Multiple TFT's not displaying

Question

I am trying to connect about 5 TFT's to my Arduino Uno and I got to the point of hooking two of them up without any issues. However, after a code upload, they stopped working. I then took all the code out and tested each in isolation and they still function. However, together only one of them displays data. The TFT's I am using are the ST7789v2 found here. https://www.waveshare.com/wiki/1.69inch_LCD_Module#Hardware_Connection_3

I have wired them up according to the schematic with "all" pins being shared except the CS pin. Each display has its own.

The code looks like this;

#include <Adafruit_GFX.h>
#include <Adafruit_ST7789.h>

#define TFT_CS1 10
#define TFT_CS2 3

#define TFT_RST        8 // Screens shared
#define TFT_DC         7 // Screens shared
#define TFT_BACKLIGHT  9 // Backlight for all screens

Adafruit_ST7789 tftHour1 = Adafruit_ST7789(TFT_CS1, TFT_DC, TFT_RST);
Adafruit_ST7789 tftHour2 = Adafruit_ST7789(TFT_CS2, TFT_DC, TFT_RST);

Adafruit_ST7789 setupScreen (Adafruit_ST7789 tft){
  tft.init(240, 280);
  tft.fillScreen(ST77XX_BLACK);
  return tft;
}

void setup(void) {
  tftHour1 = setupScreen(tftHour1);
  tftHour2 = setupScreen(tftHour2);
}

void loop(){
  tftHour1.print("1");
  tftHour2.print("2");
}

When this executes, display 2 shows output but display 1 does not. Weirdly if I take the CS from display 1 and pop it into the same line on the breadboard as CS2, and then put it back into its original position, display 1 and 2 seem to work. But only till the next power cycle or upload.

Also oddly, setting the colour of each screen works even if one is blue and the other is say green. Both change to their respective colours. But when I try to write text to them, only one of them has text.

If I comment the code for tftHour2, then tftHour1 starts showing text. If I swap the setup code for ftfHour1 and tftHour2, then the display that is last to init, is the one that shows the text and the other stays blank.

Like I said, I had all this working and then a simple font change upload and it all stopped working.

Any help on this is really appreciated.

I should mention that I am a programmer and very new to electronics so please keep it simple.

#Update. If I have separate RST pins then it works. But I need 5 screen and will run out of pins in that case. Is there a way to get around that?

Answer 1

According to the data sheet, the RST signal is accepted without an active CS, so when sharing this signal, all displays are reset if you initialize just one of them. That explains the behavior you observe.

You found the solution: Do not share the RST signal in all instances of Adafruit_ST7789.

---

Now you worry about the needed number of pins, and that is indeed an issue.

One possible solution is to use a decoder that takes n inputs to activate 1 of 2ⁿ outputs, like this:

^{simulate this circuit – Schematic created using CircuitLab}

To control 5 RST lines you will need only 3 pins. Use a binary value to select one of the unused outputs to deactivate all RST lines.

The same is true of the CS lines.

However, the library in use does not support this kind of control. Therefore, I'm afraid that you need to consider another Arduino model with more pins.

---

As a work-around, you might want to try this alternative:

Assign the same Arduino pin to all instances of Adafruit_ST7789, as in your first attempt. But do not connect it to the RST inputs of the displays. Leave the Arduino pin unconnected.

Instead, use another Arduino pin and connect that to the displays. Then during initialization, reset all displays with your own code. See the data sheet for correct levels and timing.

Or try to connect the RST inputs of all displays to VCC, as they may work without an explicit hardware reset just fine.

---

In a comment you brought up the idea of using a multiplexer.

Without any other change, you would need the selection signals as additional pins to the multiple CS pins, in your case 3 more.

Instead you can also share the CS signal in the same way:

^{simulate this circuit}

It is very important that you use multiplexers that output the inactive level as required by the displays on not selected outputs!

Even now you should instantiate as many Adafruit_ST7789 as you have displays, because each instance most probably stores internal information necessary for its correct working. But you can use the same pins on all of them.

Before you call any method of an instance, switch the multiplexers to the associated display.

To be sure that all data is completely transmitted, you might need a small delay after the called method. Perhaps the called method already blocks until all data is completely transmitted. You might try to read the library's source to learn more.