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DIALER
Circuit Description - The main elements of the Dialer Section are
the keyboard and the dialer IC HM9102. These two components
perform all of the functions of the rotary dial found in older
telephones.
The HM9102 is the tone/pulse switchable dialer with last number
redial which is fabricated with CMOS technology. It has a wide
operating voltage for both tone and pulse modes, and consumes very
low memory retention current in the on-hook state.
As the schematic diagram shows, the keyboard is laid out as a 3
column by 4 row matrix. Each of the 12 keyboard buttons is located
at the intersection of a row and column line. Pressing the button
connects the row to the column line.
connects column 1 to row 2. NOTE: For this design of telephone, the
keyboard assignment is as follows:
a) Button " * "
- In Tone Mode: Execute " * "
- In Pulse Mode: Execute Pause
b) Button "#"
- In Tone Mode: Execute "#"
- In Pulse Mode: Execute Redial
When the "#" button is pushed (only for pulse mode), the last number dialed is automatically redialed.
The functions of the dialer IC are as follows:
Power - The DC voltage at the Tip and Ring inputs is fed to the dialer IC via the diode bridge, the hook switch, resistor R14
and diode D5. Since excessive voltage may damage the IC, zener diode D7 limits the voltage to less than 4.7 volts. When
the phone goes on hook, the charge on capacitor C3 provides the small current needed for the IC memory circuits to retain
the last number dialed.
Dialing - The Central Office responds to two types of dialing signals, Pulse and Tone. In your phone, the IC turns transistor
Q3 on or off to generate dial pulses. When the switch (S3) is in the Tone position - potential pin mode - GND and Dial will
be for tone signals. When the switch (S3) in the the Pulse position - potential pin mode - VDD (4.7V) and Dial will be used
for pulse signals.
Oscillator - The IC HM9102 has a built-in oscillator (clock) which is used for tone generation and other timing functions.
Because the frequency of dialing tones must be precise for the Central Office to recognize them, a ceramic resonator (X)
used to accurately control the frequency of the IC's oscillator. This oscillator's frequency is 3.58MHz, and it is divided down
to lower frequencies inside the IC.
Hook Switch (HKS) - When the phone is off the hook, the HKS input of the IC is grounded through resistor R9. This
enables the keyboard inputs, when the phone is on the hook, a small current flows through resistors R8 and R9, this biases
the HKS input to about 2V which disables the keyboard inputs.
Mark/Space Select - This input selects the Mark to Space ratio. In this kit, the line is tied to ground, giving a Mark to Space
ratio of 40% to 60%.
Keyboard Inputs - Keyboard inputs C1 through C3 and R1 through R4
accept the column and row inputs from the keyboard (see Figure 3-4).
When a row and a column line are connected, the digit corresponding to
that row and column stored in the IC memory. The dial pulses start when
the first button is pushed. In a conventional rotary dial, the pulses are
sent as the rotor returns to its null position. This may take about one
second if a zero is dialed. The next digit is entered only when the
preceding digit is transmitted. With the HM9102 dialer IC, scanning of the
keyboard continues while the dial pulses are being transmitted as shown
in Figure 6-14. When the number is dialed, a memory section in the IC is loaded with each digit as it is dialed. Later, if
the redial button (#) is pressed, the IC automatically dials the digits you previously entered (for Pulse Mode). For Tone
Mode, button (#) - execute "#".
The button "*" for Pulse Mode - execute pause (pause time 3.6s), for Tone Mode - execute " * ".
Dialing Rate Select - For the IC HM9102, the Dialing Rate Select line is tied to ground, giving a dialing rate of 10 pulses
per second or 20pps for pulse mode and tone for tone mode.
Thus, pressing button 4
-21-
100mS
60mS
40mS
Figure 6-13
800mS
800mS
Figure 6-14
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