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GS3281 Datasheet(PDF) 39 Page - Semtech Corporation |
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GS3281 Datasheet(HTML) 39 Page - Semtech Corporation |
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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