CDB61535 datasheet(14/44 Pages) CIRRUS

14 / 44 page

The FIFO in the jitter attenuator is designed to

prevent overflow and underflow. If the jitter am-

plitude becomes very large, the read and write

pointers may get very close together. Should they

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 under-

flow. During this activity, data will never be lost.

The 32-bit FIFO in the CS61577 attenuator al-

lows it to absorb jitter with minimum data delay

in T1 and E1 switching or transmission applica-

tions. Like the CS61574, the CS61577 will

tolerate large amplitude jitter (>23 UIpp) by

tracking rather than attenuating it, preventing data

errors so that the jitter may be absorbed in exter-

nal frame buffers.

The jitter attenuator may be bypassed by pulling

XTALIN to RV+ through a 1 k

Ω resistor and pro-

viding a 1.544 MHz (or 2.048 MHz) clock on

ACLKI. RCLK may exhibit quantization jitter of

approximately 1/13 UIpp and a duty cycle of ap-

proximately 30% (70%) when the attenuator is

disabled.

Local Loopback

Local loopback is selected by taking LLOOP, pin

27, high or by setting the LLOOP register bit via

the serial interface.

The local loopback mode takes clock and data

presented on TCLK, TPOS, and TNEG (or

TDATA), sends it through the jitter attenuator and

outputs it at RCLK, RPOS and RNEG (or

RDATA). If the jitter attenuator is disabled, it is

bypassed. Inputs to the transmitter are still trans-

mitted on TTIP and TRING, unless TAOS has

been selected in which case, AMI-coded continu-

ous ones are transmitted at the TCLK frequency.

The receiver RTIP and RRING inputs are ignored

when local loopback is in effect.

Remote Loopback

Remote loopback is selected by taking RLOOP,

pin 26, high or by setting the RLOOP register bit

via the serial interface.

In remote loopback, the recovered clock and data

input on RTIP and RRING are sent through the

jitter attenuator and back out on the line via TTIP

and TRING. Selecting remote loopback overrides

any TAOS request (see Table 6). The recovered

incoming signals are also sent to RCLK, RPOS

and RNEG (or RDATA). A remote loopback oc-

c u rs i n respon se to RLOOP g oin g h igh .

Frequency in Hz

110

100

1 k

10 k

b) Maximum

Attenuation

Limit

62411 Requirements

a) Minimum Attenuation Limit

Measured Performance

Figure 12. Typical Jitter Transfer Function

RLOOP

Input

Signal

TAOS

Input

Signal

Source of

Data for

TTIP & TRING

Source of

Clock for

TTIP & TRING

TDATA

TCLK

all 1s

TCLK

RTIP & RRING

RTIP & RRING (RCLK)

Notes: 1. X = Don’t Care. The identified All Ones Select

input is ignored when the indicated loopback is

in effect.

2. Logic 1 indicates that Loopback or All Ones

option is selected.

Table 7. Interaction of RLOOP with TAOS

CS61577

DS155PP2

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