Example Baseboard I2C Power Monitor¶

Objective¶

This example demonstrates how to get data from the Baseboard I2C Power Monitor.

The power is monitored by an Analog LTC2945 part. There is a driver in Linux that makes the information about the monitored current, voltage, and power available to applications.

Note: An early version of the LTC2945 driver assumed a resistor value different from the actual resistor value used on the development kit. This difference resulted in values for the current and power which were 10 times larger than the actual values. A change to the driver was made and committed to Linux to correct this. The formulas shown below assume the newer driver is being used.

Prerequisites¶

• A serial port connected to Development Kit.

Steps¶

• The values from the LTC2945 part are made available in files located at /sys/class/hwmon/hwmon1/*
• There are 4 values available:

  Value	Description
  curr1	Current
  in1	Input voltage
  in2	Internal voltage
  power1	Power

• For each value, there are several parameters:

  Parameter	Description
  highest	highest value seen since power-on
  input	The value at the present time
  lowest	The lowest value seen since power-on
  max	The high-threshold above which should raise a max_alarm
  max_alarm	Set to 1 if the input value exceeds the max
  min	The low-threshold which should raise a min_alarm if lower values are seen
  min_alarm	Set to 1 if the input value is below the min
  reset_history	Write-only. Write a 1 to this value to clear the highest and lowest values and to clear the alarms.

• The values displayed are just integers. To convert them to real values, use the following:

  Value	Formula
  Current	Current in amps = value_shown / 1,000
  Voltage	Voltage in volts = value_shown / 1,000
  Power	Power in watts = value_shown / 1,000,000

• Example of all input values:
  

  root@mitysom-am57x:~# cd /sys/class/hwmon/hwmon1
  root@mitysom-am57x:/sys/class/hwmon/hwmon1# tail *input
  ==> curr1_input <==
  763
  
  ==> in1_input <==
  4975
  
  ==> in2_input <==
  1013
  
  ==> power1_input <==
  3793438
  root@mitysom-am57x:/sys/class/hwmon/hwmon1#
  

• Using the formulas listed above, this shows the present values of:

  Value Name	Value Shown	Formula	Present Value
  curr1_input	10600	current = value_shown / 1,000	0.763 amps
  in1_input	4975	voltage = value_shown / 1,000	4.975 volts
  in2_input	1013	voltage = value_shown / 1,000	1.013 volts
  power1_input	52735000	power = value_shown / 1,000,000	3.793 watts

• * Example of all curr1 values:
  

  root@mitysom-am57x:/sys/class/hwmon/hwmon1# root@mitysom-am57x:/sys/class/hwmon/hwmon1# tail curr1*
  tail: can't open 'curr1_reset_history': Permission denied
  ==> curr1_highest <==
  1393
  
  ==> curr1_input <==
  773
  
  ==> curr1_lowest <==
  520
  
  ==> curr1_max <==
  10238
  
  ==> curr1_max_alarm <==
  0
  
  ==> curr1_min <==
  0
  
  ==> curr1_min_alarm <==
  0
  root@mitysom-am57x:/sys/class/hwmon/hwmon1#
  
  

If the curr1_max value were to be changed to a value of 10000, then the curr1_input value would be greater than the curr1_max and the max_alarm value would be changed to 1. If the curr1_max value is changed back to 102375, then the max_alarm is not cleared until a 1 is written to curr1_reset_history. Writing a 1 to curr1_reset_history will also reset the curr1_lowest and curr1_highest values and they will initially be set to the curr1_input value and will then record the lowest and highest values.

The *_min_alarm and *_max_alarm files could be monitored in Linux if your application wants to monitor these conditions. Note that the alarm conditions only reset when a 1 is written to the appropriate *_reset_history file and will only stay clear until the alarm condition happens again.

Conclusion¶

This completes the example for the Baseboard I2C Power Monitor. The location of the driver files has been specified, the meaning of the different files has been described, and the methods for converting from the values shown to amps, volts, and watts have been provided and demonstrated.