How does the BB-400 manage power using the Power Management Unit (PMU)?
The Power Management System
The BB-400 has a sophisticated power management system, designed to perform a number of different functions primarily to prevent the Raspberry Pi Compute Module from being corrupted in the event of an external power failure. Including: a Power Management Unit (PMU), a Dual Power Supply (DPS) and a secondary Uninterruptible Power Supply (UPS), the BB-400 can quickly supply internal back-up power for a limited time after external power failure in order to allow a controlled shut-down of the device.
Power Management Unit (PMU)
The Power Management Unit, or PMU, is the heart of the internal power control for the BB-400. It coordinates the function of the primary and back-up (UPS) supplies depending on the status of external power and other parameters. In case of an external power failure, it re-routes power to the Raspberry Pi Compute Module in order to keep the system functioning long enough to initiate a controlled shut-down. Ordinarily the Raspberry Pi Compute Module cannot perform this function on its own, the PMU allows it to stay operational for an additional 10 seconds, or more (variable depending on processor power usage) to properly shut-down.
Dual Power Supply
The Dual Power Supply, or DPS, is a redundancy feature implemented in the BB-400 that allows more than one external power source to provide power for the BB-400 if desired. This supplements the internal back-up power supply (UPS) feature, providing another layer of flexibility and protection for the Raspberry Pi Compute Module. In the event that either one of the external power sources fails, the other power input can continue powering the BB-400 without any further consequence. As previously mentioned, the Raspberry Pi Compute Module can suffer data loss when its power supply is suddenly cut. The dual power supply input connector, seen on the diagram below (and also referenced in the BB-400 manual under ‘Hardware’), allows the user to wire two independent external power sources to the same terminal block. The BB-400 has a 5-pin terminal block (black connector) with 2 positive DC inputs (labelled +VA and +VB), 2 negative ground returns (both labelled -V) and a functional ground connection (labelled Func GND) which can allow the unit to be connected to earth. The external powering arrangements can thus be configured in any of the following ways.
- Pin 1 (-V), Pin 2 (+VA)
- Pin 1 (-V), Pin 2 (+VA), Pin 5 (Func GND)
- Pin 4 (-V), Pin 3 (+VB)
- Pin 4 (-V), Pin 3 (+VB), Pin 5 (Func GND)
- Pins 1&4 (-V), Pins 2&3 (+VA and +VB to separate power sources)
Pins 1&4 (-V), Pins 2&3 (+VA and +VB to separate power sources), Pin 5 (Func GND)
Note: the 'Func GND' wire on Pin 5 does not have to be connected, although it is recommended for safety.
Uninterruptible Power Supply
The Uninterruptible Power Supply, or UPS, is a hardware feature that serves to provide an internal source of limited back-up power in the event of complete external power failure. The main source of power are the two large supercapacitors (large blue capacitors that sit side-by-side on the PCB). These begin charging on initial power-up of the BB-400 (indicated by the ‘Precharge’ LED on the front panel) and take a short time to reach full charge, at which point the PMU permits the Raspberry Pi Compute Module to boot up. The PMU continuously monitors the charge in the supercapacitors and maintains their charge level while the device is operational. If external power fails completely the PMU will switch to the UPS and uses the energy stored in the supercapacitors to generate regulated internal power for the Raspberry Pi Compute Module and PMU combined. Without the UPS, the Raspberry Pi Compute Module could become corrupted when external power fails.
Do not touch anywhere near or around the supercapacitors if the device has been powered on within the last 24 hours. Accidentally shorting together the pins for both supercapacitors with either skin or jewellery can cause a rapid discharge of the supercapacitors. While the discharge voltage isn’t large enough to kill, it will cause significant burns to the contact area.