ESP32/CDP/LLDP/DHCP – Third Edition

This is the third major update to the CDP/LLDP packet sniffer (First, Second). The main processor is now the ESP32 microprocessor. Of the available packages, I’ve chosen the TTGO T-Display. I would like to give a shout-out to Colin for his assistance, and for testing what has been (at times) some cringe worthy code. There has been some pretty poor work-arounds to resolve issues, and then (hopefully) cleaned them up later. If you notice any remaining please let me know.

You can see below that the wiring is far simpler than the previous versions. This is thanks to the T-Displays extra built in options.

The major changes in the T-Display: built in TFT, charging circuit, and Bluetooth. The red mosfet is used to distribute power to peripheral devices. The reason for the mosfet is to turn off peripherals while in Deep Sleep mode. Without it, the battery (a single 18650) is drained in less than 24 hrs because the ENC and RS232 convertor never power down. With the mosfet the Deep Sleep mode uses <1mA – I cant say exactly how much as my multi-meter doesn’t swap between mA and uA automatically. Now a button on the T-Display can be used as a soft off option (Deep Sleep).

The T-Display has far more memory than the old Arduino’s, so more options can be written into it. It still performs CDP and LLDP packet capture, and it does it more reliably. This is thanks to the addition of PID checking, it now disregards DTP, VTP, and a few other packets that share the same broadcast MAC address.

// This PID is for CDP only and will filter out VTP, DTP, etc..
String PID = print_mac(EthBuffer, 20, 2);
if (PID != "20:00") {
return (0);
}

DHCP has also been tweaked by making sure the millis stays above 0 and makes sure the link is still up. If DHCP fails, you can initiate another request by “long pressing” the left button.

(Credit: @Colin) 
while (dhcpState != DHCP_STATE_BOUND && uint16_t(millis()) - start < 20000 && ENC28J60::isLinkUp()&& uint16_t(millis()) - start > 0) {
		if (isLinkUp()) DhcpStateMachine(packetReceive());
    }
    updateBroadcastAddress();
    delaycnt = 0;
    return dhcpState == DHCP_STATE_BOUND ;

It has been changed to ask for all 256 DHCP options. For those times the VLAN is correct but DHCP is way out. The common options have been setup, and it wouldn’t be that difficult to expand it to include more.

Enclosure

STL files: CDP ESP BASE, CDP ESP LID, CDP Buttons

The enclosure has the following design features:

• The screen, buttons and USB-C measurements were taken from this model: https://www.thingiverse.com/thing:4501444 Credit: https://www.thingiverse.com/pjmi/designs Licence: https://creativecommons.org/licenses/by-nc-sa/4.0/
• It’s slightly smaller and far more pocket friendly than previous models.
• Micro switches are no longer needed as each of the buttons on the T-Display have multiple functions.
• The battery is held in with a zip-tie, which also helps to keep the hot-glued mosfet in place. This does make changing battery challenging but it’s fine for now. It seems like a dirty fix, but I like it.
• During construction all pin headers and the screw terminals on the mosfet PCB are removed and wires are soldered directly to the PCB’s to save on space.
• In the base below red object is the ENC, then the mosfet, dark blue is the RS323 to TTL convertor, and purple is the battery – zip tied down using the mounting slots in the sides of the base.
• The mount used to anchor the battery was on the bottom middle, but it was found to be far too easy to snap off.
• You’ll also notice that only one edge has screws to hold the base to the lid. The tolerances between the other 3 edges is tight enough to hold the lid firmly in place.
• The gap along the top lip in the base is to allow the RS323 converter to drop through it and be mounted on the rear wall. This was a tough decision as the long walls could be bowed out to to accommodate the extra width, and the long lips toward the front become more susceptible to snapping off since they don’t connect with the rear section.
• Some lite filing may be required along the outside of the lips in the base to make sure it fits firmly but still allows the lid to be removed for maintenance.

Code Changes

There has been massive changes to the main code and some minor changes to the libraries.

• The sections of code has been broken into smaller files to make it better organised and easier to read.
• One of the cores is dedicated to Bluetooth and RS323 communication.
• CDP packets now check for the correct PID before continuing.
• The Ethercard library has been edited to allow custom SPI pins – this is due to a limitation in the TTGO T-Display. It uses the default SPI pins for the screen, and they aren’t broken out for easy of use. The edits aren’t pretty but they work.
• The GUI has more pictorial icons to make use of all the extra memory available.
• In summary, there are sections that may still contain some horrible work-arounds. The process of cleaning it all up and getting elegant solutions seems to be taking forever. Despite this, it all works, trust me 🙂

Build Environment

• Arduino <1.18.13.
• The modified ENC library: https://github.com/ModLogNet/ethercard_ESP
• The modified TFT_eSPI library: https://github.com/ESP32Home/TFT_eSPI_ttgo_t-display
• ESP32 Espressif Systems <1.0.5: https://github.com/espressif/arduino-esp32
• The INO, headers and CPP files: https://github.com/ModLogNet/ESP_ENC_CDP

When compiling I choose:

• 240Mhz (WiFi/BT), Board “ESP32 Dev Module”. 80Mhz turns out to be too slow for reliable serial communications.
• Partition Scheme: No OTA (2MB APP/2MB SPIFFS) – I ran out of space when using the Default (1.2MB APP/1.5MB SPIFFS).

Modify enc28j60.cpp to change the pins:

#define MY_CS       33
#define MY_SCLK     25
#define MY_MISO     27
#define MY_MOSI     26

RS232 to TTL Converter pins

MySerial.begin(RS323Speed, SERIAL_8N1, 32, 13); //RX, TX

Rough Bill of Materials (AUD)

• Male RS-323 to TTL convertor – $4.50 – This is likely to change as I’m looking for a thinner module.
• Mosfet breakout board – $4
• Mini ENC28J60 module – $7.5
• Passive 18650 battery holder – $2.5
• TTGO T-Display – $16
• Total: $34.5 – You will get better prices buying in bulk.

Cons

• Resolution and screen size are small – as a work around, I’ve added an option to pipe the screen details to the Bluetooth serial port, so it can be viewed in an terminal emulator on a device with a bigger screen, EG: Phone or laptop.
• Having a way of turning off the device completely but still be able to charge would have been great. I found after finishing it, the reset button can be repurposed as it cuts the power to the chip. I’m unsure if it still charges when its depressed.

UPDATE 01/05/2021

After testing the serial connection again, I’ve fixed 2 long standing bugs and another unexpected issue. The Bluetooth icon now accurately changes when connecting and disconnecting, and the randomly missing characters on the serial port is fixed. Changed the TX pin for the TTL on the ESP, apparently it has to be between pin0 and pin31.

if (event == ESP_SPP_SRV_OPEN_EVT) {
BT_STAT = 1;
}
if (event == ESP_SPP_CLOSE_EVT) {
BT_STAT = 0;
}

define SERIAL_BUFF 1278
...
MySerial.setRxBufferSize(SERIAL_BUFF);