The Dual Tone Multi-Frequency (DTMF) signal was invented by Bell Labs and was originally used for user signaling between telephones and switches in a telephone system, typically for transmitting called numbers. The DTMF signal consists of a high frequency group and a low frequency group, and the high and low frequency groups each contain four frequencies. A high frequency signal and a low frequency signal are superimposed to form a combined signal representing a number. DTMF signaling has 16 codes. The dual-tone multi-frequency dial pad is a matrix of 4&TImes;4. Each time a button is pressed, a combination of high frequency and low frequency sinusoidal signals is sent. Since there is no harmonic interference between any two frequencies, the anti-interference ability is very strong, and the false positive rate after remote transmission is very low.
The DTMF and decoding chip in the switch uses CM8870 to realize the decoding function of the dual-tone multi-frequency signal on the communication line. The chip converts the detected dual-tone multi-frequency signal into a binary four-bit code and sends it to the single-chip microcomputer to provide data flow for the single-chip microcomputer. Purpose information. A dual tone multi-frequency signal is a group of combined signals composed of a high frequency signal and a low frequency signal. Dual-tone multi-frequency signal decoding is a very important part of the switch. Whether it can accurately decode the dual-tone multi-frequency signal sent by the line is the key to establish a communication link. Its working condition directly determines the reliability of remote data communication. Sex. The dual audio decoding chip CM8870 used in this system integrates a band separation filter and a digital decoder, which can convert the received DTMF signal into 8421 code.
1CM8870 decoding function implementation1.1CM8870 chip introduction
CAMD's CM8870 dual-tone multi-frequency signal decoder is a single-chip 18-pin DIP package chip with filter and data decoding function to filter out non-audio signals from 340 to 3400 Hz and convert audio signals into binary 4-bit digital signal, the internal CMOS process greatly reduces the power consumption of the chip, and the power consumption is only 35mW. The CM8870 includes a differential input amplifier, a clock generator and a tri-state latch interface bus, which reduces the peripheral components of the chip and requires only a normal crystal oscillator to operate normally. Its characteristics are as follows: Provide DTMF signal separation filtering and decoding function; power consumption is less than 35mW; can work in the industrial temperature range; external crystal oscillator can be connected, and the internal oscillator generates the reference frequency signal; using 18-pin DIP, EIAJ, OIC, PLCC package.
The basic feature of the CM8870 circuit is to provide DTMF signal separation filtering and decoding functions, and output a corresponding 4-bit parallel binary code of 16 DTMF frequency combinations. The binary code D1~D4 of the circuit output is controlled by the data output enable segment TOE. When TOE is high level, D1~D4 output the binary code corresponding to the currently input DTMF signal; when TOE is low level, D1~D4 end It is in a high resistance state. The op amp and R1, R2, and C1 form an inverting amplifier that isolates and amplifies the input DTMF signal with its gain. K=-R2/R1, change the value of R2 to change the gain. VREF is the reference voltage output, take VDD/2=2.5V; INH and PD are internal circuit connection points, should be grounded; OSCl and OSC2 are oscillation Input and output terminals, external 3.58MHz crystal oscillator and internal oscillator generate reference frequency signal; STD is delay control output, output "1" when a set of valid dual audio signals are received, otherwise output "O"; ESt is Initial control output, if the circuit detects an identifiable tone pair, the terminal becomes high. If there is no input signal or continuous distortion, ESt returns to low level; SI/GT is control input/time monitoring Output. The functional block diagram is shown in Figure 1.
A dual tone multi-frequency signal is a combination of two different frequency signals to represent a character or number. The two frequencies are selected from the low frequency band, one is selected from the high frequency band, and each frequency band contains four different frequencies, so that a total of 16 types can be combined, and only 12 kinds are commonly used, they are O~9, and #å· and *å·. The CM8870 can decode DTMF signals into 4-bit binary codes.
1.2CM8870 collection process
The CM8870 forms a receiving circuit. Its input is a dual-tone multi-frequency signal from the analog user interface. The output is 4-bit binary data for the processor to read in from the data bus port. The process of receiving the number is as follows: after the analog signal is introduced from the IN pin, the out-of-band interference signal is initially filtered by the two-tone filter, and then the filtered signal is filtered out by the high group filter and the low group filter respectively. The high frequency and low frequency components are respectively sent to the digital detection calculation circuit after zero-crossing detection; the circuit further optimizes the audio signal to exclude external noise from being encoded by the encoder due to accidental inclusion of certain frequencies. Or affect the encoding of the encoder, causing subsequent harvest error. When the high and low frequency group signals are detected by the encoder at the same time, the ESt pin will output a high level as a flag for effectively detecting the DTMF signal, and when the DTMF signal disappears, the ESt pin will output a low level. In order to prevent external noise from being miscoded by the CM8870, the encoder requires that the encoded audio signal be maintained for a period of time, which is determined by an external RC circuit. As mentioned earlier, when the audio signal is detected, ESt outputs a high level of 1, the capacitor discharges, the voltage value on VC rises (assuming the signal exists internally throughout the required time), and when VC rises to a threshold value Vrst, The audio signal is encoded and becomes a digital signal that will be latched. At this point, GT is high, causing the voltage at the VC point to rise from the threshold to VDD. Thereafter, as long as ESt remains high GT is high, the external RC circuit returns to the initial state, and then, after a delay caused by a latch operation, the STD pin outputs a high level, indicating that the signal is latched. At this time, if you want to read the 4-bit code from Q1 to Q4, you should make TOE high and open the latch to complete the DTMF collection task.
2 single chip control CM8870 design2.1SM895l control block diagram
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