Cluster Management Bus

Turing Pi cluster management bus configuration, security and internal devices

Configure I2C and devices

Add dtoverlay to /boot/config.txt and save


Enable I2C interface in raspi-config -> Interfacing Options -> I2C and reboot

Check everything is fine with I2C and RTC

dmesg | grep i2c
[    4.414436] i2c /dev entries driver  # ok

ls /dev/*i2c*
/dev/i2c-1  # ok

dmesg | grep rtc
[    6.489206] rtc-ds1307 1-006f: registered as rtc0

ls /dev/*rtc*
/dev/rtc  /dev/rtc0

I2C tools

Install i2c-tools

sudo apt-get install i2c-tools

Now check our I2C devices on the CMB

sudo i2cdetect -y 1
     0  1  2  3  4  5  6  7  8  9  a  b  c  d  e  f
00:          -- 04 -- -- -- -- -- -- -- -- -- -- --
10: -- -- -- -- -- -- -- -- -- -- -- -- -- -- -- --
20: -- -- -- -- -- -- -- -- -- -- -- -- -- -- -- --
30: -- -- -- -- -- -- -- -- -- -- -- -- -- -- -- --
40: -- -- -- -- -- -- -- -- -- -- -- -- -- -- -- --
50: -- -- -- -- -- -- -- 57 -- -- -- -- -- -- -- --
60: -- -- -- -- -- -- -- -- -- -- -- -- -- -- -- 6f
70: -- -- -- -- -- -- -- --

You can see three I2C devices Ethernet Switch (0x04h), I2C expander (0x57h) and RTCC (0x6Fh).

The i2cget command is used to read a byte from a specified register on the I2C device. The format for this command is as follows

i2cget [-f] [-y] 1 <DEVICE_ADDRESS> <ADDRESS> [MODE]

[-f] [-y] Options:

  • -f force access to the device even if the device is still busy. However, be careful. This can cause the i2cget command to return an invalid value. We recommend avoiding this option.
  • -y disable interactive mode. Using this command will skip the prompt for confirmation from the i2cget command. By default, it will wait for the confirmation from the user before interacting with the I2C device.
  • DEVICE_ADDRESS is an I2C bus address (e.g 0x04h)
  • ADDRESS is the address on the slave from which to read data (eg. 0x00h)

The optional MODE can be one of the following:

  • b – read/write a byte of data, this is the default if left blank
  • w – read/write a word of data (two bytes)
  • c – write byte/read byte transaction

Example of reading from register 0xF2h where factory default value is 0xFFh

sudo i2cget -y 1 0x57 0xf2

Use the i2cset command to write data to an I2C device. The format of this command as follows

i2cset [-m mask] [-f] [-y] 1 <DEVICE_ADDRESS> <ADDRESS> <VALUE> [MODE]


  • -f, -y, DEVICE_ADDRESS, ADDRESS as for i2cget
  • -m mask the bit mask parameter, if specified, describes which bits of value will be actually written to data-address. Bits set to 1 in the mask are taken from VALUE, while bits set to 0 will be read from ADDRESS and thus preserved by the operation

Example of writing value 0x77h to the first byte of user space SRAM (starts from 0x00h) and then reading

sudo i2cset -y 1 0x57 0x00 0x77
sudo i2cget -y 1 0x57 0x00


As a security precaution, system devices are not exposed by default inside Docker containers. You can expose specific devices to your container using the --device option to docker run, as in

docker run --device /dev/i2c-1 app-image

or remove all restrictions with the --privileged flag

docker run --privileged app-image

User space EEPROM

I2C expander and RTCC each have 64 bytes of general purpose user memory, so combined you have 128 bytes of EEPROM. On device 0x57h user space address from 0x00h to 0x3Fh and on 0x6Fh from 0x20h to 0x5Fh.

Ethernet Switch

The onboard RTL8370 integrate all the functions of a high-speed switch system; including SRAM for packet buffering, non-blocking switch fabric, and internal register management into a single CMOS device.

Device Address: 0x04h

Register Register Description Default Value
0x00h Control Register 0x1140h
0x01h Status Register 0x7949h
0x02h PHY Identifier 1 0x001Ch
0x03h PHY Identifier 2 0xC980h
0x04h Auto-Negotiation Advertisement Register 0x0DE1h
0x05h Auto-Negotiation Link Partner Ability Register 0x0000h
0x06h Auto-Negotiation Expansion Register 0x0004h
0x07h Auto-Negotiation Page Transmit Register 0x2001h
0x08h Auto-Negotiation Link Partner Next Page Register 0x0000h
0x09h 1000Base-T Control Register 0x0E00h
0x0Ah 1000Base-T Status Register 0x0000h

Network Speed

# 0x2040: Bitmask
# 0x2000: 1000Mbps
# 0x0040: 100Mbps
# 0x0000: 10Mbps

# Set network speed to 1000Mbps
sudo i2cset -m 0x2040 -y 0x04 0x00 0x2000 w

Power Up/Down

# 0x0800: Bitmask
# 0x0800: Power down
# 0x0000: Normal operation

# Power down
sudo i2cset -m 0x0800 -y 0x04 0x00 0x0800 w

# Power up
sudo i2cset -m 0x0800 -y 0x04 0x00 0x0000 w

I2C expander

I2C expander offers users a digitally programmable alternative to hardware jumpers and mechanical switches that are being used to control power on compute nodes.

Device Address: 0x57h

Register Register Description Default Value
0x00h to 3Fh 64 bytes of general-purpose user EEPROM 0x00h
0xF2h Control Register 0xFFh
0xF8h Status Register
0xFAh to 0xFFh 6 bytes of general-purpose user SRAM

Power Management

On/off compute module through register 0xF2h

# Bits  : 76543210
# Slots : 5674321x

# Power off worker nodes 2,3,4,5,6,7
sudo i2cset -m 0xfc -y 1 0x57 0xf2 0x00

# Power on worker nodes
sudo i2cset -m 0xfc -y 1 0x57 0xf2 0xff

Read on/off status

sudo i2cget -y 1 0x57 0xf2

Compute Module status

Read compute module status through register 0xF8h

  • 1 = Installed in slot AND switched on
  • 0 = Not installed in slot OR switched off
# Bits  : 76543210
# Slots : 5674321x

sudo i2cget -y 1 0x57 0xf8
0x0e  # register 0xf2 = 0xff, all slots are power up but installed only three

Real-Time Clock/Calendar

Real-Time Clock/Calendar (RTCC) tracks time using internal counters for hours, minutes, seconds, days, months, years, and day of week. Alarms can be configured on all counters up to and including months.

SRAM and timekeeping circuitry are powered from the back-up supply when main power is lost, allowing the device to maintain accurate time and the SRAM contents. The times when the device switches over to the back-up supply and when primary power returns are both logged by the power-fail time-stamp.

Device Address: 0x6Fh

Register Register Description Default Value
0x00h to 0x06h Time & Date
0x07h to 0x09h Configuration and Trimming
0x0Ah to 0x10h Alarm 0
0x11h to 0x17h Alarm 1
0x18h to 0x1Fh Power-Fail/Power-Up Time-Stamps
0x20h to 0x5Fh 64 bytes of general-purpose user SRAM

Time & Date

Show time from hardware RTC

sudo hwclock --show
2019-10-26 20:21:03.005326+01:00

External I2C port


|  1  |  2  |  3  |  4  |
| GND | VCC | SCL | SDA |