Figure 7. Typical Keypad Interface Circuit
Keypad Strobe Register. The Keypad Strobe Register is used
to specify the output level of the strobe lines. Data written to this
register will appear on the strobe lines, assuming that the
Configuration Register has been written to set up the signals as
outputs. There is a one-to-one mapping between the bits and
the keypad strobes (i.e., bit  drives keypad strobe 0, bit
drives keypad strobe , etc.).
Keypad Return Lines Register. The Keypad Return Lines
Register is used to read the state of each of the keypad return
lines. Keypad Return 4 is multiplexed with GPIO Port A so the
related GPIO I/O select bit must be set to configure the pin as a
keypad return signal.
There is a one-to-one mapping between the bits and the keypad
return lines (i.e., bit  stores the state of keypad return line 0,
bit stores the state of keypad return line , etc.).
The annunciator circuitry on the BP generates a signal to drive a
handset buzzer. Some external circuitry may be required to
drive the buzzer depending on its electrical characteristics.
The output from the annunciator circuitry is multiplexed with
GPIO Port C. To select the PWM output to the pin, the I/O
select bit for GPIO Port C must be set to “1.” See the GPIO
section of this Data Sheet for more information.
There are two identical but separately programmable PWM
circuits on the BP. The outputs from the two circuits are toggled
at a rate of 1.95 MHz to generate the annunciator output from
the device. The annunciator output circuitry and the buzzer
driving circuitry are shown in Figure 8.
PWM Operation _____________________________________
The basic operation of each of the PWM circuits is described
The PWM uses a reference clock of 1.95 MHz to generate two
• A low frequency pulse train programmable from 200 to 500
Hz. This is the Tone signal.
• A high frequency pulse train at 39 kHz with a variable duty
cycle. This is the Mod signal.
The two pulse trains are mixed and filtered to produce the PWM
output. The frequency of this output is the frequency of Tone;
the volume is dependent on the duty cycle of Mod. The
frequency of Tone and the duty cycle of Mod are programmable
so that complex annunciator tones can be generated.
A functional block diagram for the PWM is shown in Figure 9.
June 14, 2000
Proprietary Information and Specifications are Subject to Change