Are Logic Levels Correct for This Voltage Domain?#

Logic families have specific voltage thresholds that define what counts as “high” and “low.” A 3.3V output driving a 5V input may or may not be recognized as high — it depends on the specific thresholds, not just whether the signal “looks high” on the scope. Marginal logic levels are the source of intermittent, temperature-dependent, works-on-my-bench failures.

Logic Level Thresholds#

For reliable communication: VOH > VIH and VOL < VIL. The gap between them is noise margin.

Common Logic Families#

Level Shifting: 3.3V to 5V#

A 3.3V LVCMOS output has VOH ≥ 2.4V. Whether a 5V input recognizes this as high depends on the receiver:

This is why HCT-family parts are commonly used for 3.3V-to-5V interfaces — their TTL input thresholds accept 3.3V outputs cleanly.

Measuring Logic Levels#

DC-couple the scope channel and set vertical scale to show the full logic swing. Probe the signal and trigger on an edge to get a stable display. Use cursors or auto-measurement to read the high and low levels — the flat portions when the signal is high and low.

Compare to the VOH/VOL specs (driving side) and VIH/VIL specs (receiving side). Measure at the receiver’s input pin, not the driver’s output — voltage drops across traces, connectors, and series resistors reduce the level that arrives at the receiver.

DMM DC Level Spot Check#

For quick checks on static logic levels (enable pins, chip selects, configuration pins), set the DMM to DC Volts, probe the pin vs. ground, and compare to VIH/VIL thresholds.

A DMM averages the reading — if the pin is toggling (even slowly), the DMM shows an average voltage between high and low, which lands in the undefined region and looks like a fault.

Identifying Marginal Logic Levels#

Symptoms of marginal levels:

• Works at room temperature, fails when hot or cold
• Works at one supply voltage, fails when the supply droops slightly
• Works on one board but not another (component tolerance)
• Works at low speed but fails at high speed

Diagnosis: Measure VOH at the driver with the actual load connected, then measure voltage at the receiver input pin. Calculate noise margin: NM_high = V_at_receiver - VIH_threshold. If noise margin is less than a few hundred millivolts, the interface is marginal.

Tips#

• Measure at the receiver’s input pin, not the driver’s output — trace and connector resistance drops voltage between the two points
• Temperature affects thresholds and output drive — a marginal signal at room temperature may fail at high temperature
• For open-drain buses (I2C), pull-up resistor value and bus capacitance determine actual levels

Caveats#

• Floating pins can read any voltage — if a pin reads ~1.5V on a 3.3V system and it’s not driven, it’s probably floating
• A DMM averages toggling signals — use a scope for anything that might be switching
• Capacitive loading rounds edges and reduces time the signal spends at valid levels — at high data rates, the signal may never fully reach VOH or VOL
• Open-drain buses have levels set by pull-up resistors and bus capacitance — a weak pull-up gives slow rise times and VOH that depends on RC time constant vs. data rate

In Practice#

• Measured VOH well above VIH threshold indicates healthy noise margin
• Measured VOH barely above or below VIH threshold indicates marginal interface — will fail under temperature or supply variation
• Pin reading mid-rail voltage when it should be driven indicates floating input or bus contention
• Logic level that’s correct at DC but fails at speed indicates capacitive loading slowing the edges
• Interface that works on some boards but not others suggests component tolerance pushing margins — tighten tolerances or redesign interface
• A digital output whose logic levels have degraded (higher VOL, lower VOH) commonly appears after the output has been subjected to bus contention or excessive load — the output transistors have been partially degraded by overcurrent, and their on-resistance has increased, a primitive-level consequence of a system-level configuration error.