jhochsteger / GK911

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Pinbelegung

Pico	MCP2515
26	INT
24	SCK
25	SI
21	SO
22	CS
38	GND
40	VCC1
36	VCC

Definition bestimmter Befehle

INSTRUCTIONS = {
    'RESET': 0xC0,
    'READ': 0x03,
    'WRITE': 0x02,
    'READ RX': 0x90,
    'READ STATUS': 0xA0,
}
MODES = {
    'LOOPBACK': 0x02,
    'NORMAL': 0x00,
}

Definition der Methoden zur Kommunikation mit MCP2515

def reg_write(spi, cs, address, data):
    """
    Write 1 byte to the specified register.
    """

    # Construct message (set ~W bit low, MB bit low)
    msg = bytearray()
    msg.append(INSTRUCTIONS['WRITE'])
    msg.append(address)
    msg.append(data)

    # Send out SPI message
    cs.value(0)
    spi.write(msg)
    cs.value(1)
    
def reg_reset(spi, cs):
    """
    Write 1 byte to the specified register.
    """

    # Construct message (set ~W bit low, MB bit low)
    msg = bytearray()
    msg.append(INSTRUCTIONS['RESET'])

    # Send out SPI message
    cs.value(0)
    spi.write(msg)
    cs.value(1)

def reg_set_mode(spi, cs, mode):
    """
    Write 1 byte to the specified register.
    """

    # Construct message (set ~W bit low, MB bit low)
    msg = bytearray()
    msg.append(INSTRUCTIONS['WRITE'])
    msg.append(0x0f)
    msg.append(0x07 | MODES[mode] << 5)

    # Send out SPI message
    cs.value(0)
    spi.write(msg)
    cs.value(1)

def reg_read_rx(spi, cs):
    """
    Read byte(s) from specified register. If nbytes > 1, read from consecutive
    registers.
    """

    # Construct message (set ~W bit high)
    msg = bytearray()
    msg.append(INSTRUCTIONS['READ RX'] | 0x01 << 1)

    # Send out SPI message and read
    cs.value(0)
    spi.write(msg)
    data = spi.read(8)
    cs.value(1)

    return data

def reg_read_status(spi, cs):
    """
    Read byte(s) from specified register. If nbytes > 1, read from consecutive
    registers.
    """

    # Construct message (set ~W bit high)
    msg = bytearray()
    msg.append(INSTRUCTIONS['READ STATUS'])

    # Send out SPI message and read
    cs.value(0)
    spi.write(msg)
    data = spi.read(8)
    cs.value(1)

    return data

def reg_read(spi, cs, address):
    """
    Read byte(s) from specified register. If nbytes > 1, read from consecutive
    registers.
    """

    # Construct message (set ~W bit high)
    msg = bytearray()
    msg.append(INSTRUCTIONS['READ'] | 0x01 << 1)
    msg.append(address)

    # Send out SPI message and read
    cs.value(0)
    spi.write(msg)
    data = spi.read(8)
    cs.value(1)

    return data

Ausführung zur Setzung des CAN Modes

cs.value(1)

print(reg_read_status(spi, cs))

reg_set_mode(spi, cs, 'NORMAL')

print(reg_read_status(spi, cs))

Fragestellungen

• Welche Übertragungsreichweiten und Geschwindigkeiten kommen beim CAN-Bus zum Einsatz?

  Bitrate	Kabellänge
  10 kbits/s	6,7 km
  20 kbits/s	3,3 km
  50 kbits/s	1,3 km
  125 kbits/s	530 m
  250 kbits/s	270 m
  500 kbits/s	130 m
  1 Mbits/s	40 m

• Wieviele Teilnehmer können dabei miteinander kommunizieren?

  theoretisch unendlich viele

• Welche Möglichkeiten des einfachen Deployments bieten sich an, wenn verschiedene Services zur Darstellung von Sensordaten miteinander agieren sollen?

  Raspberry Pi, Pico

• Wie können Grenzwerte definiert werden, die zur Steuerung bzw. Notifikation herangezogen werden?

Quellen

https://www.itwissen.info/CAN-Bus-controller-area-network-CAN.html#:~:text=Theoretisch%20ist%20die%20Teilnehmerzahl%20unbegrenzt,CAN%2DBus%20angeschlossen%20werden%20k%C3%B6nnen.

https://www.mikrocontroller.net/attachment/6819/canbus.pdf