Program a small Arduino Using A bigger Arduino to Display a Scrolling Text Without a Library. Sony Spresense or Arduino Uno are not that expensive and don't require a lot of power. However, if your project has limitation on power, space, or even budget, you may want to consider using Arduino Pro Mini. Unlike Arduino Pro Micro, Arduino Pro Mini does not have a USB port. You can program Arduino Pro Mini with USB to UART converter. You can also use another Arduino with USB port to program Arduino Pro Mini like we're about to do here.

We need 4 wires between WEMOS D1 Mini Matrix LED Shield and Arduino Pro Mini as follow:

WEMOS D1 Mini Matrix LED Shield - Color - Arduino Pro Mini

3V3 - Red - 3.3V

D7 - Green - A4

D5 - Yellow - A5

GND - Black - GND

Note that we are using Arduino Pro Mini 5 volt version so we had to step down the voltage using 5 diodes.

Check your Arduino operating voltage before connecting to WEMOS D1 Mini Matrix LED Shield. No voltage stepping down is needed if using Arduino Pro Mini 3.3V version. 

We need 6 Arduino male to female jumper wires plugged in the programming Arduino as follow:

Sony Spresense - Color

10 - Red (RST)

11 - Orange

12 - Yellow

13 - Green

5V - Red (Power)

GND - Black

Open Arduino IDE then File > Examples > 11.ArduinoISP > ArduinoISP. With Sony Spresense board, it is necessary to uncomment the following line:

// #define USE_OLD_STYLE_WIRING

Once that's done, press Ctrl+U to upload code into Sony Spresense or the programming Arduino you are using.

Last thing we need to do is upload the code to target Arduino. Download the code from Github, but do not press Ctrl+U in Arduino IDE because that will result in overwriting the code you already uploaded into Spresense which is needed to use it as a programmer. Instead, press Ctrl+Shift+U to upload using the programmer.