If SYSREF Not Observed
There are several settings which must be correct to achieve SYSREF outputs.
• Ensure the following settings:
– Set SYSREF_MODE to Continuous (for debugging)
– SRREQ_MODE field set to SYSREFREQ mode
– SRREQ_VCM set for DC-coupled, with about 1.1 V on SYSREFREQ_N and 1.5 V on SYSREFREQ_P
– SYSREF_DLY_BYP field set to use delay
– SYSREF_EN=1
• Ensure the frequencies of the SYSREF_DLY_DIV, SYSREF_DIV_PRE, and SYSREF_DLY_ADJ are correctly
configured. The GUI will highlight any frequency violations.
• Ensure that F
INTERPOLATOR
% F
SYSREF
= 0. The GUI will highlight the SYSREF divider in case of violations.
• Ensure that the output channel (CHx_EN/LOGIC_EN) and the SYSREF buffer (SYSOUTx_EN/
LOGISYS_EN) are enabled.
• Confirm that Windowing mode is not enabled on the
User Controls
page (SYSWND_EN=0).
• Confirm that R15[9]=1. This should be set automatically by the GUI, so this potential root cause should be
rare.
• Confirm the 1.1-V and 1.5-V source for SYSREFREQ_N and SYSREFREQ_P respectively are actually
resulting in the required voltages at the pins. If power supplies are used for these voltages, it is not
uncommon for the supplies to be unable to sink current; the 1.1-V source may not be able to sink current
from the 1.5-V supply through the internal 100-Ω impedance. An arbitrary function generator is recommended
if possible.
Troubleshooting
24
LMX1204EVM User's Guide
SNAU266 – JULY 2021
Copyright © 2021 Texas Instruments Incorporated
