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Touch-tone decoder/display program.
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TOUCH-TONE DECODER/DISPLAY PARTS LIST 147
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polls = 0 ' Clear the counter. goto again ' Poll some more. skip1: ' Tone detected: polls = 0 ' Clear the poll counter. low RS ' Get the DTMF data. low CS ' Activate 8880. dtmf = INL & %00001111 ' Strip off upper 4 bits using AND. high CS ' Deactivate 8880. gosub display ' Display the data. goto again ' Do it all again. Display: if LCDcol < LCDw then skip3 ' If not at end of LCD, don't clear screen. serout 7,N24N,[I,LCDcls,I] ' Clear the LCD screen. LCDcol = 0 ' And reset the column counter. skip3: ' Look up the symbol for the digit. if LCDcol=0 AND dtmf=16 then ret ' No spaces at first column. lookup dtmf,["D1234567890*#ABC-"] ,dtmf serout 7,N24N,[dtmf] ' Write it to the Backpack display. LCDcol = LCDcol + 1 ' Increment the column counter. ret: return
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Touch-tone decoder/display program (Continued).
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RADIO DTMF TONE DECODER/ CONTROL
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CONTENTS AT A GLANCE Basic Components DPDT Relay Silent Radio Programming Touch-Tone Control Radio DTMF Controller (Figure 11-1) Parts List Radio DTMF Controller (Figure 11-2) Parts List
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Dual-tone multifrequency (DTMF) signaling has been around for over 40 years now and
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continues to be the preferred tone signaling method. As mentioned previously the beauty of this signaling system is that two nonresonant tones out of eight are transmitted simultaneously and can be sent over land lines or wire or radio systems between and through various repeater systems around the country. The DTMF system is very reliable in that two nonharmonically related tones must be decoded at once, so that false outputs are very unlikely. The DTMF system was and still is a very powerful method of signaling and control for the phone system as well as for ham radio operators in controlling remote repeater systems or remote base transmitters over long distances. The 2-of-8 touch-tone frequencies versus numerical digits are illustrated in Table 11-1.
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Copyright 2003 by The McGraw-Hill Companies, Inc. Click Here for Terms of Use. 149
150 RADIO DTMF TONE DECODER/CONTROL
TABLE 11-1 1209
TOUCH-TONE OR DMTF TONES 1336 1477 1633
697 770 852 941
1 4 7 *
2 5 8 0
3 6 9 #
A B C D
In the past, touch-tone frequencies had to be generated and decoded by discrete tone generators and tone decoders. Encoders and decoders were quite complex, and each could fill up an entire circuit board. Today, DTMF encoding and decoding can be accomplished by the same single 20-pin integrated circuit chip. In this chapter, we will take a look at a remote control touch-tone radio decoder that can be used to remotely control your garage door or other home appliance via an amateur radio transceiver, scanner, or FRS radio. The remote control touch-tone decoder can also be used to mute the radio speaker until a specific message needs to be sent over radio. If, for example, you are sharing a frequency and you want to call a family member, when you decode the proper tone sequence your radio will allow the speaker to operate and let your specific message through.
Basic Components
The remote control radio decoder uses the BASIC STAMP 2 microprocessor and the new CM8880 DTMF encoder/decoder chip, in a 20-pin in-line DIP package, from California Micro; see App. 1 on the CD-ROM for data sheets. This encoder/decoder chip can be easily interfaced to the STAMP 2 controller and can be used to both generate and decode touch-tone tones. A block diagram of the CM8880 is shown in Fig. 11-1. The chip requires only about six external components to form a complete encoder/decoder. The CM8880 chip uses the ubiquitous 3.579-MHz color burst crystal for its oscillator. The chip produces a binary output on pins D0 through D3, which directly connects to the STAMP 2 controller; see Table 11-2. Est on pin 18 is the early steering output signal, which indicates the detection of valid tone frequencies, and StGt on pin 19 is the steering input and guard time output. A time constant based on a capacitor and a resistor determines a valid signal duration time in which to accept a tone pair. Three additional bits are used to select the modes of the CM8880: from the chip select (CS) pin, the read/write (RW) pin, and the resister select (RSO) pin; Table 11-3 depicts all the combinations of the three control pins. The CM8880 is active only when CS equals 0. The RW bit determines the data direction (1 read and 0 write), and the RS bit determines whether the transaction involves data (DTMF tones) or internal functions (i.e., 1 instructions/status and 0 data). Before you can use the CM8880, you have to set it up. The device has two control registers, A and B.
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