Programming the Arduino Ethernet Board


Following on from The case of the broken Pro Micro, I wanted to use my Arduino Ethernet Board, which is basically an Uno and an Ethernet shield fused together on one board, as an AVR programmer for the Pro Micro. For a recounting of this tale see The case of the broken Pro Micro (reprise).

However, before I could use the Arduino Ethernet board as an ISP, I needed to figure out how to program the Arduino Ethernet board, as there is no USB port. The USB-TTL adapter was a CP2102, as shown in  the Device Manager snap in of Windows 7

Device Manager
Device Manager


The product page was obviously useless, Arduino – ArduinoBoardEthernet. It says that you can use the AVR pins (with an AVR programmer) or the serial pins (with a USB-FTDI adapter), but does not list which of the six serial pins is which. This is because one is meant to use a FTDI connector, which I do not have (see also Getting started with an Arduino Ethernet board (not shield)).

Location of the AVR and Serial pins on the Arduino Ethernet Board
Location of the AVR and Serial pins on the Arduino Ethernet Board

However, from that page, one can download the Eagle files and open the Arduino_Eth08d_MU-REV3.brd file to see which pins are connected to what signal/line, which enabled me to annotate the serial pins:

Arduino Ethernet Pins
Arduino Ethernet Pins

I was able to verify this with a FTDI cable/connector pinout from the product page for a USB to FTDI connector cable, USB to 3.3v TTL PIN Header Cable

FTDI Connector
FTDI Connector

Making the connection

Connecting the USB-FTDI adapter to the serial pins,


Rx => Tx

Tx => Rx

and the corresponding 5V and GND pns, obviously. I left CTS, on the Arduino Ethernet board unconnected as there was not a corresponding pin on the USB-FTDI adapter.

Arduino Ethernet Serial Pins to USB-TTL
Arduino Ethernet Serial Pins to USB-TTL

Note that it is not necessary to connect the RTS pin on the Arduino. However, if you do not connect the DTR pin of the USB-TTL to the Arduino’s RTS pin, you must hit the reset button at the upload stage of the compile.

If your USB-TTL adapter does not have a DTR line, then you can modify it to send the reset signal, by following this guide, Mod a USB to TTL Serial Adapter (CP2102) to program Arduino Pro Mini like the FTDI Board with Auto-reset.

Uploading the sketch

However, when I used the Arduino IDE to upload Blink, while the sketch appeared to be uploaded to the board (according to the IDE), the LED did not flash.

I later realised that the Arduino Ethernet board does not use Pin 13 for the on-board LED, but rather pin 9 – note that the LED has “L9” next to it. This is because pin 13 is used for SPI timing on the Ethernet board (source: Arduino Ethernet LED 9).

Once I had modified the code, and uploaded it, then the board flashed the on-board LED, as expected.

const int kLED = 9;

// the setup function runs once when you press reset or power the board
void setup() {
// initialize digital pin 9 as an output.
pinMode(kLED, OUTPUT);

// the loop function runs over and over again forever
void loop() {
digitalWrite(kLED, HIGH); // turn the LED on (HIGH is the voltage level)
delay(500); // wait for a second
digitalWrite(kLED, LOW); // turn the LED off by making the voltage LOW
delay(500); // wait for a second


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