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DESIGN CYCLE
■ PHOTO 3. This carrier was originally designed for
an 802.15.4 radio. The carrier supplies power and
includes an FTDI USB-to-UART bridge IC.
are not designed to carry data over
great distances. So, we must ferry the
PAN packets between the PAN-based
microcontrollers and devices that
have immediate access to a WAN
(Wide Area Network) or the Internet.
There are many ways we can
move data about with a
microcontroller. Most advanced
microcontrollers contain one or more
UART, SPI, and I2C engines. I2C —
which is short for Inter IC — is
primarily used to logically
interconnect integrated circuits. The
LMX-ISM-242-LR (LMX-LR for short)
can move data over the Ether at 1
Mbps. High speed serial data transfer
is just a walk in the park for an SPI
portal. A typical UART application
will normally use data rates that are
far below 1 Mbps.
PAN devices are engineered to
move small amounts of data in
bursts. Thus, UARTS with baud rates
as low as 2400 bps can be utilized
successfully in PAN environments.
The LMX-LR transports data in a
packet that can contain anywhere
from one to 32 bytes. Even if a
packet is transmitted every second,
we can use a microcontroller’s
UART to easily move 32 bytes from
Point A to Point B with a baud rate
of 9600 bps.
If the device we’re moving data
to and from also contains a UART,
■ PHOTO 2. The Microchip RN-XV is an 802.11 b/g
Wi-Fi device that is designed to replace 802.15.4
radios. The RN-XV operates at 3. 3 VDC and is able
to connect directly to a PIC UART.
and the device and
microcontroller operate at
the same voltage level, the
microcontroller and external device
can communicate serially without the
need for any additional interface
hardware. UART intercommunication
firmware is easily written. The ideal
situation would utilize the relatively
simple UART communication process
to gain automatic access to a WAN,
LAN, or the Internet.
The ideal solution is exposed in
Photo 2. The Microchip RN-XV is a
Wi-Fi drop-in replacement for similar
802.15.4 radios. Utilizing a UART
interface and operating at 3. 3 VDC,
the RN-XV connects directly to a
PIC’s UART.
Only four physical connections
are needed to convert RS-232
protocol to wireless Ethernet. The
RN-XV comes loaded with a TCP/IP
stack, and includes integral I/O pins
and analog-to-digital converter (ADC)
inputs. Built-in networking
applications include HTTP and Telnet.
The RN-XV can also act as an FTP,
DNS, and DHCP client. UDP and
TCP protocols are supported.
RN-XV setup is simple and can
be done with a direct serial
connection or remotely via Telnet.
Initial configuration can also be
performed by forcing the RN-XV into
ad hoc mode. I chose to set up my
RN-XV with a modified 802.15.4
radio carrier. The RN-XV and
802.15.4 radio are pin compatible as
far as the power, ground, transmit,
■ SCHEMATIC 1. This is a partial
schematic of the 802.15.4 radio
carrier. To isolate the LM339 and the
RSSI LEDs, remove R7. This will free
the RN-XV GPIO5 pin for your use.
February 2013 15
