PCD3755AP datasheet(9/32 Pages) PHILIPS

9 / 32 page

1997 Apr 16

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.

xtal

23 x

()

[]

---------------------------------

where

255

≤≤

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