CS61535A datasheet(13/48 Pages) CIRRUS

13 / 48 page

When any transmit control pin (TAOS, LEN0-2

or LLOOP) is toggled, the transmitter stabilizes

within 22 bit periods. The transmitter will take

longer to stabilize when RLOOP is selected be-

cause the timing circuitry must adjust to the new

frequency.

Jitter Attenuator

The jitter attenuator is designed to reduce wander

and jitter in the transmit clock signal. It consists

of a 32 bit FIFO, a crystal oscillator, a set of load

capacitors for the crystal, and control logic. The

jitter attenuator exceeds the jitter attenuation re-

quirements of Publications 43802 and REC.

G.742. A typical jitter attenuation curve is shown

in Figure 10.

The jitter attenuator works in the following man-

ner. Data on TPOS and TNEG (or TDATA) are

written into the jitter attenuator’s FIFO by TCLK.

The rate at which data is read out of the FIFO and

transmitted is determined by the oscillator. Logic

circuits adjust the capacitive loading on the crys-

tal to set its oscillation frequency to the average

of the TCLK frequency. Signal jitter is absorbed

in the FIFO.

Jitter Tolerance of Jitter Attenuator

The FIFO in the jitter attenuator is designed to

neither overflow nor underflow. If the jitter am-

plitude becomes very large, the read and write

pointers may get very close together. Should the

pointers attempt to cross, the oscillator’s divide

by four circuit adjusts by performing a divide by

3 1/2 or divide by 4 1/2 to prevent the overflow

or underflow. When a divide by 3 1/2 or 4 1/2

occurs, the data bit will be driven on to the line

either an eighth bit period early or an eighth bit

period late.

When the TCLK frequency is close to the center

frequency of the crystal oscillator, the high fre-

quency jitter tolerance is 23 UI before the divide

by 3 1/2 or 4 1/2 circuitry is activated. As the

center frequency of the oscillator and the TCLK

frequency deviate from one another, the jitter tol-

erance is reduced. As this frequency deviation

becomes large, the maximum jitter tolerance at

high frequencies is reduced to 12 UI before the

underflow/overflow circuitry is activated. In ap-

plication, it is unlikely that the oscillator center

frequency will be precisely aligned with the

Frequency in Hz

100

1 k

10 k

b) Maximum

Attenuation

Limit

AT&T 62411

Requirements

a) Minimum Attenuation Limit

Measured Performance

Figure 10. Typical Jitter Attenuation Curve

269 ns

244 ns

194 ns

219 ns

488 ns

Nominal Pulse

100

110

120

-10

-20

Percent of

nominal

peak

voltage

Figure 9 . Mask of the Pulse at the 2048 kbps Interface

CS61535A

DS40F2

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