Hardware Monitoring

A Raspberry Pi runs safely without any cooling solutions. When it's about to get too hot, it throttles back the CPU frequency and voltage, in order to protect the hardware. If that doesn't help, the Pi will halt.

If course, you don't want your Pi to reduce the CPU speed, as it severely impacts performance. One of the features of Homebridge RPi is to monitor the Pi hardware, so you can check if additional cooling measures are needed. Homebridge RPi uses the vcgencmd command-line tool, to get the Pi hardware status. It's using following commands:

Command	Description
vcgencmd measure_temp	Report the CPU temperature.
vcgencmd measure_clock arm	Report the CPU frequency.
vcgencmd measure_volts	Report the CPU voltage.
vcgencmd get_throttled	Report the system status.

CPU Temperature

The measure_temp command returns the CPU temperature in degrees Celsius, with a 0.1°C resolution:

vcgencmd measure_temp

temp=55.8'C

You want to make sure the CPU temperature stays below 80°C (or 60°C for the Pi 3B+). For more info, see Frequency Management and Thermal Control.

CPU Frequency

The measure_clock arm returns the CPU frequency in Hz:

vcgencmd measure_clock arm

frequency(45)=1400000000

This is on a Pi 3B+ running at 1400 MHz.

I haven't been able to find an overview of all Pi models with their maximum CPU speed, but I have found the following:

Model	CPU Frequency
Pi 3B	1200 MHz
Pi 3B+	1400 MHz (1200 MHz above 60°C)
Pi 4B	1500 MHz (1800 MHz for rev 1.4 on bullseye)

Note that the Pi also reduces the CPU speed to 600 MHz when there's little to no system load.

CPU Voltage

The measure_volts reports the CPU voltage in V, with a resolution of 0.1 mV:

vcgencmd measure_volts

volt=1.3688V

I'm not sure what the voltage is supposed to be, but I typically find the following values:

Model	CPU Voltage
Pi 3B	1.2000 V
Pi 3B+	1.3688 V
Pi 4B	0.8375 V

System Status

The get_throttled returns the system status as a bitmap:

vcgencmd get_throttled

throttled=0x80000

The bitmap contains two words. The low-end word indicates the current conditions. The corresponding bits are cleared as soon as the condition no longer occurs. The high-end word indicates whether a condition has occurred. The corresponding bits are cleared only on reboot.

So far, I've uncovered the following status bits:

Bit	Value	Meaning
0	0x1	Under-voltage active.
1	0x2	ARM frequency capped.
2	0x4	Currently throttled.
3	0x8	Soft temperature limit active (Pi 3B+).
16	0x10000	Under-voltage has occurred.
17	0x20000	ARM frequency capped has occurred.
18	0x40000	Throttling has occurred.
19	0x80000	Soft temperature limit has occurred (Pi 3B+).