Working with the Beaglebone Black

Overview

This document covers the Kubos Linux features which are specific to the Beaglebone Black target.

Please refer to Using Kubos Linux for a general guide to using Kubos Linux.

Reference Documents

Kubos Documentation

Debug Console Connection

As documented in section 7.5 of the Beaglebone Black System Reference Manual, an FTDI cable can be connected to the serial debug connector in order to establish a debug console connection.

This connection will be passed through to a Kubos Vagrant image as /dev/FTDI.

Status LEDs

There are four LEDs present on the Beaglebone Black which give some indication of what state the board is in. When there is only one blinking LED, the board is running Kubos Linux and the system is currently idle. The LEDs will blink in correspondence with CPU and MMC activity. If all LEDs are solid, then the system has reached some kind of locked error state.

Peripherals

The Beaglebone Black has several different ports available for interacting with peripheral devices. Currently, users should interact with these devices using the standard Linux functions. A Kubos HAL will be added in the future to abstract this process.

Note

Kubos Linux for the Pumpkin MBM2 can be used instead of Kubos Linux for the Beaglebone Black. In this case, some buses and pins won’t be available, since they aren’t exposed in the MBM2’s CSK headers, or are dedicated to other uses. See the Peripherals section for more information.

ADC

The Beaglebone Black has seven analog input pins available:

Name Pin
AIN0 P9.39
AIN1 P9.40
AIN2 P9.37
AIN3 P9.38
AIN4 P9.33
AIN5 P9.36
AIN6 P9.35

The pins are available through the Linux device /sys/bus/iio/devices/iio\:device0/.

A single raw output value can be read from each of the pins via /sys/bus/iio/devices/iio\:device0/in_voltage{n}_raw, where {n} corresponds to the AIN number of the pin.

Information about setting up continuous data gathering can be found in this guide from TI.

To convert the raw ADC value to a voltage, use this equation:

\[V_{in} = \frac{D * (2^n - 1)}{V_{ref}}\]

Where:

  • \(D\) = Raw ADC value
  • \(n\) = Number of ADC resolution bits
  • \(V_{ref}\) = Reference voltage

The Beaglebone Black uses 12 resolution bits and a reference voltage of 1.8V, so the resulting equation is

\[V_{in} = \frac{D * (4095)}{1.8}\]

Ethernet

The Beaglebone Black provides an ethernet port which can be used for things like inter-system communication.

The ethernet port is configured to have support for static IPv4 addressing and can be used with SSH via the included Dropbear package.

Kubos Linux currently guarantees support for TCP, UDP, and SCTP. Other protocols might be supported by default, but have not been verified.

Resources

Note

By default, Windows Firewall will block many incoming packet types. This may impact testing.

Configuration

The static IP address can be updated by editing the /etc/network/interfaces file. By default the address is 10.0.2.20.

Examples

A couple example programs using the ethernet port can be found in the examples folder of the kubos repo:

GPIO

The Beaglebone Black has many GPIO pins available for general use. Pinout diagrams are available on the Beaglebone website.

Any pin that is not dedicated to a previously mentioned peripheral is available for use.

CLI and Script Interface

To interact with a pin from the command line or from a script, the user will first need to generate the pin’s device name:

$ echo {pin} > /sys/class/gpio/export

For example, to interact with pin P8.11, which corresponds with GPIO_45, the user will use:

$ echo 45 > /sys/class/gpio/export

Once this command has been issued, the pin will be defined to the system as ‘/sys/class/gpio/gpio{pin}’. The user can then set and check the pins direction and value.

Set pin as output:
$ echo out > /sys/class/gpio/gpio45/direction

Set pin's value to 1:
$ echo 1 > /sys/class/gpio/gpio45/value

Get pins's value:
$ cat /sys/class/gpio/gpio45/value

Once finished, the pin can be released:

$ echo {pin} > /sys/class/gpio/unexport

Application Interface

This functionality can also be used from a user’s application with Linux’s sysfs interface.

