Forum Discussion
New ContributorInvestigation of HIGH-Level Voltage Fluctuations on MAX 10 3.3 V LVTTL Output Signals
New Contributor1. The issue is a variation in the high-level voltage of a pulse signal in the 1 MHz range.
2. An active probe is used, with the GND connection made through the shortest possible via.
3. No additional load is applied.
We also confirmed with TI that noise from the LVDS signal may couple through the IC's internal circuitry.
Since this phenomenon has not been observed on signals connected to other devices, we believe this is the likely cause.
Regular ContributorMay I know what is your main concern on this voltage fluctuations?
I checked the datasheet of the external device you connected and found that 2.8 V is still a valid high for the DS90CR287. The TxIN pins are LVTTL/LVCMOS inputs, and the datasheet specifies VIH at 2.0 V minimum (DS90CR287, SNLS056, https://www.ti.com/lit/ds/symlink/ds90cr287.pdf). Correct me if I'm wrong.
- y-fujiwara
As you mentioned, this phenomenon does not affect the circuit's operation.
We investigated it as a precaution because we noticed an unusual feature in the waveform.
The phenomenon appears to be caused by internal noise coupling within the IC. Therefore, if there are no effective countermeasures available, we believe it should be acceptable to leave the design as it is.
- AqidAyman_Altera
I understand. Thank you for the clarification.
From the MAX 10 side, there are several signal integrity levers worth examining if you have the chance to do it as a software-only change without board respins required.
Two immediate knobs to try:
- Reduce drive strength. The output buffer for each MAX 10 I/O pin has a programmable current strength control. For 3.3 V LVTTL, the available settings are 8 mA and 4 mA. Programmable Current Strength Try to drop from 8 mA to 4 mA.
- Enable slow slew rate. Slow slew rate reduces system noise and crosstalk, but adds a nominal delay to the rising and falling edges. MAX 10 General Purpose I/O User Guide If timing margin permits, this is a low-risk change that you can experiment.
