M5913 datasheet(5/17 Pages) STMICROELECTRONICS

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the

signaling

function.

TSX is a timeslot

strobe/buffer enable output which gates the PCM

word onto the PCM highway when an external

buffer is used to drive the line.

Data is transmitted on the highway at DX on the

first eight positive transitions of CLKX following

the rising edge of FSX. Similarly, on the receive

side, data is received on the first eight falling

edges of CLKR. The frequency of CLKX and CLKR

is selected by the CLKSEL pin to be either 1.536,

1.544 or 2.048MHz. No other frequency of opera-

tion is allowed in the fixed data rate mode.

Variable Data Rate Mode

Variable data rate timing is selected by connect-

ing DCLKR to the bit clock for the receive PCM

highway rather than to VBB. It employes master

clocks CLKX and CLKR, bit clocks DCLKR and

DCLKX and frame synchronization clocks FSR

and FSX.

Variable data rate timing allows for a flexible data

frequency. It provides the ability to vary the fre-

quency of the bit clocks, from 64kHz to 4096MHz.

Master clocks inputs are still restricted to 1.536,

1.544, or 2.048MHz.

In this mode, DCLKR and DCLKX become the

data clocks for the receive and transmit PCM

highways. While FSX is high, PCM data from DX

is transmitted onto the highway on the next eight

consecutive positive transitions of DCLKX. Simi-

larly, while FSR is high, each PCM bit from the

highway is received by DR on the next eight con-

secutive negative transitions of DCLKR.

On the transmit side, the PCM word will be re-

peated in all remaining timeslots in the 125

µs

frame as long as DCLKX is pulsed and FSX is

held high. This feature allows the PCM word to be

transmitted to the PCM highway more than once

per frame, if desired, and is only available in the

variable data rate mode. Conversely, signaling is

only allowed in the fixed data rate mode since the

variable mode provides no means with which to

specify a signaling frame.

Precision Voltage References

No external components are required with the

combochip to provide the voltage reference func-

tion. Voltage references are generated on-chip

and are calibrated during the manufacturing proc-

ess. The technique use the bandgap principle to

derive a temperature and bias stable reference

voltage. These references determine the gain and

dynamic range characteristics of the device.

Separate references are supplied to the transmit

and receive sections. Transmit and receive sec-

tion are trimmed independently in the filter stages

to a final precision value. With this method the

combochip can achieve manufacturing tolerances

of typically

± 0.04dB in absolute gain for each half

channel, providing the user a significant margin

for error in other board components.

Conversion Laws

The M5913 is designed to operate in both

µ-law

and A-law systems. The user can select either

conversion law according to the voltage present

on the SIGX/ASEL pin . In each case the coder

and decoder process a companded 8-bit PCM

word following CCITT recommandation G.711 for

µ-law and A-law conversion. If A-law operation is

desired, SIGX should be tied to VBB. Thus, signal-

ing is not allowed during A-law operation. If

µ =

255-law operation is selected, then SIGX is a TTL

level input which modifies the LSB on the PCM

output in signaling frames

TRANSMIT OPERATION

Transmit Filter

The input section provides gain adjustment in the

passband by means of an on-chip uncommitted

operational amplifier. This operational amplifier

has a common mode range of 2.17V, a maximum

DC offset of 25mV, a minimum voltage gain of

5000, and a unity gain bandwidth of typically

1MHz. Gain of up to 20dB can be set without de-

grading the performance of the filter. The load im-

pedance to ground (GRDA) at the amplifier output

(GSX) must be greater than 10k

Ω in parallel high

less than 50pF. The input signal on lead VFXI+

can be either AC or DC coupled. The input op

amp can also be used in the inverting mode or

differential amplifier mode (see figure 3).

A low pass anti-aliasing section is included on-

chip. This section typically provides 35dB attenu-

ation at the sampling frequency. No external com-

ponents are required to provide the necessary

anti-aliasing function for the switched capacitor

section of the transmit filter.

The passband section provides flatness and stop-

band attenuation which fulfills the AT&T D3/D4

channel bank transmission specification and

CCITT recommendation G.712.

The M5913 specifications meet or exceed digital

class 5 central office switching systems require-

ments. The transmit filter transfer characteristics

and specifications will be within the limits shown

the relative table.

A high pass section configuration was chosen to

reject low frequency noise from 50 and 60Hz

power lines, 17Hz European electric railroads,

ringing frequencies and their harmonics, and

other low frequency noise.

Even though there is high rejection at these fre-

quencies, the sharpness of the band edge gives

low attenuation at 200Hz. This feature allows the

use of low-cost transformer hybrids without exter-

nal components.

M5913

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