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Interfaces and Connectivity

This page documents how the CM5's interfaces are exposed on the HALPI2 carrier board. For everyday use of the built-in CAN FD and RS-485 ports, see the Interfaces and Connectivity user guide.

Serial ports (UART)

The Compute Module 5 reaches the 40-pin header through its RP1 I/O controller, which exposes five UARTs (uart0–uart4). Each UART is wired to one fixed GPIO pair — unlike earlier Pi models, the pins cannot be remapped. The login console is a separate dedicated debug UART (/dev/ttyAMA10) and is not one of these.

UART TX / RX Header pins Linux device Availability on HALPI2
uart0 GPIO14 / 15 8 / 10 /dev/ttyAMA0 Free. Conventional HAT serial port; used for GNSS HATs.
uart1 GPIO0 / 1 27 / 28 /dev/ttyAMA1 Free. These are the HAT ID EEPROM pins (ID_SD / ID_SC).
uart2 GPIO4 / 5 7 / 29 /dev/ttyAMA2 Free.
uart3 GPIO8 / 9 24 / 21 /dev/ttyAMA3 Used by the CAN FD controller (SPI0).
uart4 GPIO12 / 13 32 / 33 /dev/ttyAMA4 Used by RS-485.

Enabling a UART

Add the matching -pi5 overlay to /boot/firmware/config.txt and reboot:

dtoverlay=uart2-pi5

uart0 is enabled with dtparam=uart0=on instead. (On a CM5 the firmware redirects the plain uartN overlays to their uartN-pi5 equivalents, so either name works; the -pi5 form is used here for clarity.)

Hardware flow control is opt-in with the ctsrts parameter, and the overlays can drive an RS-485 transceiver's enable line directly with the rs485 parameter:

dtoverlay=uart2-pi5,ctsrts

CTS/RTS occupy the next GPIO pair, which on HALPI2 is often already in use:

UART CTS / RTS Conflicts with
uart1 GPIO2 / 3 System I2C bus (I2C1)
uart2 GPIO6 / 7 CAN FD chip-select
uart3 GPIO10 / 11 CAN FD SPI bus
uart4 GPIO14 / 15 uart0

uart1 is therefore practical only as a TX/RX-only port.

Freeing an occupied UART

uart3 and uart4 overlap the onboard CAN FD and RS-485 interfaces:

  • uart3 shares the SPI0 bus with the CAN FD controller — GPIO9 is the controller's data output (SDO). Using uart3 requires disabling the CAN interface and a hardware modification, and is not supported on the standard board.
  • uart4 is the RS-485 port. Removing the board's RX-enable jumper disconnects the RS-485 receiver from GPIO13, freeing uart4 for general use. RS-485 is then unavailable.

See Disabling built-in interfaces for the hardware steps.

Verifying

After rebooting, confirm the device node exists and the pins carry the expected function:

ls /dev/ttyAMA*
pinctrl funcs | grep -iE 'txd|rxd'
pinctrl 4 5

The selected pins should report their UART function (a2 for uart1–uart4, a4 for uart0).

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