Hardware Reference
In-Depth Information
3.3V Regulator
Most Arduinos (excluding the Due) operate at 5V logic levels; 0V indicates a
logical low, and 5V indicates a logical high. The XBee, however, operates at 3.3V
logic level, and it must be supplied with 3.3V power. Although the Arduino does
have a small 3.3V regulator onboard, it does not supply enough current for the
XBee, so most shields implement an LDO (low dropout) linear regulator that
drops the 5V supply down to 3.3V for feeding into the VCC pin of the XBee.
Logic Level Shifting
The UART TX and RX pins of the Arduino and the XBee need to be connected;
here too, however, you need to consider the fact that the XBee is a 3.3V part.
Data transmitted from the XBee to the Arduino does not need to be level shifted
(although some shields will do it anyways). This is because 3.3V is still above
the threshold to be read as a logical high by the Arduino RX I/O pin. The data
transmitted from the Arduino to the XBee, however, must be shifted down to
3.3V before it can be fed into the DI I/O pin of the XBee. Different shields use
different methods to accomplish this.
Associate LED and RSSI LED
Most shields have an “associate” LED that blinks whenever the XBee is powered
up and in use as a simple serial pass-though. It is generally used when running
the XBee in API mode, which you do not do in this chapter.
The RSSI LED, also present on most XBee shields, lights up briefly when data
is being received.
UART Selection Jumper or Switch
The XBee radio communicates with your Arduino via a serial Universal
Asynchronous Receiver/Transmitter (UART) connection (RX and TX). In the
case of the Arduinos other than the Mega and Due, there is only one available
UART that is duplexed to the USB serial connection that you use for commu-
nicating with your computer for programming and debugging. The Leonardo
(and similar boards) has just one UART, but it can be dedicated to the RX/TX
pins, because the USB programming interface connects to the microcontroller
unit (MCU) directly. In the case of the Uno, this raises a question: How can the
XBee module and your computer's interface both be connected to the Arduino's
single UART at the same time? When the shield is attached, the connection of
the RX and TX pins looks like the diagram shown in Figure 11-5.
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