821004J QUAD NON-PROGRAMMABLE PCM CODEC
INDUSTRIAL TEMPERATURE RANGE
FUNCTIONAL DESCRIPTION
Transmit PCM Interface
The 821004J contains four channel PCM CODEC with on chip digital
filters. It provides the four-wire solution for the subscriber line circuitry in
digital switches. The device converts analog voice signal to digital PCM
data, and converts digital PCM data back to analog signal. Digital filters
are used to bandlimit the voice signals during the conversion. Either A-law
or m-law is supported by the 821004J. The law selection is performed by A/
m pin.
The transmit PCM interface clocks out 1 byte (8 bits) PCM data out of
DX pin every 125 ms. The transmit logic, synchronized by the Transmit
Frame Sync signal (FSXn), controls the data transmission. The FSXn
pulse identifies the transmit time slot of the PCM frame for Channel N.
The PCM Data is transmitted serially on DX pin with the Most Significant
Bit (MSB) first. When the PCM data is being output on DX pin, the TSC
signal will be pulled low.
The frequency of the master clock (MCLK) can be 2.048 MHz, 4.096
MHz, or 8.192 MHz. Internal circuitry determines the master clock frequency
automatically.
The serial PCM data for four channels are time multiplexed via two pins,
DX and DR. The time slots of the four channels are determined by the
individual Frame Sync signals at rates from 256 kHz to 8.192 MHz. For
each channel, the 821004J provides a transmit Frame Sync signal and a
receive Frame Sync signal.
Receive Signal Processing
In the receive path, the PCM code is received at the rate of 8,000 samples
per second. The PCM code is expanded and sent to the DSP for
interpolation.A receive filter is implemented in the DSP as a digital lowpass
filter. The filtered signal is then sent to an oversampling DAC. The DAC
output is post-filtered and delivered at VOUT pin by an amplifier. The
amplifier can drive resistive load higher than 2 KW.
Each channel of the 821004J can be powered down independently to
save power consumption. The Channel Power Down Pins PDN1-4 configure
channels to be active (power-on) or standby (power-down) separately.
Receive PCM Interface
The receive PCM interface clocks 1 byte (8 bits) PCM data into DR pin
every 125 ms. The receive logic, synchronized by the Receive Frame Sync
signal (FSRn), controls the data receiving process. The FSRn pulse
identifies the receive time slot of the PCM frame for Channel N. The PCM
Data is received serially on DR pin with the Most Significant Bit (MSB)
first.
Signal Processing
High performance oversampling Analog-to-Digital Converters (ADC) and
Digital-to-Analog Converters (DAC) are used in the 821004J to provide the
required conversion accuracy. The associated decimation and interpo-
lation filtering are realized with both dedicated hardware and Digital Signal
Processor (DSP). The DSP also handles all other necessary functions such
as PCM bandpass filtering and sample rate conversion.
Hardware Gain Setting In Transmit Path
The transmit gain of the 821004J for each channel can be set by 2
resistors, RREF and RTXn (as shown in Figure 1), according to the follow-
ing equation:
Transmit Signal Processing
In the transmit path, the analog input signal is received by the ADC and
converted into digital data. The digital output of the oversampling ADC is
decimated and sent to the DSP. The transmit filter is implemented in the
DSP as a digital bandpass filter. The filtered signal is further decimated
and compressed to PCM format.
3´RREF
Gt =
RTXn
The receive gain of 821004J is fixed and equal to 1.
821004J
to
SLIC
VTX
CTX1
RTX1
A/D
IREF
VIN1
VREF
to
IREF
RREF1
CFIL
IREF1
Bal
Net
VREF1
to
SLIC
RSN
RRX1
CRX1
VOUT1
VREF
D/A
Figure 1. 821004J Transmit Gain Setting for Channel 1
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