MOTOROLA
9-2
MC68HC05F8
DTMF/MELODY GENERATOR
9
sampling rate of the desired “staircase sine wave”. This pulse train then clocks a divide-by-28
binary counter (PLA scanner) whose 28 decoded outputs sequentially scan 28 memory locations
of a 28x6 sine wave generator (PLA) in 28 time steps (M). The 6 resulting digital sine wave bits
are then fed separately to a 6-bit resistor ladder to produce a current signal.
The method for generating a square wave tone in a path is similar to that of a sine wave tone
except that only the most signicant bit of a sine wave PLA is fed to the 6-bit resistor ladder (the
other 5 bits are masked by the Sine/Square wave select) to produce a current signal. The resulting
square wave tone has exactly the same frequency and phase as a sine wave tone for the same
frequency control register value.
After obtaining the current signals from the row and column paths, the row current signal is rst
attenuated by 2dB, and is then summed with the column current signal, and is nally fed to an
active 7KHz low pass lter to reduce harmonic distortion. The resulting DTMF or melody signal is
output to the TONEOUT pin, which is normally buffered to drive a buzzer.
The generator provides not only DTMF and melody but also a square wave pacier tone (Tone).
This signal is also extracted from the most signicant bit of the sine wave PLA of the row path, but
is not subjected to the lter. The ToneX signal is output to the TONEX pin, which is normally
connected to a loudspeaker.
Figure 9-1 DTMF/Melody Generator Block Diagram
Row Frequency
Divider
PLA
Scanner
Data
Validator
Row Frequency Control Register
Column Frequency Control Register
Column Frequency
Divider
TGER
TGEC
MS1
MS0
Sinewave PLA
28 x 6-bits
PLA
Scanner
Sinewave PLA
28 x 6-bits
Sine/Square
Wave Select
Sine/Square
Wave Select
MUX
6-bit Resistor
Ladder
6-bit Resistor
Ladder
High Group
Pre-emphasis
+
3.58MHz
÷ 2
TONEX
TONEOUT
current summer
+
active low-pass lter
MSB
LSB
Tone Control
register bits 4 & 5
Tone Control
register bits 6 & 7
5
6
5
6
TPG
80
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Freescale Semiconductor, Inc.
For More Information On This Product,
Go to: www.freescale.com
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Freescale Semiconductor, Inc.
For More Information On This Product,
Go to: www.freescale.com