MT91L60/61
In the event of PWRST, the MT91L60/61 defaults
such that the side-tone path is off, all programmable
gains are set to 0dB and ITU-T
µ-Law
is selected.
Further, the digital port is set to SSI mode operation
at 2048 kb/s and the FDI and driver sections are
powered up. (See Microport section.)
The internal architecture is fully differential to provide
the best possible noise rejection as well as to allow a
wide dynamic range from a single 3 volt supply
design. This fully differential architecture is
continued into the Transducer Interface section to
provide full chip realization of these capabilities for
the handset functions.
A reference voltage (V
Ref
), for the conversion
requirements of the Codec section, and a bias
voltage (V
Bias
), for biasing the internal analog
sections, are both generated on-chip. V
Bias
is also
brought to an external pin so that it may be used for
biasing external gain setting amplifiers. A 0.1µF
capacitor must be connected from V
Bias
to analog
ground at all times. Although V
Ref
may only be used
internally, a 0.1µF capacitor must be connected from
V
Ref
to ground. The analog ground reference point
for these two capacitors must be physically the same
point. To facilitate this the V
Ref
and V
Bias
pins are
situated on adjacent pins.
The transmit filter is designed to meet ITU-T G.714
specifications. The nominal gain for this filter is 0dB
(gain control = 0dB). Gain control allows the output
signal to be increased up to 7dB. An anti-aliasing
filter is included. This is a second order lowpass
implementation with a corner frequency at 25 kHz.
The receive filter is designed to meet ITU-T G.714
specifications. The nominal gain for this filter is 0dB
(gain control = 0dB). Gain control allows the output
signal to be attenuated up to 7dB. Filter response is
peaked to compensate for the sinx/x attenuation
caused by the 8 kHz sampling rate.
Side-tone is derived from the input of the Tx filter and
is not subject to the gain control of the Tx filter
section. Side-tone is summed into the receive
handset transducer driver path after the Rx filter gain
control section so that Rx gain adjustment will not
affect side-tone levels. The side-tone path may be
enabled/disabled with the gain control bits located in
Gain Control Register 2 (address 01h).
Transmit and receive filter gains are controlled by the
TxFG
0
-TxFG
2
and RxFG
0
-RxFG
2
control bits,
respectively. These are located in Gain Control
Preliminary Information
Register 1 (address 00h). Transmit filter gain is
adjustable from 0dB to +7dB and receive filter gain
from 0dB to -7dB, both in 1dB increments.
Side-tone filter gain is controlled by the STG
0
-STG
2
control bits located in Gain Control Register 2
(address 01h). Side-tone gain is adjustable from
-9.96dB to +9.96dB in 3.32dB increments.
Companding law selection for the Filter/Codec is
provided by the A/µ companding control bit while
the coding scheme is controlled by the Smag/ITU-T
control bit. The A/µ control bit is logically OR’ed with
the A/µ pin providing access in both controller and
controllerless modes. Both A/µ and Smag/ITU-T
reside in Control Register 2 (address 04h). Table 1
illustrates these choices.
Code
+ Full Scale
+ Zero
-Zero
(quiet code)
- Full Scale
Sign/
Magnitude
1111 1111
1000 0000
0000 0000
0111 1111
ITU-T (G.711)
µ-Law
1000 0000
1111 1111
0111 1111
0000 0000
A-Law
1010 1010
1101 0101
0101 0101
0010 1010
Table 1
Transducer Interfaces
Standard handset transducer interfaces are provided
by the MT91L60/61. These are:
• The handset microphone inputs (transmitter),
pins M+/M-. The nominal transmit path gain may
be adjusted to either 6.0 dB or 15.3 dB. Control of
this gain is provided by the TxINC control bit
(Gain Control register 1, address 00h).
• The handset speaker outputs (receiver), pins
HSPKR+/HSPKR-. This internally compensated
fully differential output driver is capable of driving
the load shown in Figure 3. The nominal receive
path gain may be adjusted to either 0 dB, -6 dB or
-12 dB. Control of this gain is provided by the
RxINC control bit (Gain Control register 1,
address 00h). This gain adjustment is in addition
to the programmable gain provided by the receive
filter.
Microport
The serial microport, compatible with Intel MCS-51
(mode 0), Motorola SPI (CPOL=0,CPHA=0) and
Intel® and MCS-51® are registered trademarks of Intel Corporation
Motorola® and SPI® are registered trademarks of Motorola Corporation
National® and Microwire® are trademarks of National Semiconductor Corporation
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