5.a. Connecting Power
The two main ways of powering the Wixel are the USB port and the VIN pin. The schematic of the Wixel’s power system is shown below:
|Wixel power system schematic diagram.|
VIN Power Input
The Wixel can be powered from VIN if you connect a 2.7–6.5 V power supply (e.g. battery or regulator) to the GND and VIN pins. The negative terminal should be connected to GND. The positive terminal should be connected to VIN. It is okay to have both USB and VIN connected at the same time.
|The Wixel can be powered from an external power source connected to VIN.|
USB Power Input
The Wixel can be powered from USB if you connect a USB cable and leave VIN disconnected. The Wixel will draw its power from USB if VIN is disconnected or it is below about 4 V.
|The Wixel can be powered from USB.|
3V3 Power Output
The Wixel’s 3V3 pin gives access to the output of the Wixel’s 3.3 V regulator. If the Wixel’s power supply drops below approximately 3.5 V, the 3V3 output will be less than 3.3 V. Normally this output can provide up to 150 mA, but if the Wixel’s power supply is above 5 V then it is limited to 100 mA. You can use 3V3 to power your own 3.3 V devices.
VALT Power Output
VALT provides access to the input pin of the Wixel’s 3.3 V regulator, which is connected through a diode to VIN or to the USB bus voltage, depending on which power source is connected. You can use VALT to power your own circuits as long as you do not draw more than about 500 mA.
For example, if you leave VIN disconnected, VALT can power 5V devices.
Low Power Considerations
The CC2511F32 is capable of a sleep mode (PM2) where the chip draws less than 1 μA and is still capable of waking itself up. Without modifying the Wixel’s hardware, it is possible to power it from VIN and get the current consumption down to around 100 μA. Most of that current is consumed by the Wixel’s 3.3 V regulator. To get rid of it, you will need to sever the output pin of the regulator and power the Wixel directly from the 3V3 pin with a 2.0–3.6 V power supply. For details on how to do this, please contact Pololu.
Please note that currently none of the Wixel apps support low power modes, so the Wixel will draw approximately 30 mA at all times. To make your Wixel operate with low power you would have to write your own app or modify one of the existing apps. You will need to make sure that all of the I/O lines are either outputs or get pulled high or low: an input line at an intermediate voltage can consume several microamps of extra current.