[VoIP] MF tone generator

[Donald Froula]

I thought there might be some interest in a "Blue Box"
circuit I designed using a 68HC705C8 microcontroller.
I have the schematic and S-records for programming the
chip (or I'd be happy to program a blank one).

It might be useful for getting a toll test board
running or other applications.

Don

Design Notes
============

The microprocessor runs at 4 MHz.

It is powered directly from three 1.5 V mercury button
cells in series. Current draw at nominal
volume is only 12 ma (idle) to 17 ma (tone being
generated).

One of the 8-bit 68HC705C8 ports is wired to an R-2R
resistive network made with precision 10K/20K
resistors. The output of the network (input to the
LM386N audio amp) is the sum of the
binary-weighted output of the port (0xFF = 5 volts,
0x00 = 0 volts). It is very linear.

Each button drives an input port on the
microcontroller. No keyboard scanning is used.

To generate tones, an interrupt service routine driven
by an on-chip timer samples a sign wave
table at 12KHz. The index amount into the table
determines the frequency. Two indices are
run. The retrieved values are then summed and divided
by 2. The result is sent to the 8-bit
output port, which generates the dual-tone waveform.
Tone duration is accurate to a single
sampling interval. Frequency acuracy is typically
within 1 Hz.

The microcontroller automatically shuts off the
amplifier and goes into low-power mode
(microamps) after 5 seconds of inactivity.

Three modes of operation are supported.

In MF burst mode (power-up default) the digit buttons
each generate an MF pulse of the
maximum recommended duration (75 ms.). An additional
button can be wired to its own pin on the
micro for 2600, or * and # may be depressed together
in the MF modes. 2600 plays as long as the
button is depressed. The * key alone is KP. The # key
alone is ST. The KP burst is timed to 120 ms.
The ST burst is 75 ms. Pressing keys 1 and 3 together
play the STP tone. 4 and 6 play the ST2P tone.
7 and 9 play the ST3P tone.

Depressing buttons 7 and 8 together toggle in and out
of MF continuous mode. A low to high or
high to low beep confirms the mode change. In
continuous mode, the MF digits play as long as the
key is depressed.


KEYPRESS             DIGIT OR                       
MF FREQ                CCITT
                            CODE                      
        IN Hz                 SYS NO. 5

1                              1                      
        700 +900
2                              2                      
        700 +1100
3                              3                      
        900 +1100
4                              4                      
        700 +1300
5                              5                      
        900 +1300
6                              6                      
        1100 +1300
7                              7                      
        700 +1500
8                              8                      
        900 +1500
9                              9                      
        1100 +1500
0                              0                      
        1300 +1500
*                              KP                     
       1100+ 1700                  KP1
#                             ST                      
       1500+ 1700
1+3                          STP                      
     900+ 1700                   Code 12
4+6                          ST2P                     
    1300+ 1700                  KP2
7+9                          ST3P                     
    700+ 1700                    Code 11

KP = Preparatory for digits
ST = End of pulsing
STP, ST2P, ST3P = Control codes


Depressing buttons 8 and 0 together toggle in and out
of DTMF mode. Beeps confirm the mode change.
In DTMF mode, the 2600 button is not active. The digit
buttons play their corresponding tones in
continuous mode (burst mode is not supported in DTMF
mode). Depressing the first and third buttons
in each row play the DTMF A, B, C, and D tones.

                                         DIGIT OR     
                  DTMF FREQ
KEYPRESS	                            CODE             
                   IN Hz
1                                              1      
                       697   + 12092
2                                              2      
                       697 + 1336
3                                              3      
                       697 + 1477
4                                              4      
                       770 + 1209
5                                              5      
                       770 + 1336
6                                              6      
                       770 + 1477
7                                              7      
                       852 + 1209
8                                              8      
                       852 + 1336
9                                              9      
                       852 + 1477
0                                              0      
                       941 + 1336
*                                              *      
                       941 + 1209 
#                                             #       
                      941 + 1477
1+3                                          A        
                     697 + 1633
4+6                                          B        
                     770 + 1633
7+9                                          C        
                     852 + 1633
*+#                                          D        
                     941 + 1633


Depressing buttons 8 and 9 together toggle in and out
of calibration mode. Beeps confirm the mode
change. In this mode buttons 1-6 play the MF master
tones alone; 700, 900, 1100, 1300, 1500, and 1700.
Button 8 plays a 1004 Hz test tone. 

A power down resets the box out of calibration mode,
but the other mode settings are retained.

In any mode:

Pressing buttons 1 and 2 together generates a standard
dial tone.

Pressing buttons 2 and 3 together generates a standard
2-second/4-second ringback tone.

Pressing buttons 4 and 5 together generates a standard
busy signal (60 ppm).

Pressing buttons 5 and 6 together generates a standard
reorder (120 ppm).

Pressing buttons 4 and 7 together generates an
intercept SIT tone sequence (subscriber disconnected).

Pressing buttons 5 and 8 together generates an
inter-LATA all circuits busy SIT tone sequence.

Pressing buttons 6 and 9 together generates an
inter-LATA reorder SIT tone sequence (mangled MF
addressing indication).