GS3281 datasheet(39/97 Pages) SEMTECH

39 / 97 page

GS3281

Final Data Sheet

Rev.7

PDS-061453

January 2019

39 of 97

Semtech

Proprietary & Confidential

www.semtech.com

4.7.3 Output Driver Data Rate Selection

By default, the GS3281 uses the output driver and slew rate group settings for the data

rate to which the CDR is locked.

When the CDR is unlocked it will use the Bypass rate group:

 CFG_OUTPUT<n>_CD_BYPASS_DRIVER_SWING

 CFG_OUTPUT<n>_CD_BYPASS_PREEMPH_WIDTH

 CFG_OUTPUT<n>_CD_BYPASS_PREEMPH_AMPL

 CFG_OUTPUT<n>_CD_BYPASS_PREEMPH_PWRDWN

If required, manual selection of the output driver and slew rate group is possible using

the following steps:

1. Set CTRL_OUTPUT<n>_AUTO_SLEW = 0

2. Set CTRL_OUTPUT<n>_MANUAL_SLEW to the desired rate group. The slew rate

options are as follows:

0 = SD/MADI

1 = HD/3G

4.7.4 Amplitude and Pre-Emphasis Control

The two output drivers offer very granular amplitude and pre-emphasis control. For

optimal loss compensation, both the pre-emphasis pulse amplitude and the

pre-emphasis pulse width can be independently configured on both output drivers.

This extra flexibility provides a mechanism to better shape the pre-emphasis gain to

match the frequency loss response of interconnect composed of trace, connector and

via losses. The swing and pre-emphasis can be independently configured for specific

data rates.

Note: The parameters referred to within this section are linked to their respective

registers in Table 4-8. For a complete list of registers and functions, please see Section 5.

The output swing can be configured for the following three rate groups:

CFG_OUTPUT<n>_CD_SD_DRIVER_SWING (MADI and SD)

CFG_OUTPUT<n>_CD_HD_DRIVER_SWING (HD and 3G)

CFG_OUTPUT<n>_CD_BYPASS_DRIVER_SWING (Bypass)

The output pre-emphasis can be configured for the following two rate groups:

CFG_OUTPUT<n>_CD_HD_PREEMPH_WIDTH (HD and 3G)

CFG_OUTPUT<n>_CD_HD_PREEMPH_AMPL (HD and 3G)

CFG_OUTPUT<n>_CD_BYPASS_PREEMPH_WIDTH (Bypass)

CFG_OUTPUT<n>_CD_BYPASS_PREEMPH_AMPL (Bypass)

The output driver swing and pre-emphasis will use the rate specific swing configuration

when the CDR is locked to that rate. The default swing setting is ~800mVpp single

ended into an external 75Ω load, and is adjustable in each of the output swing

parameters listed above. Applications where maximum output swing and pre-emphasis

range are desired, it is recommended that the output supplies VCCO_0 and VCCO_1 be

connected to a 3.3V supply. For most applications with short trace between GS3281 and

output BNC, 2.5V power supply can be used.

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