This add-on board for OVMS (Open Vehicle Monitoring System) adds Single-Wire CAN (GMLAN) functionality to OVMS 3.0 module.
This board uses the same MCP2515 CAN controller as OVMS module, but instead of SN65 transceiver it employs TH8056 which fully supports SWCAN capabilities. While SN65 (and other CAN transceivers) can be used to receive/transmit CAN messages on SWCAN bus, they unfortunately are unable to wake the car with the so called High-Voltage Wake Up (which pulls CAN bus line up to 12V). Thus OVMS hasn't been so useful so far with those cars with most of the interesting communication done on SWCAN bus (such as GM/Chevrolet brands).
This add-on board adds the full support for SWCAN communication and allows such remote functionality as locking/unlocking the car, cabinet/engine preheating, polling environmental variables (such as cabin temperature) even when the car is asleep. The SWCAN circuitry is heavily inspired by same implementation found on Macchina M2 interface board. However this board is designed to connect directly to the OVMS extension header and to fit inside the existing plastic case on top of the optional GSM/GPS module.
In addition to SWCAN bus functionality, there are also 3 LEDS, various debugging test points and a 13 pin header to facilitate easier connection to logic analyzer.
As for software, see my GitHub repository which has patches for OVMS to enable support for this add-on board. It contains also additional functionality for Chevrolet Volt / Opel Ampera.
To physically connect the SWCAN bus lines to the board, there are several ways.
- Use the 2-pin SWCAN header on the board. (SWCAN_LO is GND by standard)
- Connect the SWCAN_HI to pin 19 (GEP_1) on DB25 extension port.
- Create OBD cable that connects the SWCAN bus to the unused pin 1 on OVMS DB9 port. Then solder a jumper wire from that pin 1 to the GEP_1 on OVMS extension header.
- Substitute the CAN2 with SWCAN: Create OBD cable so that GND is attached to CAN2_LO and SWCAN_HI to CAN2_HI. Then connect pin 16 (CAN2_HI) to pin 19 on DB25 (or use the dongle mentioned below). This actually then replaces the third CAN bus (CAN2) and the second internal MCP2515, which is put to sleep with modified OVMS code. This approach doesn't require internal modification of the OVMS module.
The cable I use with my Opel Ampera / Chevrolet Volt uses the approach #4:
| OBD-pin | DB9-pin | Comment |
|---|---|---|
| 1 | 8 | SWCAN_HI --> CAN2_HI |
| 4 | 6 | CHASSIS & SIGNAL GND --> CAN2_LO |
| 4 | 3 | CHASSIS & SIGNAL GND |
| 6 | 7 | CAN0_HI (Primary High-Speed CAN) |
| 14 | 2 | CAN0_LO (Primary High-Speed CAN) |
| 12 | 5 | CAN1_HI (Secondary High-Speed CAN) |
| 13 | 4 | CAN1_LO (Secondary High-Speed CAN) |
| 16 | 9 | +12V |
The TH8056 transceiver modes (normal, sleep, high-voltage, high speed) are controlled either indirectly with MCP2515 controller (default setting, changeable with M0_SEL and M1_SEL jumpers) or directly with MAX7317 EGPIO pins. This latter approach was a backup option in case MCP2515 turned out to be finicky, but it seems this is not the case. So, the available MAX7317 pins can be used for other uses, for example for test buttons connected to GEP_4 and GEP_3 (connect jumpers BTN1_SEL and BTN2_SEL in this case).
In case you want to debug the SPI bus with all the CS lines from various peripherals connected at the same time, it's easier to solder jumper wires from the ESP32 to GEP #2-#4, which are routed on the the add-on board to the 13 pin header. Then hooking/unhooking a logic analyzer wires becomes a breeze. Note that the CS line of MCP2515 of this add-on board is wired already to the header, so no need for soldering if you need to debug only the SPI traffic on the SWCAN controller.
To allow doing a hardware reset on MCP2515 via MAX7317, connect RESET_SEL jumper and then drive EGPIO_3 low.
Included in this repository is also an optional debugging dongle that contains following functionality:
- Placeholders for 3 hardware buttons, of which the first two (BTN1 and BTN2) can be tailored in varied pull-up/pull-down/RC debouncing configurations. They can be polled with MAX7317. BTN3 is a pull-down button connected to EXP_1/SWCAN_INT that can induce interrupt if needed (for example to awake the OVMS from sleep state for testing purposes)
- STATUS/TX/RX leds (wired in parallel with internal LEDS) so that they can be seen also with the case on.
- 12V / EXT_12V and 3.3V pins
- CAN0, CAN1, CAN2 and SWCAN bus pins
- By default the CAN2_HI is connected to SWCAN_HI to avoid internal jumper wires. This connection can be severed by disconnecting the SWC_SEL jumper with a sharpie.