TX Pattern Generator

Versal Adaptive SoC GTM Transceivers Architecture Manual (AM017)
Document ID
AM017
Release Date
2024-09-05
Revision
1.1 English

Pseudo-random sequences (PRBS) are commonly used to test the signal integrity of high-speed links. These sequences appear random but have specific properties that can be used to measure the quality of a link. The GTM transceiver pattern generator block can generate several industry-standard PRBS patterns listed in the following table.

Table 1. Supported PRBS Patterns
Name Polynomial Length of Sequence Description
PRBS-7 1 + x6 + x7 27 – 1 bits Used to test channels with 8B/10B.
PRBS-9 1 + x5 + x9 29 – 1 bits ITU-T Recommendation O.150, Section 5.1. PRBS-9 is one of the recommended test patterns for SFP+.
PRBS-13 1 + x + x2 + x12 + x13 29 – 1 bits IEEE Std P802.3bs 120.5.11.2.1 test requires PRBS13Q test pattern. OIF2014.230 CEI-56G-VSR-PAM4 specification requires QPRBS13-CEI test pattern.
PRBS-15 1 + x14 + x15 215 – 1 bits PRBS-15 is often used for jitter measurement because it is the longest pattern the Agilent DCA-J sampling scope can handle.
PRBS-23 1 + x18 + x23 223 – 1 bits PRBS-23 is often used for non-8B/10B encoding schemes. It is one of the recommended test patterns in the SONET specification.
PRBS-31 1 + x28 + x31 231 – 1 bits PRBS-31 is often used for non-8B/10B encoding schemes. It is a recommended PRBS test pattern for 10 Gigabit Ethernet. The pattern generator output does not include the inverter specified by IEEE802.3-2018 for PRBS31.
Important: For PAM4 modulation, QPRBS/PRBSQ patterns are supported by sending a conventional PRBS pattern with PAM4 and Gray Coding based on the OIF2014.230 CEI-56G-VSR-PAM4 specification and IEEE Std P802.3bs. The PRBSQ pattern, as defined in IEEE 802.3bs-2017 120.5.11.2.1, is a Gray-coded PRBS pattern repeated twice. It is equivalent to the QPRBS-CEI pattern as defined in OIF-CEI-04.0 CEI-56G-VSR-PAM4 Clause 16. QPRBS, as defined in IEEE 802.3-2015 94.2.9.3, is PRBS followed by inverted PRBS. The A_TX/RXQPRBSEN is used to enable QPRBS.

The PRBS31 pattern is not IEEE802.3-2018 compliant due to non-inverted output. This means to generate an OIF/IEEE Std 802.3 compliant PRBS31Q pattern, a soft generator that can provide an IEEE Std 802.3-2018 compliant PRBS31 must be used prior to Gray encoding. One such PRBS generator and checker is demonstrated in An Attribute-Programmable PRBS Generator and Checker (XAPP884).

The following table describes the relationship between QPRBSEN, Gray Coding bypass, and the associated data pattern.
Table 2. PRBS Patterns for PAM4
TXQPRBSEN TX_GRAY_BYP_EN PRBS Pattern
0 0 PRBSQ
0 1 PRBS
1 1 QPRBS

In addition to PRBS patterns, the GTM transceiver supports a 64 UI square wave test pattern as shown in the following figure, an alternating 1'b0 and 1'b1 (NRZ clock) test pattern. Clocking patterns are usually used to check PLL random jitter often done with a spectrum analyzer.

Note: For PAM4 modulation, an alternating 1'b0 and 1'b1 test pattern will not be a square wave due to the amplitude modulation mapping.
Figure 1. 64 UI Square Wave

The error insertion function is also supported to verify link connection for jitter tolerance tests. When an inverted PRBS pattern is necessary, the CH[0/1]_TXPOLARITY signal is used to control polarity.

Figure 2. TX Pattern Generator Block

The pattern generator might control when an injection of an error in the generated pattern occurs by choosing between two modes. In LEVEL mode, the generator injects an error after any cycle in which CH*_TXPRBSFORCEERR is High. In EDGE mode, an error is injected only after CH*_TXPRBSFORCEERR has just gone High after being Low in the previous cycle. The mode selection can be done using the TX_PRBS_FORCE_MODE attribute. The following figure shows the error injection in both operating modes.

Figure 3. LEVEL and EDGE Mode Error Injection