PCD3349 datasheet(7/20 Pages) PHILIPS

7 / 20 page

1998 May 11

Philips Semiconductors

Product speciﬁcation

8-bit microcontroller with DTMF generator

PCD3349A

6.2

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 is dependent

upon the decimal value ‘x’ held in the frequency registers

(HGF and LGF), and this may be calculated as follows:

; where 60

≤ x ≤ 255.

The frequency limitation given by x

≥ 60 is due to the

low-pass filters which would attenuate higher frequency

sine waves.

6.3

DTMF frequencies

Assuming an oscillator frequency fxtal = 3.58 MHz, the

DTMF standard frequencies can be implemented as

shown in Table 3.

The relationship between telephone keyboard symbols

and the frequency register contents are given in Table 4.

xtal

23 x

()

[]

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

Table 3

DTMF standard frequencies and their

implementation; value = LGF, HGF contents

Table 4

Dialling symbols, corresponding DTMF

frequency pairs and frequency registers content

VALUE

(HEX)

FREQUENCY (Hz)

DEVIATION

STANDARD

GENERATED

(%)

(Hz)

697

697.90

0.13

0.90

770

770.46

0.06

0.46

852

850.45

−0.18 −1.55

941

943.23

0.24

2.23

1209

1206.45

−0.21 −2.55

1336

1341.66

0.42

5.66

1477

1482.21

0.35

5.21

1633

1638.24

0.32

5.24

TELEPHONE

KEYBOARD

SYMBOLS

DTMF FREQ.

PAIRS

(Hz)

LGF

VALUE

(HEX)

HGF

VALUE

(HEX)

(941, 1336)

(697, 1209)

(697, 1336)

(697, 1477)

(770, 1209)

(770, 1336)

(770, 1477)

(852, 1209)

(852, 1336)

(852, 1477)

(697, 1633)

(770, 1633)

(852, 1633)

(941, 1633)

•

(941, 1209)

(941, 1477)

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