MSC8252 datasheet(42/68 Pages) FREESCALE

42 / 68 page

MSC8252 Dual-Core Digital Signal Processor Data Sheet, Rev. 3

Electrical Characteristics

Freescale Semiconductor

2.6.2.2

PCI Express AC Physical Layer Specifications

The AC requirements for PCI Express implementations have separate requirements for the Tx and Rx lines. The MSC8252

supports a 2.5 Gbps PCI Express interface defined by the PCI Express Base Specification, Revision 1.0a. The transmitter

specifications are defined in Table 25 and the receiver specifications are defined in Table 26. The parameters are specified at

the component pins. the AC timing specifications do not include REF_CLK jitter.

Note:

Specifications are valid at the recommended operating conditions listed in Table 3.

Table 25. PCI Express (2.5 Gbps) Differential Transmitter (Tx) Output AC Specifications

Parameter

Symbol

Min

Typical

Max

Units

Notes

Unit interval

399.88

400.00

400.12

Minimum Tx eye width

TTX-EYE

0.70

—

2, 3

Maximum time between the jitter median and

maximum deviation from the median.

TTX-EYE-MEDIAN-

to-MAX-JITTER

—

0.15

3, 4

AC coupling capacitor

CTX

—

200

Notes:

Each UI is 400 ps ± 300 ppm. UI does not account for spread spectrum clock dictated variations. No test load is necessarily

associated with this value.

The maximum transmitter jitter can be derived as TTX-MAX-JITTER = 1 – TTX-EYE = 0.3 UI.

Specified at the measurement point into a timing and voltage compliance test load as shown in Figure 8 and measured over

any 250 consecutive Tx UIs. A TTX-EYE = 0.70 UI provides for a total sum of deterministic and random jitter budget of

TTX-JITTER-MAX = 0.30 UI for the transmitter collected over any 250 consecutive Tx UIs. The TTX-EYE-MEDIAN-to-MAX-JITTER

median is less than half of the total Tx jitter budget collected over any 250 consecutive Tx UIs. It should be noted that the

median is not the same as the mean. The jitter median describes the point in time where the number of jitter points on either

side is approximately equal as opposed to the averaged time value. Jitter is defined as the measurement variation of the

crossing points (VTX-DIFFp-p = 0 V) in relation to a recovered Tx UI. A recovered Tx UI is calculated over 3500 consecutive

unit intervals of sample data.

Jitter is measured using all edges of the 250 consecutive UI in the center of the 3500 UI used for calculating the Tx UI.

All transmitters shall be AC-coupled. The AC coupling is required either within the media or within the transmitting component

itself. The SerDes transmitter does not have built-in Tx capacitance. An external AC coupling capacitor is required.

Table 26. PCI Express (2.5 Gbps) Differential Receiver (Rx) Input AC Specifications

Parameter

Symbol

Min

Typical

Max

Units

Notes

Unit Interval

399.88

400.00

400.12

Minimum receiver eye width

TRX-EYE

0.4

—

2, 3, 4

Maximum time between the jitter median

and maximum deviation from the median.

TRX-EYE-MEDIAN-to-MAX

-JITTER

—

0.3

3, 4, 5

Notes:

Each UI is 400 ps ± 300 ppm. UI does not account for spread spectrum clock dictated variations. No test load is necessarily

associated with this value.

The maximum interconnect media and transmitter jitter that can be tolerated by the receiver can be derived as

TRX-MAX-JITTER =1 – TRX-EYE = 0.6 UI.

Specified at the measurement point and measured over any 250 consecutive UIs. The test load in Figure 8 should be used as

the Rx device when taking measurements. If the clocks to the Rx and Tx are not derived from the same reference clock, the

Tx UI recovered from 3500 consecutive UI must be used as a reference for the eye diagram.

A TRX-EYE = 0.40 UI provides for a total sum of 0.60 UI deterministic and random jitter budget for the transmitter and

interconnect collected any 250 consecutive UIs. The TRX-EYE-MEDIAN-to-MAX-JITTER specification ensures a jitter distribution in

which the median and the maximum deviation from the median is less than half of the total. UI jitter budget collected over any

250 consecutive Tx UIs. It should be noted that the median is not the same as the mean. The jitter median describes the point

in time where the number of jitter points on either side is approximately equal as opposed to the averaged time value. If the

clocks to the Rx and Tx are not derived from the same reference clock, the Tx UI recovered from 3500 consecutive UI must

be used as the reference for the eye diagram.

Jitter is defined as the measurement variation of the crossing points (VRX-DIFFp-p = 0 V) in relation to a recovered Tx UI. A

recovered Tx UI is calculated over 3500 consecutive unit intervals of sample data. Jitter is measured using all edges of the

250 consecutive UI in the center of the 3500 UI used for calculating the Tx UI. It is recommended that the recovered Tx UI is

calculated using all edges in the 3500 consecutive UI interval with a fit algorithm using a minimization merit function. Least

squares and median deviation fits have worked well with experimental and simulated data.

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