An example program might look like this:

#include <sys/stat.h>
#include <sys/types.h>
#include <fcntl.h>
#include <stdio.h>
#include <stdlib.h>
#include <unistd.h>

int fd;
int pin = 45;
int value = 1;

/* Define the pin to the system */
fd = open("/sys/class/gpio/export", O_WRONLY);
write(fd, &pin, sizeof(pin));
close(fd);

/* Set the pin's direction */
fd = open("/sys/class/gpio/gpio45/direction", O_WRONLY);
write(fd, "out", 3);
close(fd);

/* Set the pin's value */
fd = open("/sys/class/gpio/gpio45/value", O_WRONLY);
write(fd, &value, 1);
close(fd);

/* Read the value back */
fd = open("/sys/class/gpio/gpio45/value", O_RDONLY);
char strValue[3];
read(fd, strValue, 3);
value = atoi(strValue);
close(fd);

/* Release the pin */
fd = open("/sys/class/gpio/unexport", O_WRONLY);
write(fd, &pin, sizeof(pin));
close(fd);

I2C

The Beaglebone Black has two user-accessible I2C buses.

Linux Device Kubos Device SCL Pin SDA Pin
/dev/i2c-1 K_I2C1 P9.17 P9.18
/dev/i2c-2 K_I2C2 P9.19 P9.20

I2C Standards Doc

Kubos Linux is currently configured to support the I2C standard-mode speed of 100kHz.

For examples and instructions, see the I2C HAL documentation.

SPI

The Beaglebone has one SPI bus available with a pre-allocated chip select pin.

SPI Bus 1

Name Pin
MOSI P9.30
MISO P9.29
SCLK P9.31
CS0 P9.28

Users can interact a device on this bus using Linux’s spidev interface The device name will be /dev/spidev1.0.

An example user program to read a value might look like this:

#include <fcntl.h>
#include <unistd.h>
#include <sys/ioctl.h>
#include <linux/types.h>
#include <linux/spi/spidev.h>

#define SPI_DEV "/dev/spidev1.0"

int fd;
uint8_t mode = SPI_MODE_0;
uint8_t bits = 8;
uint32_t speed = 100000;
uint16_t delay = 0;

uint8_t register, shift_reg;
uint8_t value;

fd = open(SPI_DEV, O_RDWR);

/* Register to read from */
register = 0xD0;

struct spi_ioc_transfer tr = {
    .tx_buf = (unsigned long)&register,
    .rx_buf = (unsigned long)&register,
    .len = 1,
    .speed_hz = speed,
    .bits_per_word = bits,
    .cs_change = 0,
    .delay_usecs = delay,
};

/* Send request to read */
ioctl(fd, SPI_IOC_MESSAGE(1), &tr);

/* Setup buffer to read to */
tr.tx_buf = &value;
tr.rx_buf = &value;

/* Read data */
ioctl(fd, SPI_IOC_MESSAGE(1), &tr);

close(fd);

UART

The Beaglebone Black has 5 UART ports available for use:

Linux Device TX Pin RX Pin RTS Pin CTS Pin
/dev/ttyS1 P9.24 P9.26    
/dev/ttyS2 P9.21 P9.22    
/dev/ttyS3 P9.42   P8.34 P8.36
/dev/ttyS4 P9.13 P9.11 P8.33 P8.35
/dev/ttyS5 (SLIP) P8.37 P8.38 P8.32 P8.31

Note

/dev/ttyS3 (UART3) is TX-only. /dev/ttyS1 and /dev/ttyS2 do not have RTS/CTS due to pin conflicts with other buses.

Users can interact with these ports in their applications using Linux’s termios interface.

A tutorial on this interface can be found here

Additionally, the ports can be used from the command line:

The stty -F {device} [parameters] command can be used to configure the port. For example, this command will set the baud rate of /dev/ttyS1 to 4800:

$ stty -F /dev/ttyS1 4800

The echo command can be used to transmit basic data out of the TX pin. For example:

$ echo "Hello!" > /dev/ttyS1

The cat command can be used to read any data from the RX pin. For example:

$ cat < /dev/ttyS1

The /dev/ttyS5 device has been preconfigured to be used for SLIP connections. Please refer to the SLIP instructions for more information.

User Data Partitions

The Beaglebone Black has two user data partitions available, one on each storage device.

eMMC

The user partition on the eMMC device is used as the primary user data storage area. All system-related /home/ paths will reside here.

/home/system/usr/bin

This directory is included in the system’s PATH, so applications placed here can be called directly from anywhere, without needing to know the full file path.

/home/system/etc/init.d

All user-application initialization scripts live under this directory. The naming format is ‘S{run-level}{application}’.

microSD

/home/microsd

This directory points to a partition on the microSD device included with the base Beaglebone Black board