Arduino applications with ILI9341 library (Part 1/2)
By Giovanni Carrera
Introduction
The main purpose of this project is to build a
system based on chip ATMEGA328P, Arduino compatible, and interface it with a
2.2" QVGA TFT display using the library ILI9341. As an alternative to the self-build
system you can use an Arduino Pro 3.3 V/ 8 MHz board or similar.
This system
works well and can be very useful for many applications, as data-loggers,
graphic terminal, etc. A great advantage of this system is to operate at 3.3 V
with a modest power (180 mW). It is very suitable to be powered by a lithium
3.6V rechargeable battery. The regulator adopted still works well with a
typical drop-out of about 50mV at low currents.
The display
After buying a TFT graphic display for few euros
I decided to use it with Arduino. As often happens, the device was not at all
documented by the seller neither by the manufacturer. However, the most salient
features were screen-printed on the board. The graphics resolution was very
good, QVGA means ¼ VGA, i.e. half of the pixels on each side, then 240x320
points to 64K or 256K colors. The interface used is the SPI. On the back of the
card it is also a connector for SD card, very suitable to load images or save
data. After searching on various
forums and sellers in the internet, not without some difficulty, I were able to
locate the object, finding a very synthetic scheme, a data sheet of the display
and its controller ILI9341 of Ilitek. By
forum came out the link that allowed to download the library Seeed ILI9341 2.2
TFT + SD by Albert Miao, Loovee and Visweswara (https://github.com/gmtii/ili9341-arduino). The next photo shows the look of
the TFT display used.
The first thing to do was to find out the power
supply module, indicated by the abbreviation Vcc. I had to supply the module to
5 or 3.3V? Not even the scheme was to help. On it was indicated only the output
voltage of the regulator: 3V, typical supply voltage of the display and the
controller, as is apparent from the respective data sheet. On the card was a smd
regulator whose name was different from that of the diagram and I did not find
its data sheet in the internet. Surely it had to be a low-dropout regulator. I then,
decided to supply the module with a voltage of 3.3V as a voltage drop of 0.3V
is usually well tolerated by regulators with low drop-out, for currents of
relatively modest also the use of 5V could have overheat, if not destroy, the
tiny controller.
Hardware
Having already experienced the
self-construction on the bread boards using the Arduino chip ATMEGA328P
programmed with boot-loader, I decided for a version powered to 3.3V instead of the classic 5 V.
This is possible because the chip ATmega328P works also at lower voltages
(1.8V), and has a less consumption. But the quartz in this case should be 8 MHz
instead of 16, I have tried to overclock to 16 without having problems. The 3.3
V power supply simplifies interfacing of the display with the CPU, which
otherwise require level converters. I programmed the chip with the Arduino
bootloader, but it is easy to buy the chips already programmed. To load the
sketch I used a USB serial converter module connected to a PC. The following
figure shows the prototype and component layout.
To show the details of the card the display
module was removed, it is connected to the board via a 9-pin strip connector
(TFT interface) and a 4-wire cable and connector (SD interface). The display is
fixed to the board by means of two nylon spacers. On the right it can be see
the module that provides to convert the signals of the UART to USB. Below you
can see the complete scheme of my system.
You can see the three filters RC low-pass
anti-aliasing, with values below the pulsation of cut is equal to: w=1/(RC), which corresponds to a
frequency of about 340Hz. The three diodes act as protection for voltages
greater than 3.3V. The low dropout regulator used is a LM3940-3.3, but you can
replace it with an equivalent. The current consumption with display on and is about 50mA.
The image below shows the prototype in
operation and with the sketch of test loads, which displays the measures of
three analog channels.
When using the Arduino IDE to compile the
sketch and send it to the system, it must indicate that the board used (Tools
menu) is a "Arduino Pro" clocked by 16MHz and ATmega328 processor.
This is a card without USB interface very similar to that proposed.
As showed in the scheme, the first three analog
channels are wired to the terminal blocks because I want to use the prototype to show these
measurements on the display and store the data on an SD card.
This little board is used only to program the
card. The module serial / USB should work with the signals Tx, Rx and DTR as
well as provide the 5V to our regulator. Only for programming you have to disconnect
the external power source using the jumper W1 (P position). DTR is used to give
the reset. Of course you must know what chip this board uses and load its
driver for your Windows. I used the module Pololu USB01A, with the chip CP2101,
but you can use similar modules. The following figure shows the wiring between
the module and the connector J1, always made with a small board breadboard.
It is obvious that the Tx signal of this module
should be connected to the Rx of the Arduino as well as Rx should be linked to
the corresponding Tx, as you do with a null modem serial.
TFT wirings.
The
module has a 9-pin 0.1 " strip connector already welded. with the comb already welded. The table shows the
connections and the pin assignments for Arduino I / O . Continues in Part 2
TFT board
J2
Pin
|
function
|
Arduino
Pin I/O
|
CPU Pin
|
1
|
Vcc (+3.3V)
|
||
2
|
Gnd
|
||
3
|
Cs
|
D5
|
11
|
4
|
Reset
|
D4
|
6
|
5
|
D/C
|
D6
|
12
|
6
|
SDI (MOSI)
|
D11
|
17
|
7
|
Sck
|
D13
|
19
|
8
|
Led
|
D7
|
13
|
9
|
SDO (MISO)
|
D12
|
18
|
