by Fred Eady
Wire is a wonderful thing. There’s not much that
is more reliable than a short piece of stranded
or solid copper wire between the ends of an
electrical connection. However, there are situations where
electrically tying devices together with long runs of wire is
impractical. Light does a good job of replacing copper
when the conditions are right, but if one needs to move
electrons reliably over a relatively long distance there’s no
better conductor than the Earth’s magnetic field.
Odds are that if you’re not using copper wire to
supply power to a device, you’re using that wire to
convey a signal. If your signaling environment allows the
communicating devices to place their sensors in plain sight
of each other and if the signaling environment can bounce
light around between the sensors with a minimum of
spectral loss, a beam of modulated light is as good as a
signal-carrying piece of copper wire.
On the other hand, if the devices that wish to
communicate can’t visibly see each other or the distance
between the devices is large, light may not be the most
reliable means of carrying a signal from one device to
the other. In this case, short-distance signaling using
low-power RF is a better bet.
A Smart Data Radio
What do you get when you mix a Microchip
PIC16F690 microcontroller with a Texas Instruments
CC1100 1 GHz transceiver IC and a fingertip full of 0402
PHOTO 1.R3 and R4 are SMT jumpers that determine the baud
rate which is set for 19200 bps in this shot. From what I can
match up with the PIC’s pinout, R2 and C14 are the MCLR reset
circuit, while R5, C11, C12, and X1 form the PIC’s system clock
oscillator. Bulk capacitor C1 is guarding the input voltage.
34 SERVO 10.2008
SMT components? The itty-bitty EmbedRF data radio you
see in Photo 1. The EmbedRF can replace up to 80 50-foot
copper signal wires as it is pinned for four 16-bit analog-to-digital (A-to-D) inputs, a digital output, and a digital input.
Alternately, you can program the EmbedRF to present two
A-to-D inputs, one digital input, and two digital outputs.
The EmbedRF’s A-to-D input voltage levels and digital I/O
logic levels can be included in the RF transmit stream or
completely ignored and replaced with user-defined data.
The digital input and one of the digital outputs can be
optionally configured as part of a three-wire EUSART-based
serial interface (RXD, TXD, and GND).
The EmbedRF’s CC1100 transceiver IC dominating the
view in Photo 2 is hard-coded to run at 915 MHz in point-to-point or point-to-multipoint modes. All of the A-to-D
inputs and the serial interface (RXD and TXD) are tied to
the on-board PIC. Data communications between the PIC
and the CC1100 is performed using a SPI (Serial Peripheral
Interface) data link.
The EmbedRF has the ability to transmit a 17-byte
packet in intervals of 0.25 to 12. 75 seconds. The interval
granularity is 50 milliseconds per interval bit. Only 11 of
the 17 bytes in the packet are loaded with data that the
user can access as the first six bytes of the packet contain
the network ID and transmitting device ID. The network ID
is hidden during transmission to provide network security.
Of the 11 transmitted data bytes, only 10 of the 11 data
bytes are user customizable via the EmbedRF’s EUSART-based serial interface. The last byte in the 17-byte packet
is a packet counter byte that is incremented each time a
new RF packet is transmitted by a transmit-enabled
EmbedRF device.
A minimum of two EmbedRF devices are required
to form a network. Each EmbedRF network node has
the capability of transmitting only, receiving only, or
transmitting and receiving. A set of unique device and
network IDs allows EmbedRF nodes to selectively
communicate with each other. An EmbedRF network node
can have a unique transmit device ID and a separate
unique receive device ID. Device IDs can range from zero
(0x000000) to 16,777,215 (0xFFFFFF). The same 24-bit ID
PHOTO 2. The unmarked termination to the right of VDD
is the RSET pin, which is tied to the junction of MCLR
components R2 and C14 in Photo 1. The antenna is
covered by the SN000189 label.
