LVCMOS Input Erroneously Detects Logic 1 When Left Floating

Updated Nov 1, 2023

Issue Details

  • I am testing a PFI line on my NI hardware, which uses LVCMOS. When I leave the input floating, I erroneously detect a logic 1. Is my device damaged?
  • I have a device that uses a 3.3V LVCMOS input. It sporadically switches between a high and low signal. Is this expected?
  • When I don't have anything connected to my LVCMOS input, I randomly measure a logic 1. Is my device broken?

Solution

When a LVCMOS input is left floating i.e. there is no load connected, the input is in a indeterminate state. Indeterminate states mean that the input can either be in a high or low state, which cannot be accurately predicted. This is due to the fact that LVCMOS (and TTL) signals are not intended to be used when floating.

If your device is sporadically detecting a logic 1, the device is unlikely to be broken or damaged. Instead, connect a load to the input to verify a stable signal. If the device continues to randomly switch between a logic high and low, contact NI Support.

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