5.7. Power

The 3pi+ 32U4 control board includes battery terminal connections that provide access to power from the 3pi+ chassis’s four-AAA battery compartment. We recommend using rechargeable AAA 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 reverse protection circuit and then a power switching circuit controlled by the on-board pushbutton. The output of the power switching circuit is designated VSW.

VSW provides power to the on-board motor voltage regulator, and that regulator’s output (VM) powers 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 analog pin 1 (PF6). The divider outputs a voltage that is equal to one third of the battery voltage, which will be safely below the ATmega32U4’s maximum analog input voltage of 5 V. The readBatteryMillivolts() function in the Pololu3piPlus32U4 library can be used to determine the battery voltage from this reading.

Power switch circuit

The 3pi+ 32U4 control board uses the patented latching circuit from the Pololu pushbutton power switch, which provides a solid-state power switch for your robot controlled with the on-board pushbutton. By default, this pushbutton can be used to toggle power: one push turns on power and another turns it off. Alternatively, a separate pushbutton can be connected to the PWRA and PWRB pins and used instead. Multiple pushbuttons can be wired in parallel for multiple control points, and each of the parallel pushbuttons, including the one on the board itself, will be able to turn the switch on or off. The latching circuit performs some button debouncing, but pushbuttons with excessive bouncing (several ms) might not function well with it.

Alternatively, to disable the pushbutton, you can cut the button jumper labeled Btn Jmp; this allows you to connect a slide or toggle switch to control the board’s power instead. The switch should be wired such that it connects the SW pin to GND when it is closed, and a set of three through-holes along the left edge of the board provide a convenient place to do so (the third hole is not connected to anything but helps accommodate 3-pin switches).

The power switch circuit also offers several alternate pushbutton connection options that result in push-on-only or push-off-only operation, and additional inputs enable further power control options like allowing your robot to turn off its own power. These advanced control options are available through the button connection pins and four control inputs:

PIN	Description
PWRA	Connect through momentary switch to pin “PWRB” for standard push-on/push-off operation. Connect through momentary switch to ground for on-only operation.
PWRB	Connect through momentary switch to pin “PWRA” for standard push-on/push-off operation.
ON	A high pulse (> 1 V) on this pin turns on the switch circuit. This pin only functions when pushbutton operation is enabled (i.e. the button jumper has not been cut).
OFF	A high pulse (> 1 V) on this pin turns off the switch circuit (e.g. allowing a powered device to shut off its own power). This pin only functions when pushbutton operation is enabled.
CTRL	With pushbutton operation enabled, this pin directly determines the state of the switch circuit. A high pulse (> 1 V) on this pin turns on the switch; a low pulse (e.g. driving the pin low with a microcontroller output line or pushing a button connected from this pin to ground) turns the switch off. Leave this pin disconnected or floating when not trying to set the switch state. Note that this pin should not be driven high at the same time the “OFF” pin is driven high.
SW	With pushbutton operation disabled (button jumper cut), this pin controls the state of the switch circuit: driving it low turns the switch on, while letting it float turns the switch off. Connect through slide or toggle switch to ground for on/off operation. Leave this pin disconnected or floating for proper pushbutton operation. We recommend only ever driving this pin low or leaving it floating; this pin should never be driven high while the slide switch is in the “On” position.

Motor voltage regulator

VSW supplies power to a regulator that provides 8 V for the 3pi+’s DRV8838 motor drivers. This regulated motor voltage helps keep the performance of the motors consistent as the batteries discharge and their voltage drops. However, the condition of the batteries can still have an impact on motor performance in some situations; see Section 5.3 for more details.

The 3pi+’s motor voltage regulator is designed to cut out at a higher voltage than its 5 V logic voltage regulator. This way, if there is a significant transient drop in battery voltage due to the motors drawing a large amount of current, the motor voltage regulator will turn off and ensure that the battery voltage does not continue to fall. (Otherwise, the battery voltage could drop low enough to disable the logic regulator and cause the ATmega32U4 to reset.)

5 V and 3.3 V regulators

VSW also supplies power to a 5 V regulator, whose output is designated R5V. This output is not directly user-accessible, but when available, it is the preferred source for the logic power selection circuit described below. The rest of the regulator’s achievable output current, which depends on input voltage and ambient conditions, can be used to power other devices via the selection circuit’s output (5V). Under typical conditions, roughly 0.7 A of current is available from the 5 V regulator.

The 3pi+ 32U4 control board also contains a 3.3 V LDO that draws its power from the output of the logic power selection circuit. The output of the 3.3 V regulator is designated 3V3 and is used to supply the on-board inertial sensors and level shifters.

Logic power selection

The 3pi+ 32U4 control board’s power selection circuit uses the TPS2113A power multiplexer from Texas Instruments to choose whether its 5 V supply (designated 5V) is sourced from USB or the batteries via the 5 V regulator, enabling the control board to safely and seamlessly transition between them. The TPS2113A is configured to select regulated battery power (R5V) unless the regulator output falls below about 4.5 V. If this happens, it will select the higher of the two sources, which will typically be the USB 5 V bus voltage if the control board is connected to USB.

Consequently, when 3pi+ 32U4 is connected to a computer via USB, it will receive 5 V logic power 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.

The currently selected source is indicated by the STAT pin; this pin is an open-drain output that is low if the external power source is selected and high-impedance if the USB supply is selected. The current limit of the TPS2113A is set to about 1.9 A nominally. For more information about the power multiplexer, see the TPS2113A datasheet (1k redirect).

The 5 V output of the selection circuit is used to supply the control board’s ATmega32U4 microcontroller, logic power for the DRV8838 motor drivers, the reflectance sensors and bump sensors, and the encoders.

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.
• VM is the output of the on-board 8 V motor voltage regulator.
• R5V is the output of the on-board 5 V logic voltage regulator. (This output is not user-accessible.)
• 5V is the output of the TPS2113A power multiplexer circuit which is connected to R5V by default, but switches to 5 V USB power if R5V is too low.
• 3V3 is the output of the 3.3 V LDO regulator.

See Section 5.8 for a diagram of the board’s power access points.