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LouisG
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Joined: 19/03/2016
Location: Australia
Posts: 121

Posted: 03:41pm 01 Feb 2023

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Hi guys,

I am expecting to receive 3.5" screens shortly that have ILI9488 controllers. I've been reading on the forum about the "issue" surrounding the MISO pin. Could I pose a few questions for which I haven't managed to find (or must have missed) answers?

Does the MISO pin on the LCD panel connect to a MOSI pin at the Pico, similar to the way a serial Tx pin connects to an Rx pin?

Regarding the ILI9488 controller, the manual says not to connect the MISO pin. I assume this is the MISO pin for the display. Does this mean that the SD_MISO and the T_DO pins on the LCD panel need also not be connected?

In the command [OPTION SYSTEM SPI CLOCK, MOSI, MISO] will I put a 0 for MISO (or MOSI)?

What facility or capability will I be losing by not connecting the MISO pin? What I really need is display, touch and SD card.

Will I now find myself with an extra Pico pin at my disposal?

... Louis

lizby
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Joined: 17/05/2016
Location: United States
Posts: 3015

Posted: 03:54pm 01 Feb 2023

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Which LCDs? Some ILI9488 modules use shift registers which are incompatible with MISO usage.

"MISO" (Master-In, Slave-Out) connections are not crossed over like Tx & Rx--MISO goes to MISO and MOSI to MOSI.

Without MISO, you can't BLIT from the LCD.

PicoMite, Armmite F4, SensorKits, MMBasic Hardware, Games, etc. on fruitoftheshed

Mixtel90

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Joined: 05/10/2019
Location: United Kingdom
Posts: 5725

Posted: 04:12pm 01 Feb 2023

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In each case MISO/MOSI is as seen from the Pico<ite, so it will send on MOSI and receive on MISO - it's always regarded as the master device. Slave devices should always be labelled from their own point of view, so a slave will send on MISO and receive on MOSI.

The problem with the ILI9488 chip is that it doesn't release the MISO pin correctly. You can use MISO normally for the SD card and Touch, they aren't affected by the fault. You might be able to connect the display MISO as well, providing that you do it via a 680R resistor (or use one of the other hacks on this forum). Get everything working first though. If you try connecting it without doing something like this then it will prevent the SD card and Touch from working. Touch and SD card won't work without MOSI.

You do need to specify a pin for MOSI in the OPTION, even if you don't use it. No, you don't get to use the "spare" one AFAIK. :(

MISO is what it says - it enables the master to receive data from the slave. Without it the master can't read stuff back e.g. it can't read the pixels back from the display. There have been recent changes to the PicoMite firmware which may allow it, but at one time you couldn't use transparent text or sprites without MISO (possibly other things - I've never had a ILI9488).

Mick

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Preliminary MMBasic docs & my PCB designs

phil99

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Joined: 11/02/2018
Location: Australia
Posts: 1781

Posted: 10:25pm 01 Feb 2023

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One way to add the 680R

That will allow touch and SD to share MISO on the Pico. If you find SD access is unreliable here is another method.

Footnote added 2024-04-15 12:07 by phil99
The 4" IPS ILI9488 is different to the panel above. The resistors near the SD socket are 4.7k pull-ups. R8 on SDO must be removed if the SD card is sharing SPI with the LCD panel. Otherwise the 10k (can be 4.7k to 10k) is unable to pass SDO to MISO.

You can use other diodes provided they have sufficient junction capacitance.
eg 1N400x series rectifiers work but 1N914 and 1N4148 don't unless you add 22pF to 150pF in parallel.

LouisG
Senior Member

Joined: 19/03/2016
Location: Australia
Posts: 121

Posted: 12:44pm 02 Feb 2023

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Thanks guys. Much clearer now. Thanks Phil for the marked up photos.

The screens arrived this morning. Now to put it all into practice!

... Louis
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matherp
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Joined: 11/12/2012
Location: United Kingdom
Posts: 8578

Posted: 03:44pm 02 Feb 2023

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Definitely recommend the 10K + diode option

stanleyella

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Joined: 25/06/2022
Location: United Kingdom
Posts: 1642

Posted: 04:07pm 02 Feb 2023

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After help from this forum I used a 680R resistor then it worked.

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