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BU8872 Datasheet(PDF) 5 Page - Rohm |
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BU8872 Datasheet(HTML) 5 Page - Rohm |
5 / 10 page 196 Communication ICs BU8872 / BU8872FS FCircuit operation A DTMF signal is supplied to the INPUT pin and applied to a pair of 6th-order bandpass filters, which separate the DTMF signal into its high (COL) and low (ROW) frequen- cies. The separated tones are converted into square waves and fed to a digital detector. (See the block dia- gram.) The digital detector checks the two tones to see if they are within the valid DTMF frequency bands. If they are, it sends a DETECT signal to the steering circuit, and sends the appropriate column and row address signals to a code converter. The code converter encodes the received and detected DTMF signal, and outputs an ENABLE signal to the steering circuit. Based on the DETECT and ENABLE signals, the steer- ing circuit outputs an Early Steering (ESt) signal, which sets the ESt pin to HIGH, indicating that a valid DTMF signal has been detected. If a series of pulses is input at the ACK pin while ESt is HIGH, a decoded DTMF signal is output to the SD pin as a binary code. (See Figure 2 for the overall timing.) If a pulse sequence is input at the ACK pin, the data is latched at the rising edge of the first pulse by a parallel- serial converter, and at the same time, the LSB is output from the SD pin. Following this, three bits of data are out- put from the SD pin for each bit of each pulse in the pulse sequence input from the ACK pin. As a result, a total of four bits of data are output for the four pulses. (See Fig- ure 3 for the ACK and SD timing.) If the pulse sequence input to the ACK pin consists of three or fewer pulses, the next DTMF input cannot be de- coded properly. Any ACK pulses in excess of four are ig- nored until ESt goes HIGH again. Table 1 shows the format of serial data output from the SD pin. |
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