DA9191A.000
July 31, 1997
FUNCTION
◆ TX Audio
The TX audio block is in the power down mode after
a reset. The block is set to the operation mode with
the AUDIOP bit of register 07HEX. The same bit also
controls the RX audio block. The TX audio block
starts with switch M2, which is controlled by bits 4
and 5 in register 00HEX. Switch M2 connects one of
the following blocks as an input source: 1) After a
reset the input source is connected to signal ground.
2) In the second position the input is connected to
the microphone amplifier A6. The gain of amplifier A6
is determined with external components. The
maximum gain is 30 dB. 3) In the third position the
input is connected to EXTMIC, which is for external
accessories. 4) The fourth position selects the DTMF
generator. The DTMF generator is controlled with
registers 05HEX and 06HEX. Register 05HEX controls
the low frequencies and register 06HEX controls the
high frequencies. A detailed description of how to
use these registers is in the REGISTERS section.
The DTMF generator is in the power down mode
after a reset and can be set up with the DTMFTP bit
GC3. The gain is
-3dB...+3dB with 16 steps
according to bits 0..3 in register 00HEX. The amplified
signal is then fed to lowpass filter F9LP and highpass
filter F9HP, which are 4th order SC filters. During
normal operation the output of filter F9HP is
connected to output MICOUT. With switch SYS0-
SYS1 the highpass filter can be transferred to the
output of GC5 after the signal has been compressed
and pre-emphasized. Bit 6 in register 12HEX and bit 6
in register 14HEX control switch SYS0-SYS1. Connect
MICOUT through
a
22nF capacitor to the
compressor input COMPIN. The MICOUT can also
be used as a source for the side tone amplifier and
for detecting the audio level with the uncommitted Op
Amp (See APPLICATIONS section). The compressor
is an SC audio type with a 2:1 compressing ratio.
The detailed values are found in the ELECTRICAL
CHARACTERISTICS section under Compressor.
The compressor can be bypassed internally with
switch S12 or externally with S13. The bypass gain is
0dB (100 mVrms). The compressor requires one
external 22nF capacitor between pins CAMP20 and
CWCIN. Another 22nF capacitor is needed between
pins CAMP2IN and COUT. This capacitor also
serves as an external DC blocking capacitor between
the compressor output and gain control GC4 input.
of register 07HEX
.
The signal is then filtered with anti-aliasing filter F8.
The gain at 1kHz is 0dB and the cutoff frequency is
15kHz. After the filter the signal is amplified with
DTMF
Generator
F9HP
MIC
GC3
F8
F9LP
A6
SYS0
SYS1
MICSGND
GC4
F10
S14
S12
MICFB
S13
EXTMIC
2:1
SYS0
SYS1
M2
COMPRESSOR
S7
PRE-EMPHASIS
GC9
F14
GC6
F13
GC7
S19
F11
GC5
S16
LIMITER
S15
+
TX
TXTST
SYS0
SYS1
TX data, ST
S20
Audio Transmit Path
19
MAS [ MICRO ANALOG SYSTEMS ]
