Open-Vehicle-Monitoring-System: Module based on Lilygo T-Call A7670E V1.0/V1.1

The Lilygo T-Call modules provide most of the hardware required for an OVMS module:

• ESP32-WROVER E with 4MB Flash and 8MB PSRAM
• Simcom A7670E 4G modem with GPS
• Included antennas for 4G and GPS
• USB-C port for programming and powering during testing
• ext 5V powering possible via pin
• Price is around 35€ + tax

This module supports only 2 CAN buses. This might limit the suitability for your car!

OVMS Requirements

• Connection to OBD-II connector
• Powering via 12V of the OBD-II connector
• OBD-II to DSub-9 cable

Communication with the car via CAN-bus requires

• Physical layer(s) and CAN controller
• 12V to 5V DC/DC converter and including filtering

Hardware

The Lilygo board is placed onto a motherboard, either into a socket, or the pins can be directly soldered to the board. The motherboard provides the voltage for the Lilygo board and the DSub-9 connector to connect to the OBD-II. The board revision B is suitable for the T-Call version 1.0 and 1.1 (different GPIO mapping).

The image shows the module, the antennas and the 3D printed case (Freecad and Step files availybe in the CAD folder). The motherboard slides into slots and the lid will hold it in place. The lid allows to store the GPS antenna, but allows to add SMA connectors for external antennas as well. The 4G antenna can be stored in a 1mm high compartment inside the case.

The Lilygo based module and case. The GPS-antenna is placed into a corresponding slot in the lid of the case

The chosen orientation (Simcom 7670 on the top) of the Lilygo board allows for easy access to the antenna connectors. The ESP32 WROVER E is in this case located on the bottom side.

Features of the motherboard

The Lilygo T-Call is placed on a motherboard

• size 49 x 98 mm²
• DSub-9 connector (pinout compatible standard OVMS module) to connect to the OBD-II
• 12V car battery input from the OBD-II, or a separate power souce
• Fuse and filtering (common mode choke and capacitors) of the input voltage
• DC/DC converter to provide the Lilygo board with 5V
  • Auto shutdown of the DC/DC converter at a voltage below 11.7V to prevent the drainage of the battery
  • A voltage of more than 11.9V is required to switch on again
• Two CAN busses are implemented
  • ESP32 internal requires only a physical layer
  • Second bus with an MCP2515 controller
  • Solder jumpers to enable 120 Ω bus termination resistors
• Voltage divider to measure the battery voltage
• The schematic and board layout is provided for Autodesk Eagle
• The Gerber and bill of material is provided for the board production and assembly at JLCPCB
• Nearly all components are placed on the bottom side of the PCB. This allows for an easy access, even when the Lilygo board is soldered to the motherboard
• 3D printable case to house the motherboard as well as the antennas
  • sufficient space in the lid to add SMA connectors for external antennas

Top and bottom side of the module. Only a single CAN bus is assembled, because the module is used for a car with a single bus. Therefore the second physical layer and the MCP2515 is left empty

Limitations

• The flash memory size of only 4MB Flash does no allow Over-the-Air updates of the firmware. So, each change requires access via the USB-C port
• Only two CAN buses
• No GPIO extender
• No SD-Card support

Assembly related hints

The Lilygo module can in principle be soldered directly to the board by using the provided pin headers. The board has to be mounted with the Simcom modem on the top side. Using a precision socket and corresponding pin headers, the module can easliy be removed again, but should be secured by non conducting screws.

The DC-DC converter output voltage is defined by R8 and R9. The reference voltage is 0.8V, so with R8=100k and R9=18k, the output voltage is about 5.2V. The Schottky diode reduces this by about 0.4V, hence the Lilygo module is powered by 4.8V. The diode is necessary to allow to provide power via the USB-C port at the same time, i.e. when flashing a new firmware.

The DC-DC converter exhibits an enable input, which requires a minimum voltage to power on of 1.2V and to power off of 1.0V. The enable voltage is derived from the input voltage by means of the Zener diode Z1 and the voltage divider R3/R11. For a Zener voltage of 10V, the default values R3=100k and R11=220k lead to a minimal power on voltage of 11.8V and a maximal power off voltage of 11.5V. In case a different Zener voltage is used, the divider has to be adjusted accordingly.

Firmware

Except for the above mentioned limitation, the module is compatible with the standard OVMS firmware, BUT the pinout requires a remapping of the ESP32 pins. This is implemented in the OVMS repository.

• Follow the standard OVMS documentation to clone the above repository, install the required additional packages and install the required Espressif IDF
• In the folder ./vehicle/OVMS.V3 copy support/sdkconfig.lilygo_tc to sdkconfig and support/partition4M.csv to partition4M.csv
• Run make menuconfig and go to Component config -> Opene Vehicle Monitoring System (OVMS) -> Hardware support.
  • Hardware Version should be 3.1
  • Enable GPIO remapping under Choose GPIO Mapping
  • Now a new menu entry appears in Hardware support called File with GPIO mapping
    • For the above sdkconfig, the default file is lilygo_tc_v11.h. This is valid for the Rev B motherboard and a Lilygo A7670E T-Call V1.1
    • The GPIO mappings files are located in the folder components/gpio_maps
• Be sure, that the following OVMS components are disabled: MAX7317, external SWCAN, Over-The-Air update, SD card
• Disable the support for all cars (vehicle support) except for the one you are interested in. This is required to meet the limited space in the Flash memory for the firmware
  • For the default partitions4M.csv, the available space for the firmware is 0x2f0000 = 3080192B = 2.94MB. The full firmware requires about 3.5MB.
  • The support for the Nissan Leaf and Renault Twizzy should always be disabled, since these do not compile without the MAX7317 support.
• Follow the standard documentation to compile and flash the firmware via USB
  • Check the size of the firmware: ls -la build/ovms3.bin
• If the flash process is failing due to a too large firmware
  • disable some additional cars under Vehicle support
  • if no corresponding scripts are utilized, disable the Javascript support (under OVMS3 library suppoprt). This saves about 220kB
  • disable components which are not needed, like the ZIP archive support (OVMS3 library support) and the OVMS3 component options Reverse Engineering tools, ECU scanning, File editor, Plugins
• The above limitation for the firmware size can be reduced, by using a modified partition table (partitions4M_full.csv) from this repository
  • This reduces the FAT partition, the storage for the OVMS configuration, from 1MB to 512kB, which might cause problems in your setup!
  • In this case the available space for the firmware is 0x370000 = 3604480B = 3.44MB

Alternative GPIO mapping

Alternative GPIO mapping files are available and can be placed in the folder components/gpio_maps.

Check the Readme for details about the naming of the GPIO pins and the format of the mapping files.

Known Issues

• An outdated firmware version of the Simcom A7670E can cause connection problems. The firmware can be updated according to the Modem Firmware Update Guide. This requires to solder an USB cable to the corresponding pins on the Lilygo board (see here).

Disclaimer

This design is provided as is. Use at your own risk.

Enjoy!