Autopilot Connections

There are two connections between the Flight Controller and the Pi. USB and Serial.

USB

The Pi CM4 has one USB 2.0 OTG interface. It is muxed between the external micro USB port for flashing and a built in USB hub. The USB hub connects to the flight controller, USB C port, and the two USB JST-GH ports.

The USB connection to the autopilot is muxed with the external Pi micro USB port. When a micro USB cable is connected, the autopilot is disconnected from the Pi and the USB port on the Pi switches from host to device mode. After the micro USB cable is disconnected, a reboot is required to switch the USB port back to a host and connect to the autopilot and external ports.

For the USB port to be enabled on the ARKV6X flight controller, the VBUS_SENSE pin must be driven high from the Pi. It is connected to pin GPIO27. There is a helper python script in ark_companion_scripts to drive the pin high.

https://github.com/ARK-Electronics/ark_companion_scripts/blob/main/platform/pi/scripts/vbus_enable.py

#!/usr/bin/env python

# Copyright (c) 2019-2022, NVIDIA CORPORATION. All rights reserved.
# Permission is hereby granted, free of charge, to any person obtaining a
# copy of this software and associated documentation files (the "Software"),
# to deal in the Software without restriction, including without limitation
# the rights to use, copy, modify, merge, publish, distribute, sublicense,
# and/or sell copies of the Software, and to permit persons to whom the
# Software is furnished to do so, subject to the following conditions:
#
# The above copyright notice and this permission notice shall be included in
# all copies or substantial portions of the Software.
#
# THE SOFTWARE IS PROVIDED "AS IS", WITHOUT WARRANTY OF ANY KIND, EXPRESS OR
# IMPLIED, INCLUDING BUT NOT LIMITED TO THE WARRANTIES OF MERCHANTABILITY,
# FITNESS FOR A PARTICULAR PURPOSE AND NONINFRINGEMENT.  IN NO EVENT SHALL
# THE AUTHORS OR COPYRIGHT HOLDERS BE LIABLE FOR ANY CLAIM, DAMAGES OR OTHER
# LIABILITY, WHETHER IN AN ACTION OF CONTRACT, TORT OR OTHERWISE, ARISING
# FROM, OUT OF OR IN CONNECTION WITH THE SOFTWARE OR THE USE OR OTHER
# DEALINGS IN THE SOFTWARE.

import RPi.GPIO as GPIO
import time

# Pin Definitions
vbus_det_pin = 27

def main():
    # Pin Setup:
    GPIO.setwarnings(False)
    GPIO.setmode(GPIO.BCM)  # BCM pin-numbering scheme from Raspberry Pi
    # set pin as an output pin with optional initial state of HIGH
    GPIO.setup(vbus_det_pin, GPIO.OUT, initial=GPIO.HIGH)

    value = GPIO.HIGH
    print("Outputting {} to pin {}".format(value, vbus_det_pin))
    GPIO.output(vbus_det_pin, value)

if __name__ == '__main__':
    main()

UART

The UART connection between Pi and the autopilot is on Pi UART4 which appears as /dev/ttyAMA4 in Linux. This goes to Telem2 on the Flight Controller. This connection has been tested to 3Mbps.

Flow control needs to be set to "Force Off" when using Telem2 for MAVLINK. MAV_X_FLOW_CTRL set to "0".

Flight Controller Reset

The flight controller can be hard reset using the reset signal connected to Pi pin GPIO25. The Pi cannot reset the flight controller while it is armed. The reset signal is gated by the nARMED signal from the flight controller.

There are a couple of helper scripts to reset the flight controller. One to drive the VBUS_SENSE pin high before the reset to wait in the bootloader for 5 seconds. This is useful when updating the flight controller firmware and a hard reboot is needed. The other fast reset script drives the VBUS_SENSE pin low before the reset to avoid the wait.

https://github.com/ARK-Electronics/ark_companion_scripts/blob/main/platform/pi/scripts/reset_fmu_wait_bl.py

