6.8. Power

The Zumo 2040 main board includes battery terminal connections that provide access to power from the Zumo chassis’s four-AA battery compartment. We recommend using rechargeable AA NiMH cells, which results in a nominal voltage of 4.8 V (1.2 V per cell). You can also use alkaline cells, which would nominally give you 6 V.

The negative battery voltage is connected to GND. The positive battery voltage is designated VBAT. VBAT feeds into a circuit that provides reverse protection and power switching controlled by the on-board power switch. The output of the power switching circuit is designated VSW.

VSW provides power to the motors through the DRV8838 motor drivers, so the motors can only operate if the batteries are installed and the power switch circuit is on.

The reverse protected and switched battery voltage on VSW can be monitored through a voltage divider that is connected to GP26. The divider outputs a voltage that is equal to 1/11 of the battery voltage. The Zumo 2040 Robot Libraries and Example Code provide functions that can be used to determine the battery voltage from this reading. GP26 is also used to control the line sensor infrared emitters (see Section 6.5); the low divider ratio ensures that the emitters are effectively off even while the pin is being used as an input to measure the battery voltage.

3.3 V logic power circuit

The Zumo 2040’s logic power can come from either its batteries or its USB connection. When VSW is available, it powers the logic boost regulator, whose output is designated VBST and is normally 8.3 V. Both VBST and the 5 V USB bus voltage VBUS are connected through diodes to a supply called VB/VU (in other words, VB/VU is the result of ORing VBST and VBUS together).

VB/VU provides power for the logic buck regulator, which converts it into the 3.3 V logic voltage (designated 3V3) that supplies the control board’s logic circuitry, including the RP2040, sensors, RGB LEDs, and buzzer. The rest of the 3.3 V regulator’s achievable output current can be used to power other devices; under typical conditions, up to 1.5 A of current is available from 3V3 when the Zumo is running on battery power.

Since VBST (when present) is normally higher than VBUS, the Zumo will prefer to draw logic power from its batteries over USB when both are present, but it will still receive logic power from USB even when the power switch circuit is off. This can be useful if you want to upload or test a program without drawing power from the batteries and without operating the motors. It is safe to have USB connected and battery power switched on at the same time.

Topology of the Zumo 2040 Main Board power circuits.

Power distribution

• VBAT is connected to the battery contact labeled BAT+ and provides a direct connection to the battery supply.
• VSW is the battery voltage after reverse-voltage protection and the power switch circuit; this powers the motors.
• VBST is the output of the on-board 8.3 V logic boost regulator.
• VB/VU is the input for the 3.3 V logic buck regulator; it normally comes from VBST through a diode, but it can come from 5 V USB power (VBUS) if VBST is not present or too low.
• 3V3 is the output of the 3.3 V logic buck regulator.

See Section 6.9 for a diagram of the board’s power access points.