Data Sheet

Warning: You must never connect different power sources to multiple Power In locations at the same

time, as doing so will create a short between the supplies.

VIN power output (or alternative input): When power is supplied through the Power In pins, the VIN pin can be

used as an output to supply reverse-protected and switched power to other devices. Alternatively, the external supply

can be connected directly between VIN and GND, bypassing the reverse-voltage protection and power switch.

5V power output: This pin provides access to the board’s 5 V supply, which comes from either the USB 5 V bus

voltage or the onboard switching regulator, depending on which power sources are connected and enabled. Note that

some of the available current on the 5 V line is used by the board itself (typically at least 30 mA, but potentially much

more if peripherals like the buzzer are active) or used to provide current for the GPIO pins or 3.3 V regulator (see

below).

3V3 power output: This pin gives access to the output of the onboard 3.3 V linear regulator. The microSD card, if

present, draws power from the 3.3 V line; the remainder (up to a few hundred milliamps) is available for powering

external circuits or devices.

When the A-Star 32U4 Prime is being powered through Power In, the sum of the 5V output current, 3V3 output

current, GPIO output current, and current used by the board itself should not exceed the maximum current that the

switching regulator can provide.

In a battery-powered application, it might be useful for the A-Star to monitor the battery’s voltage level. The

BATLEV pin provides access to a voltage divider that outputs a fraction of the VIN voltage (one-third on the

LV, one-eighth on the SV), and this voltage can be read by connecting it to the adjacent analog pin 1 (A1)

(or another analog input). The readBatteryMillivoltsLV() and readBatteryMillivoltsSV() functions in the

AStar32U4Prime library can be used to determine the battery voltage from this reading.

Battery level jumper on the A-Star

32U4 Prime.

5. A-Star 32U4 Prime Page 26 of 47