Silicon Chip – May 2019

siliconchip.com.au Australia’s electronics magazine May 2019 87

have been so popular. That would give

us more screen real estate and more

pixels, without using up any more

I/O pins.

And that’s just what we found.

We have been aware of the existence

of 3.2in (8cm) and 3.5in (9cm) touch-

screen modules for some time, but in

the past, all the ones we’d seen had a

parallel interface.

That’s good for providing a fast up-

date rate, but it requires a micro with

a parallel interface and plenty of pins

to use efficiently.

So we went searching for similar se-

rial-controlled screens, and we found

two vendors in AliExpress offering

just that (see http://www.aliexpress.com/

item//32954128438.html and http://www.

aliexpress.com/item//32954240862.

html).

We bought one from each to test.

There are several different variants

of this type of display around, with dif-

ferent connectors and interfaces, but

all use 0.1in (2.54mm) pitch header

pins to connect to the controller board.

Many sellers indicated that they use

the ILI9488 controller IC, although, as

we found out later, this is not always

the case.

They all come with either a full-

size SD or microSD socket onboard,

and many have a resistive touch panel

too. We particularly wanted to get the

touchscreen variants since that obvi-

ates the need to fit any buttons or other

controls in most cases.

Once we got the screens, it took

quite a bit of effort to get them work-

ing (for reasons we’ll explain later),

but we got there in the end. Later on,

we’ll give you download links to our

software and source code, so that you

can do it too.

We also decided to try out some oth-

er similar screens, one from Altronics

(because it was easy to get) and another

which is designed to plug straight into

an Arduino, since that one is really

easy to get up and running if Arduino

is your platform of choice.

This article assumes that you are

familiar with either the Arduino In-

tegrated Development Environment

(IDE) or Micromite BASIC and the

various possible methods of uploading

MMBasic code to a Micromite.

If you are not, we suggest that you try

working on simpler projects with these

platforms before diving into this one.

We have designed a small breakout

board to connect the ‘universal’ 3.5in

serial touchscreen (ie, the one that does

not come as a ‘shield’) to an Arduino.

We’ll describe this board below.

This breakout board also works with

the 2.8in touchscreen that we’ve used

so often in the past in the Micromite

LCD BackPack.

Contestant number one: 3.5-

inch serial touchscreen

The 3.5in serial touchscreens we

sourced look very similar to the 2.8in

touchscreen used in the very popu-

lar Micromite LCD BackPack project

(February 2016; siliconchip.com.au/

Article/9812).

The screen is not only bigger but it

also has a substantially higher resolu-

tion, at 480x320 pixels (0.15MP) com-

pared to 320x240 pixels (0.07MP). So

Fig.1: this excerpt from the XPT2046 datasheet shows a typical circuit for

the chip and demonstrates how the touch panel can be viewed as a variable

resistor network.

Contestant number one:

we recommend that you

use this 3.5in display panel as it works

with either a Micromite or Arduino (once you build

our breakout board). We cut off the pin which is now missing, as it

was causing a conflict between the touch and display controllers, but that is no

longer necessary with the revised breakout board we present in this article.