C. connecting buttons and starting the bootloader, Section 5.c, Section – Pololu Wixel User Manual
Page 25
Making serial connections between a
Wixel and a 5V microcontroller.
Making serial connections between a
Wixel and a 3.3V microcontroller.
To connect your microcontroller to a Wixel for serial communication, make these connections:
• GND: Connect the ground (also known as GND or VSS) of your microcontroller to one of the GND pins on
the Wixel. This connection is required.
• TX: If you want the microcontroller to be able to receive serial bytes from the Wixel, connect the Wixel’s TX
line to the microcontroller’s RX line.
• RX: If you want your microcontroller to be able to send serial bytes to the Wixel, connect the
microcontroller’s TX line to the Wixel’s RX line. The Wixel’s RX line is not 5 V tolerant. If your microcontroller
is running at 5 V (or any voltage significantly above 3.3 V) then you will need to add extra components to ensure
that your microcontroller never drives the Wixel’s RX line higher than 3.3 V. A simple voltage divider consisting
of 2 resistors as shown in the diagram above will suffice. The Wixel’s RX line has an internal 20 kΩ pull-up
resistor.
• RST: If you want the microcontroller to be able to reset the Wixel, then connect the Wixel’s RST line to
any free general-purpose I/O (GPIO) line on the microcontroller. The microcontroller can drive this line low to
reset the Wixel and then stop driving the line to release the Wixel from reset. The Wixel’s RST line is not 5 V
tolerant. If your microcontroller is running at 5 V (or any voltage significantly above 3.3 V) then you must avoid
driving the Wixel’s RST line high. If that is not possible, then you could put a diode between the Wixel’s RST
line and the microcontroller’s GPIO to prevent current from flowing in the wrong direction (from the GPIO).
The connection to RST is optional and not required for sending or receiving data.
Please refer to the documentation of your specific Wixel app to determine the location of the TX pin(s) and RX pin(s).
Note: The Wixel does not support the RS-232 voltage levels typically used by DB9 serial ports. The
Wixel’s I/O lines, including the RX and TX lines, operate on voltages between 0 and 3.3 V are not 5 V
tolerant. To connect the Wixel to an RS-232 serial signal, you will need additional level-shifting and
inverting hardware like the
(RS-232 serial
is inverted; the Wixel’s serial interface expects non-inverted serial).
5.c. Connecting Buttons and Starting the Bootloader
In order to load a new app or new settings onto your Wixel (or read the Wixel’s flash memory) you will need to get it
into bootloader mode. Most Wixel apps support a special USB command for putting the Wixel into bootloader mode
and the Wixel Configuration Utility can send that command automatically when you try to access the Wixel’s flash.
However you might find yourself in a situation where that method will not work. This can happen for two reasons:
• You accidentally loaded a malfunctioning program onto the Wixel that is incapable of responding to the
special USB command.
Pololu Wixel User's Guide
© 2001–2014 Pololu Corporation
5. Connecting Your Wixels
Page 25 of 64