MSC8254_11 Datasheet, PDF (43/68 Pages) Freescale Semiconductor, Inc – Quad-Core Digital Signal Processor

Electrical Characteristics
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 MSC8254
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
UI
399.88
400.00
400.12
ps
1
Minimum Tx eye width
T_TX-EYE
0.70
—
—
UI
2, 3
Maximum time between the jitter median and
T_{TX-EYE-MEDIAN-}
—
maximum deviation from the median.
to-MAX-JITTER
—
0.15
UI
3, 4
AC coupling capacitor
C_TX
75
—
200
nF
5
Notes: 1. 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.
2. The maximum transmitter jitter can be derived as T_{TX-MAX-JITTER} = 1 – T_TX-EYE = 0.3 UI.
3. 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 T_TX-EYE = 0.70 UI provides for a total sum of deterministic and random jitter budget of
T_{TX-JITTER-MAX} = 0.30 UI for the transmitter collected over any 250 consecutive Tx UIs. The T_{TX-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 (V_TX-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.
4. Jitter is measured using all edges of the 250 consecutive UI in the center of the 3500 UI used for calculating the Tx UI.
5. 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
UI
399.88
400.00
400.12
ps
1
Minimum receiver eye width
T_RX-EYE
0.4
—
—
UI
2, 3, 4
Maximum time between the jitter median
T_{RX-EYE-MEDIAN-to-MAX}
—
and maximum deviation from the median.
-JITTER
—
0.3
UI
3, 4, 5
Notes: 1. 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.
2. The maximum interconnect media and transmitter jitter that can be tolerated by the receiver can be derived as
T_{RX-MAX-JITTER} = 1 – T_RX-EYE = 0.6 UI.
3. 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.
4. A T_RX-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 T_{RX-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.
5. Jitter is defined as the measurement variation of the crossing points (V_RX-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.
MSC8254 Quad-Core Digital Signal Processor Data Sheet, Rev. 6
Freescale Semiconductor
43