Issue 149 December 2002
Wireless Temperature Sensor Stew

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rfPICDEM1

Future Electronics’s Technical Solutions Management Group and Microchip have teamed up to make designing and implementing wireless sensor applications dead easy. All of the math and plumbing that’s common to putting RF boxes together has been done for you. The plumbing refers to the RF part of the demo board. In my FM radio days, I called the transmitter techs plumbers, because they worked with wave guides and tuned cavities just like real plumbers worked with kitchen pipes. The only real difference was that the radio plumbers didn’t do toilets and they never got wet.

Figure 1 is a block diagram of the rfPICDEM1 wireless sensor demo board set. The receiver and transmitter are separate, microcontroller-based units. As you know, Microchip has branched out its suite of products. Now, in addition to the standard line of microcontrollers, this set of rfPIC demo boards includes a Microchip LDO voltage regulator (TC55) and temperature sensor (TC74).

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Figure 1—It’s too bad you can’t click on the boxes to go directly to companies’ web sites for the datasheets. You can get a copy of the CD-ROM that contains this image from your Future Electronics or Microchip representative.

The object of the rfPIC wireless sensor solution demo kit is to show us how to use the Microchip rfPIC12C509 to send temperature data collected by a Microchip TC74 temperature sensor over a short-haul 433-MHz RF link to a smart receiver using a Philips UAA3201 RF receiver IC front-ending a PIC16C925 with an integrated LCD controller. Whew! That was a mouthful, but it isn’t as complicated as that last sentence makes it seem.

The transmitter is based on the rfPIC12C509AG that’s driving a loop antenna, which is etched onto the transmitter circuit board. The rfPIC12C509AG is really a standard PIC12C509 coupled with a 433-MHz amplitude shift-keying (ASK) transmitter. The RF side of the rfPIC12C509AG consists of a crystal oscillator, an integral phase-locked loop, mode control logic, and an open-collector differential-output power amplifier. As you can see in Figure 2, the RF section and the PIC microcontroller electronics are logically separate, although they reside inside the same physical package.

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Figure 2—As you can see, the RF section and the PIC microcontroller electronics are clearly separate from each other.

Normally, ASK modulation is performed by alternately changing the amplitude of the carrier in a pulse width modulation fashion (i.e., the carrier wave is modulated for a period of time to signal a logic 1 and left idle for a period of time to indicate a logic 0). Instead of employing the more common PWM method, the rfPICDEM1 uses Manchester encoding.

In Manchester encoding, a logic 0 is seen as a transition from zero to one at the center of the bit time. Conversely, a logic 1 is indicated by a one-to-zero transition at the center of the bit time. This results in a synchronous bitstream with an encoded clock signal that doesn’t always transition physically at bit boundaries; instead, it always transitions logically at the center of each bit time.

When powered up, the transmitter sleeps most of the time to conserve the battery. When it awakes, the PIC12C509 part of the rfPIC reads the TC74 using I2C signaling. If the temperature has changed, the new temperature is transmitted. There are two push buttons on the transmitter. These push buttons are used to wake up the transmitter, set the station ID, generate a test (calibration) tone, or initiate the transmission of the current temperature and transmitter switch status.

The transmitter code was written to automatically change the transmitter station ID (zero through seven) when SW1 is pressed and held. Also, because each TC74 has a permanent I2C address that depends on the part number of the sensor, the transmitter code uses I2C signaling to automatically determine the I2C address of the current on-board TC74.

A message sent by the rfPICDEM1 transmitter consists of 32 bits of header data (zeros) followed by a 4-bit sync frame (ones) and 32 bits of data. The data field contains the station ID (8 bits), the temperature (8 bits), a 4-bit sensor type, 2 bits of message type, 2 bits of button status, and an 8-bit checksum. This entire message is transmitted three times each time the transmit event is invoked.

The rfPICDEM1 receiver can detect and support up to 16 remote transmitters. With the push of a button on the receiver, you can select one of four modes of operation. You can display the temperature local to the receiver, the temperature at the remote transmitter, or the difference between the local and remote temperatures can be exhibited. Auto mode displays the local temperature followed by all of the received remote temperatures. Two LEDs on the receiver show the status of the remote push buttons.

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Photo 1—Just another sunny day. Note the etched loop antenna on the top of the smaller transmitter board. The PIC16C925 that comes loaded with the receiver firmware is a one-time programmable (OTP) part and can be replaced with a windowed-reprogrammable part for firmware development.

As you can see in Photo 1, both boards are battery powered, and the receiver has on-board provisions for in-circuit serial programming (ICSP). An 8-pin socket pad area on the transmitter allows you to use a separate 8-pin PIC or PIC in-circuit emulator to control the RF section of the rfPIC. On the transmitter board, "X" symbols mark the spots where the traces can be cut to isolate the optional 8-pin PIC pads from the rfPIC12C509AG. For a close-up view of these markings, take a look at Photo 2.

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Photo 2—"X" marks the spot. A cut of the trace at every "X" logically separates the rfPIC’s microcontroller I/O from the RF section. Only two lines from the microcontroller side are used to control the RF side.