IDT821064 QUAD PROGRAMMABLE PCM CODEC WITH GCI INTERFACE
INDUSTRIAL TEMPERATURE RANGE
The Frame Sync (FSC) pulse identifies the beginning of the Transmit
and Receive frames and all GCI time slots are referenced to it. The Data
Clock (DCL) is either 2.048 MHz or 4.096MHz, the internal circuit of
IDT821064 monitors this input to determine which frequency is being
used. The internal timing will be adjusted according to the DCL frequency
so that DU and DD operate at 2M rate.
IDT821064 allows both compressed and linear data format coding/
decoding. VDS bit in Global Regiser 5 makes the selection of voice data
format.
FUNCTIONAL DESCRIPTION
The IDT821064 is a four-channel PCM CODEC with on-chip digital
filters. It provides the four-wire solution for the subscriber line circuitry in
digital switches. The IDT821064 converts analog voice signals to digital
PCM samples and digital PCM samples back to analog voice signals.
Digital filters are used to bandlimit the voice signals during conversion.
High performance oversampling Analog-to-Digital Converters (ADC) and
Digital-to-Analog Converters (DAC) in the IDT821064 provide the required
conversion accuracy. The associated decimation and interpolation filters
are realized with both dedicated hardware and Digital Signal Processor
(DSP). The DSP also handles all other necessary functions such as
PCM bandpass filtering, sample rate conversion and PCM companding.
See the Functional Block Diagram.
COMPRESSED GCI MODE
In GCI compressed mode, one GCI frame consists of 8 GCI time
slots, the Data Upstream Interface transmits four 8-bit bytes per GCI
time slot. They are:
- Two voice data bytes from the A-law or m-law compressor for two
different channel, for easy description, we name the two channels as
channel A and channel B. The compressed voice data bytes for channel
A and B are 8-bit wide;
In the transmit path, the analog voice signal input from VIN pin is
converted to PCM code by ADC, DSP and PCM companding circuits.
Band-limiting functions as specified in ITU-T are implemented by digital
filters. At last the fully processed signal is transferred to the GCI interface,
in a compressed or linear signal presentation.
In the receive path, the digital signal is received via the GCI interface.
Then it is expanded and sent to the DSP for interpolation and receive
channel filtering function. The filtered signal is then sent to an
oversampling DAC. The DAC output is post-filtered and then delivered
at VOUT pin by a power amplifier. The amplifier can drive resistive load
higher than 300 W AC.
- One Monitor channel byte, which is used for reading control data
from the device for Channel A and B;
- One C/I channel byte, which contains a 6 bit width C/I channel sub-
byte together with an MX bit and an MR bit. All real time signaling infor-
mation is carried on the C/I channel sub-byte. The MX (Monitor Trans-
mit) bit and MR (Monitor Receive) bit are used for handshaking func-
tions for Channel A and B. Both MX and MR are active low.
Transmit logic controls the transmission of data onto the GCI bus.
The data structure of the Data Downstream is as same as that of
Upstream. The Data Downstream Interface logic controls the reception
of data bytes from the GCI bus. The two compressed voice channel
data bytes of the GCI time slot are transferred to the A-law or m-law ex-
pansion logic circuit. The expanded data is passed to the receive pathof
the signal processor. The Monitor Channel and C/I Channel bytes are trans-
ferred to the GCI control logic for processing.
GCI INTERFACE
The General Control Interface (GCI) provides communication of both
control and voice data between the GCI highway and SLIC over a pair of
pins on the IDT821064. The IDT821064 sends Data Upstream out of
the DU pin and receives Data Downstream on the DD pin. DCL and FS
are two input clock signals providing Data Clock (DCL) and Frame
Synchronization (FS) information for the device. A complete GCI frame
is sent upstream on DU pin and received downstream on DD pin every
125 ms.
Figure 1 shows the overall compressed GCI frame structure.
125 ms
FSC
DCL
TS0
TS0
TS1
TS1
TS2
TS2
TS3
TS4
TS4
TS5
TS5
TS6
TS6
TS7
TS7
DD
DU
Detail
TS3
Detail
M M
Voice Channel A
Voice Channel A
Voice Channel B
Voice Channel B
Monitor Channel C/I Channel
Monitor Channel C/I Channel
DD
R
X
M M
DU
R
X
Figure 1. Compressed GCI Frame Structure
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