RA8875 off of one microcontroller, can it be

by paulstoffregen on Sun Aug 09, 2015 9:24 am

To use more than one RA8875 on the same SPI bus, you must deal with the RA8875's lack of tri-state on MISO.

For background, read issue #2 here:
http://www.dorkbotpdx.org/blog/paul/bet ... in_3_steps

Perhaps Adafruit will add the buffer to their RA8875 products? Or maybe they already have? I do not know... but I'm sure you could look at the schematic to check. Or maybe someone from Adafruit can comment whether their products now have MISO tri-state?
Last edited by paulstoffregen on Sun Aug 09, 2015 9:46 am, edited 1 time in total.

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Re: Driving 2X RA8875 off of one microcontroller, can it be

by adafruit_support_rick on Sun Aug 09, 2015 9:28 am

We have not yet updated the RA8875. You still need to use the buffer chip.

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Re: Driving 2X RA8875 off of one microcontroller, can it be

by eph_raim on Sun Aug 09, 2015 9:45 am

Im planning to use an arduino mega connected to two (2) ra 8875 driver board to connect to two (2) 5 inch no touchscreen lcd .. the output will only be just text thats inpputted from a keyboard.. so what should i add ti the setup?? i just saw an image of two ra 8875 with 7 inch lcd on this fprum connected to an arduino..

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Re: Driving 2X RA8875 off of one microcontroller, can it be

by paulstoffregen on Sun Aug 09, 2015 9:52 am

eph_raim wrote:Im planning to use an arduino mega connected to two (2) ra 8875 driver board ..... so what should i add ti the setup??


For two, you will need 74HC125 or 74AHCT125 or similar tri-state buffer, as I described and showed in a photo on this message:

viewtopic.php?f=47&t=60533&start=45#p323058

I believe you can buy it from Adafruit, here:

https://www.adafruit.com/products/1787

Of course, the ideal situation would be if Adafruit were to integrate the buffer onto their RA8875 breakout board, like they did with the CC3000 breakout..... maybe Adafruit could still make that happen someday (hint, hint)

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Re: Driving 2X RA8875 off of one microcontroller, can it be

by adafruit_support_rick on Sun Aug 09, 2015 9:55 am

Also, see this post:
viewtopic.php?f=47&t=60533&start=60#p323604

Paul - I'll ping product development about this again...

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Re: Driving 2X RA8875 off of one microcontroller, can it be

by adafruit2 on Sun Aug 09, 2015 4:58 pm

we still have tons of PCBs and its not a terribly popular product so not sure when we'll do a revision. How about we do a tutorial and indicate to people to put in a 2K resistor in series to MISO, would that do it?

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Re: Driving 2X RA8875 off of one microcontroller, can it be

by paulstoffregen on Sun Aug 09, 2015 5:25 pm

adafruit2 wrote:...and indicate to people to put in a 2K resistor in series to MISO, would that do it?


If you're willing to accept an undefined logic level at Arduino's MISO pin when accessing two or more of these displays, then why bother with the MISO wire at all? Summing the signals with resistors isn't going to give you a valid MISO signal when 2 are connected. You might as well just leave it unconnected.

Currently, it seems your library uses a simple write-only approach to the RA8875, so it seems likely to work without MISO. Of course, I haven't actually tested disconnecting MISO... this is just my hunch from reading the code and my understanding of this chip.

But not connecting MISO would close off any possibility of improving some parts of the code, especially this very unfortunate delay while using larger fonts:

https://github.com/adafruit/Adafruit_RA ... 5.cpp#L368

I believe the "proper" approach would involve polling the RA8875's STSR status register until bit 7 is cleared. Then again, the library hasn't changed much since it's initial release. If this isn't a very popular product, maybe it'll never merit that sort of engineering effort, so MISO will never actually get used?

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Re: Driving 2X RA8875 off of one microcontroller, can it be

by local_dani_21 on Sun Aug 09, 2015 6:16 pm

adafruit2 wrote:we still have tons of PCBs and its not a terribly popular product so not sure when we'll do a revision. How about we do a tutorial and indicate to people to put in a 2K resistor in series to MISO, would that do it?


No wonder the driver and it's displays don't become more popular with all the problems about RA8875's MISO line in your forum ... and then there are the problems with the LITE input that won't work properly so the displays cannot be dimmed without a humming/beeping.

I would love a revised RA8875 board with the above problems solved.

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