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PCD3755E Datasheet(PDF) 9 Page - NXP Semiconductors |
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PCD3755E Datasheet(HTML) 9 Page - NXP Semiconductors |
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9 / 32 page  1997 Apr 16 9 Philips Semiconductors Product specification 8-bit microcontrollers with DTMF generator, 8 kbytes OTP and 128 bytes EEPROM PCD3755A; PCD3755E; PCD3755F 6.2 Melody output (P1.7/MDY) The melody output (P1.7/MDY) is very useful for generating musical tones when a purely sinusoidal signal is not required, such as for ringer applications. The square wave (duty cycle = 12 ⁄23 or 52%) will include the attenuated harmonics of the base frequency, which is defined by the contents of the HGF register (Table 3). However, even higher frequency tones may be produced since the low-pass filtering on the TONE output is not applied to the P1.7/MDY output. This results in the minimum decimal value x in the HGF register being 2 for the P1.7/MDY output, rather than 60 for the TONE output - the value shown in equation (1). A sinusoidal TONE output is produced at the same time as the melody square wave, but due to the filtering, the higher frequency sine waves with x < 60 will not appear at the TONE output. Since the melody output is shared with P1.7, the port flip-flop of P1.7 has to be set HIGH before using the melody output. This is to avoid conflicts between melody and port outputs. The melody output drive depends on the configuration of port P1.7/MDY; see Chapter 14, Table 24. 6.3 Frequency registers The two frequency registers HGF and LGF define two frequencies. From these, the digital sine synthesizers together with the Digital-to-Analog Converters (DACs) construct two sine waves. Their amplitudes are precisely scaled according to the bandgap voltage reference. This ensures tone output levels independent of supply voltage and temperature. The amplitude of the Low group frequency sine wave is attenuated by 2 dB compared to the amplitude of the High group frequency sine wave. The two sine waves are summed and then filtered by an on-chip switched capacitor and RC low-pass filters. These guarantee that all DTMF tones generated fulfil the CEPT recommendations with respect to amplitude, frequency deviation, total harmonic distortion and suppression of unwanted frequency components. The value 00H in a frequency register stops the corresponding digital sine synthesizer. If both frequency registers contain 00H, the whole frequency generator is shut off, resulting in lower power consumption. The frequency of the sine wave generated ‘f’ is dependent on the clock frequency ‘fxtal’ and the decimal value ‘x’ held in the frequency registers (HGF and LGF). The variables are related by the equation: (1) The frequency limitation given by x ≥ 60 is due to the low-pass filters which would attenuate higher frequency sine waves. f f xtal 23 x 2 + () [] --------------------------------- = where 60 x 255 ≤≤ |
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