Data Sheet

example, digitalRead(15) and digitalRead(SCK) are equivalent. If you have an SMT-only version of the A-Star,

it is possible to solder a shrouded box header [http://www.pololu.com/product/854] to this location; the outlines on the

board indicate the correct connector orientation.

Power

The A-Star 32U4 Prime can either be powered directly from the USB 5 V supply or from an external voltage source,

which is regulated to 5 V by its onboard switching regulator. The slide switch on A* controls whether the external

source is connected to the input of the regulator, providing a convenient way to switch off external power to the A-

Star without unplugging any connections. The adjacent set of three pins provides a place to connect your own power

switch: to enable external power, connect the middle pin to ground (accessible through the upper pin).

When the A-Star is connected to a computer via USB, it will receive 5V power even when the power

switch is off. This can be useful if you want to upload or test a program without drawing external

power (to avoid draining a battery pack, for example).

The board’s power selection circuit uses the TPS2113A power multiplexer [http://www.pololu.com/product/2596] from

Texas Instruments to choose whether its 5 V supply is sourced from USB or an external supply via the regulator,

allowing both sources to be connected at the same time and enabling the A-Star to safely and seamlessly transition

between them. The TPS2113A is configured to select external power 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 A* is connected to USB. The currently selected source is indicated by the STAT pin in the middle of the board;

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. For more information about the power

multiplexer, see the TPS2113A datasheet [http://www.pololu.com/file/download/tps2113a.pdf?file_id=0J771] (1MB pdf).

In some situations, it might be undesirable for the A-Star 32U4 Prime to draw power from an external source when

it is connected to USB, even if the power switch is left on. If this is the case, the regulator can be disabled by driving

the regulator shutdown pin, SHDN, high; this shuts down the regulator and causes the power mux to fall back to

USB power. For example, this could allow a battery-powered system to automatically turn off the regulator while it

is connected to a computer.

The input voltage range of the regulator depends on the particular version of the A-Star 32U4 Prime:

• LV: 2.7 V to 11.8 V (see Section 5.3 for regulator details)

• SV: 5 V to 36 V (see Section 5.4 for regulator details)

Reverse-protected and switched power inputs: The connection points labeled “Power In” are power inputs with

reverse-voltage protection that are controlled by the power switch. These are the recommended pins to use when

connecting an external power supply because they allow the A-Star’s reverse-voltage protection circuit to help

prevent it from being damaged by accidentally-reversed power connections, and they enable the A-Star’s power

switch to disconnect the supply.

A number of different connectors can be soldered to the board and used to provide power, including a DC barrel jack

[http://www.pololu.com/product/1139] or a 3.5 mm screw terminal block [http://www.pololu.com/product/2444] (the assembled

versions of the board come with one or both of these populated).

5. A-Star 32U4 Prime Page 25 of 47