DA9191A.000
July 31, 1997
FUNCTIONS
The signal is fed to the SC-type audio expander with
expansion ratio 1:2. The expander can also be
bypassed with switch S5. The switch is controlled
with bit S5 in register 03HEX. After switch S5 the
signal goes to the EXPOUT output pin. An external
capacitor is used to connect the signal to the
RAUDIN input and to filter F6. Filter F6 is a 4th order
SC-type lowpass filter. The gain at 1kHz is 0dB. After
filtering the signal and the external accessory input
EINR can be summed with switch S8. The control bit
for the switch is in register 03HEX. The summed signal
is amplified with GC2, which has a -15dB...+15dB
gain with 16 steps. The gain is controlled with bits
0..3 in register 03HEX. The amplified signal can be
summed with SIDETONE. The side tone can be
switched on with S9, which has a control bit in
register 02HEX. The side tone input has an internal Op
Amp with feedback signal SIDEFB. The gain of the
Op Amp is controlled by external components.
COMPIN is normally used as an input for the side
tone amplifier circuit.
After filtering the signal can be connected to three
amplifiers with switches M1, S10 and S11. The
control bits are located in registers 02HEX and 03HEX
.
A4 is the earphone amplifier, which is a single input
differential output amplifier. Amplifier A5 is for
external accessories and is capable of driving a
capacitive load. The load capacitance is 1nF and the
block has a 4.82dB gain. The third amplifier A3 is a
buzzer driver. It drives the signal to the power
transistor, which drives the buzzer. The buzzer
represents a high inductive load of 1.2mH/25ohm.
The block stabilizes the current flow through the
external buzzer which depends on the current gain
factor of the external bipolar transistor. The emitter
resistor of the transistor must be 6.8 ohms. Three
current values (peak values) can be chosen with
switch M1:10mA, 66mA and 160mA. The tolerance
of the external resistor will directly affect the buzzer
current. The RX audio block can be set into power
down mode together with the TX audio block. The
AUDIOP bit in Register 07HEX is used for this
purpose. The blocks are in power down mode after a
reset.
Filter F7 is a second order low pass filter. The cutoff
frequency is 20kHz and the gain at 1kHz is 0dB.
18
MAS [ MICRO ANALOG SYSTEMS ]
