Powering the Meadow F7 Micro
NOTE: This document is a work-in-progress
The Meadow F7 Micro development board is designed such that it can be powered by supplying the appropriate voltage to either the
USB connector, or the
3.3V power rails.
Battery Charging Circuit
Supplying voltage via either the
USB connector or
5V rail is effectively the same; it will output
3.3V on the
3V3 power rail, and enable the battery charging circuit, which will charge any standard
3.7V LiPo/LiIon battery.
To use a battery, you can either hook it to the JST-PH battery connector, or wire it directly to the
GND pins on the header. Both Adafruit and SparkFun have a good selection of LiPo/LiIon batteries that will work.
The battery charging circuit will supply a battery with up to
200mA of current (at up to
If you supply voltage only to the
3.3V power rail, the board will operate as expected, but the battery charging circuit will not be enabled and the
5V power rail will only be at
When powered by either
USB connector or the
5V rail, the amount of combined
3.3V current available onboard (including the
3V3 power rail, MCUs, and IO peripherals) is limited, and is known as the power budget.
When powered via the
USB connector, the budget is limited only by the
3.3V power regulator, which is good for
800mA of output. However, on revision
1.c of the board, when power input comes from the
5V rail, it’s limited by a diode that has a
500mA maximum power throughput. Therefore, the onboard
3.3V power budget is as follows:
However, in practice, a typical USB port is only rated to deliver
500mA of power. Some USB charging adapters will deliver much more than this, however.
You can manually upgrade a
v1.c board to the
v1.d version that can handle
800mA of current via the
5V rail by swapping out the following diode with a
Nominal Power Usage
You should generally reserve up to
400mA of the power budget for onboard functionality including both MCUs, RAM, and flash. This leaves, at a minimum,
100mA for peripherals, including anything drawing power from the IOs on the board.
In addition to the overall power budget, the amount of power being delivered to peripherals via the IO pins must be considered. There is both an overall maximum that the MCU can drive, as well as a per pin maximum.
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Battery Charger Usage
The battery charging circuit is hooked directly to the
USB power rail. When powering Meadow via USB and charging a battery, the battery charging circut will pull up to
200mA. This means you should subtract
200mA from your USB power budget. For example, if you’re powering Meadow with a USB power supply that can deliver
5V, you should subtract
200mA from the USB power budget. The Meadow board will have
The battery charging circut is also connected to the
5V rail via a diode. You can charge a battery when powering Meadow via the
5V rail. However, this will cause up to
200mA to flow through the
5V rail reducing your
5V power budget. This is important because Meadow has a current limit on the
1.c boards and
1.d boards. You’ll only be able to safely use and additional
1.c boards and
1.d boards regardless of the avalaible current of the external power supply.
Solar + Battery Power
The board can be adequately powered by a solar panel that outputs a minimum of
6V, but it’s best to pair a solar panel with a battery in order to provide backup power when solar power is not available.
Real-Time Clock (RTC)
The STM32F7 is equipped with a real-time clock (RTC), which, when set, will retain the system time as long as the the board has power. If the board will have intermittent power, as when powered by a solar panel, having a battery hooked up to the board will ensure the RTC will not lose the time.
The reset pin is used to do an MCU system reset. If you pull this pin
GND) momentarily, the MCU will reboot, clearing out it’s volatile registers. The
RST button on the board does exactly this.
Note that as long as the board still has power, the RTC will continue to keep time without resetting.
3.3V Power Rail (
3.3V power rail is exposed via the
3V3 header pin.
Analog Reference (
The analog reference (
AREF) pin provides a reference voltage for the Analog to Digital Converter (ADC) to compare against. Typically, this should be supplied with
3.3V, so as a convenience, the
AREF pin is actually connected to the
3.3V rail via
0Ω resistor that is located next to the
D08 pin, just below the main MCU:
If you need to provide a different analog reference voltage, make sure to remove that resistor before hooking
AREF to your voltage reference.
The ground rail (
GND) provides a common
0V voltage sink and reference. It’s important to make sure all connected peripherals are tied into this ground, otherwise they may not operate correctly.
The battery pin (
BAT) provides an alternative positive terminal connection for an external battery or power source to the built-in JST-PH battery connector. If using the
BAT pin, make sure to tie the negative terminal of the battery to the board ground (
The enable pin (
EN) serves as a sort of power switch for the board. By default, it is pulled
3.3V), but when pulled
0V), it will disconnect most of the power to the MCU. However, it will not disconnect backup power to the MCU, so that it will keep the RTC going and keeping time.
To create a power switch for the development board, hook the
EN pin to a switch that sinks to
0V) when in the
OFF position, as shown in the following schematic:
5V Power Rail (
5V power rail is exposed via the
5V header pin.