#!/usr/bin/env python

# Copyright (c) 2019-2022, NVIDIA CORPORATION. All rights reserved.
# Permission is hereby granted, free of charge, to any person obtaining a
# copy of this software and associated documentation files (the "Software"),
# to deal in the Software without restriction, including without limitation
# the rights to use, copy, modify, merge, publish, distribute, sublicense,
# and/or sell copies of the Software, and to permit persons to whom the
# Software is furnished to do so, subject to the following conditions:
#
# The above copyright notice and this permission notice shall be included in
# all copies or substantial portions of the Software.
#
# THE SOFTWARE IS PROVIDED "AS IS", WITHOUT WARRANTY OF ANY KIND, EXPRESS OR
# IMPLIED, INCLUDING BUT NOT LIMITED TO THE WARRANTIES OF MERCHANTABILITY,
# FITNESS FOR A PARTICULAR PURPOSE AND NONINFRINGEMENT.  IN NO EVENT SHALL
# THE AUTHORS OR COPYRIGHT HOLDERS BE LIABLE FOR ANY CLAIM, DAMAGES OR OTHER
# LIABILITY, WHETHER IN AN ACTION OF CONTRACT, TORT OR OTHERWISE, ARISING
# FROM, OUT OF OR IN CONNECTION WITH THE SOFTWARE OR THE USE OR OTHER
# DEALINGS IN THE SOFTWARE.

import RPi.GPIO as GPIO
import time

# Pin Definitions
reset_pin = 25
vbus_det_pin = 27

def main():
    # Pin Setup:
    GPIO.setwarnings(False)
    GPIO.setmode(GPIO.BCM)  # BCM pin-numbering scheme from Raspberry Pi
    # set pin as an output pin with optional initial state of HIGH
    GPIO.setup(reset_pin, GPIO.OUT, initial=GPIO.HIGH)
    GPIO.setup(vbus_det_pin, GPIO.OUT, initial=GPIO.HIGH)

    print("Resetting Flight Controller!")

    GPIO.output(reset_pin, GPIO.HIGH)
    time.sleep(0.1)
    GPIO.output(reset_pin, GPIO.LOW)

    # Enable VBUS immediatly to catch bootloader and wait
    GPIO.output(vbus_det_pin, GPIO.HIGH)

    time.sleep(1)

if __name__ == '__main__':
    main()

https://github.com/ARK-Electronics/ark_companion_scripts/blob/main/platform/pi/scripts/reset_fmu_fast.py

#!/usr/bin/env python

# Copyright (c) 2019-2022, NVIDIA CORPORATION. All rights reserved.
# Permission is hereby granted, free of charge, to any person obtaining a
# copy of this software and associated documentation files (the "Software"),
# to deal in the Software without restriction, including without limitation
# the rights to use, copy, modify, merge, publish, distribute, sublicense,
# and/or sell copies of the Software, and to permit persons to whom the
# Software is furnished to do so, subject to the following conditions:
#
# The above copyright notice and this permission notice shall be included in
# all copies or substantial portions of the Software.
#
# THE SOFTWARE IS PROVIDED "AS IS", WITHOUT WARRANTY OF ANY KIND, EXPRESS OR
# IMPLIED, INCLUDING BUT NOT LIMITED TO THE WARRANTIES OF MERCHANTABILITY,
# FITNESS FOR A PARTICULAR PURPOSE AND NONINFRINGEMENT.  IN NO EVENT SHALL
# THE AUTHORS OR COPYRIGHT HOLDERS BE LIABLE FOR ANY CLAIM, DAMAGES OR OTHER
# LIABILITY, WHETHER IN AN ACTION OF CONTRACT, TORT OR OTHERWISE, ARISING
# FROM, OUT OF OR IN CONNECTION WITH THE SOFTWARE OR THE USE OR OTHER
# DEALINGS IN THE SOFTWARE.

import RPi.GPIO as GPIO
import time

# Pin Definitions
reset_pin = 25
vbus_det_pin = 27

def main():
    # Pin Setup:
    GPIO.setwarnings(False)
    GPIO.setmode(GPIO.BCM)  # BCM pin-numbering scheme from Raspberry Pi
    # set pin as an output pin with optional initial state of HIGH
    GPIO.setup(reset_pin, GPIO.OUT, initial=GPIO.HIGH)
    GPIO.setup(vbus_det_pin, GPIO.OUT, initial=GPIO.HIGH)

    # Disable vbus detect for a faster reset
    GPIO.output(vbus_det_pin, GPIO.LOW)

    print("Resetting Flight Controller!")

    GPIO.output(reset_pin, GPIO.HIGH)
    time.sleep(0.1)
    GPIO.output(reset_pin, GPIO.LOW)

    # Do not enable VBUS, skips bootloader
    time.sleep(1)
    GPIO.output(vbus_det_pin, GPIO.HIGH)

if __name__ == '__main__':
    main()