XC68HC705F32 Motorola, XC68HC705F32 Datasheet - Page 76

• Manufacturer Part Number: XC68HC705F32
• Description: MICROCONTROLLER
• Manufacturer: Motorola

7.2
Functional description
As shown in
Figure
7-1, the DMG consists of 2 tone generation paths (the column and row paths).
One path generates the row tone and the other the column tone, whose frequencies are
determined by the values in the frequency control registers FCR and FCC respectively. The tones
allowed at the TNO output are single/dual sine/square wave tones of DTMF and melody
frequencies, whereas at the TNX output, only single square wave tones are allowed. The method
of tone generation for the two paths is almost the same, and is described as follows.
To generate a sine wave tone with programmable frequency in a path, the internal clock (i.e. the
3.58MHz/2) is first divided by a frequency divider according to a number on the register (FCR or
FCC). The output of the divider is a periodic pulse train whose frequency is the sampling rate of
the desired ‘staircase sine wave’. This pulse train, in turn, clocks a divide-by-28 binary counter
(PLA scanner) whose 28 decoded outputs scan sequentially 28 memory locations of a 28x6 sine
7
wave generator (PLA) in 28 time steps (M). The six 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 significant bit of a sine wave PLA is fed to the 6-bit resistor ladder to
produce a current signal (the other 5 least significant bits are masked by the sine/square wave
select). Using this method, a square wave tone can be produced which has exactly the same
frequency and phase as a sine wave tone, and uses the same frequency control register value.
After obtaining the current signals from the row and column paths, the row current signal is first
attenuated by 2dB. It is then summed with the column current signal, and is finally fed to an active
7 KHz low pass filter to reduce harmonic distortion (note that square wave tones are also passed
through this filter). The resulting DTMF or melody signal is output through the TNO pin which is
normally connected to a speech circuit.
The generator provides not only DTMF and melody but also a square wave pacifier tone (ToneX).
This signal is also extracted from the most significant bit of the sine wave PLA of the row path, but
is not passed through the filter. The ToneX signal is output through the TNX pin which is normally
connected to a loudspeaker.
TPG
MOTOROLA
DTMF/MELODY GENERATOR
MC68HC05F32
7-2