8
INDUSTRIAL TEMPERATURE RANGEIDT821034 QUAD PCM CODEC WITH PROGRAMMABLE GAIN
Bit Name Description
7 Register Indicator Always ‘0’
6 -- Reserved, always ‘0’
5 -- Reserved, always ‘0’
4 O_4 Data Output data on O_4 pin of the selected channel
3 O_3 Data Output data on O_3 pin of the selected channel
2 O_2 Data Output data on O_2 pin of the selected channel
1 I/O_1 Data Output data on I/O_1 pin (if defined as an output) of the selected channel
0 I/O _0 Data Output data on I/O_0 pin (if defined as an output) of the selected channel
Table 4. Definition of SLIC Control Register
Bit Name Description
7 I/On_0 Image Mapped to I/On_0 pin of the selected channel n
6 I/On_1 Image Mapped to I/On_1 pin of the selected channel n
5 On_2 Image Mapped to On_2 pin of the selected channel n
4 On_3 Image Mapped to On_3 pin of the selected channel n
3 On_4 Image Mapped to On_4 pin of the selected channel n
2 I/O1_0 Image Always mapped to the I/O1_0 pin
1 I/O2_0 Image Always mapped to the I/O2_0 pin
0 I/O3_0 Image Always mapped to the I/O3_0 pin
Table 5. Definition of SLIC Status Register
APPLICATION NOTE
The IDT821034 is mainly used in line card application. Figure 5 shows
a typical system with telephony line interface.
The IDT821034 offers not only encoding/decoding function, but also a
signaling channel, which can simplify the circuit design of the control
interface. In addition, the dynamic time slot assignment of IDT821034
reduces the hardware requirement for PCM interface. The device also
supports 8.192 Mbps PCM data rate, which can increase the time slot
density up to 128.
Signal to total distortion ratio (both STD
X
and STD
R
) are guaranteed
over -55 dBm0 to +3 dBm0 range with a specific gain setting (0 dB for both
transmit path and receive path). Since there is a finite noise floor associated
with the quantization effect of both data converters and digital filter
coefficients, the overall signal to total distortion ratio of each path is a function
of the gain setting. In system design, attention should be paid to the gain
setting for the best signal to total distortion performance.
Generally, a channel gain of a line-card system is contributed
by both SLIC and CODEC. In a system design using IDT821034, the
SLIC gain should be taken into account to optimize the SNR. In the transmit
path of IDT821034, there are two resistors (R1 and R3 in Figure 5)
which enable the analog gain to be adjusted around 0 dB. Further gain
adjustment can be obtained by programming the DSP filters. Since this
adjustment is close to 0 dB, the SNR remains at the optimum value. In
the receive path of IDT821034, analog gain adjustment is not available.
Thus, the adjustment of CODEC gain will be performed only by
programming the DSP filters. In this way, the SLIC gain should be such
that the DSP gain is closest to 0 dB. This will maximize the achievable
SNR in the overall system. For example, if the design target for receive
path gain is -3.5 dB and -7 dB for local and long distance calls
respectively, the recommended solution is to set SLIC gain at -3.5 dB.
As a result, the gain of CODEC, which is adjusted by programming DSP
coefficients, will be 0 dB and -3.5 dB.
